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wbochar
August 2nd, 2014, 08:09 AM
I've got a PET that was really in bad shape (bird nest and all), he was in storage for a while and last week I pulled him out to work on him.

He worked fine for a few days, the first thing was getting the RAM to work properly -- there was few bad chips. Testing was swapping around till I got around 6k. the last two chips I swapped into video ram to see if both were bad. Both were bad.

I've got a site that I am keeping my notes at, which while I was updating them -- the screen died.

PET 2001-8, found in Toronto, Canada
6/8k Working
Has Full PET ASCII Keyboard, Non-Tape Drive model.
Logic Assembly: 320132
Has Original ROM's

So I wasn't sure if something on the main board died, so i started checking ROM's and Ram etc..

Well This is boot up...

CH(A) (TOP) is Pin 1 off of the 2114 SRAM in the first bank.
CH(B) (BOTTOM) is PHI2

19696

Well, it's got a pulse.. hah.

I think it's actually the display system itself. The monitor is not powering up or showing the signs when it's powered off.. The Fading white dot..

It looks like the tube doesn't power up at all. With an induction amplifier, aiming it at ANY CRT will be noisy as hell. My 1084 monitor generates a lot of noise, and a PET monitor should be loud... not a peep.

Opening the back:

It looks ok?
19697

19698

19699

This last pic is with the light off, to see that the tube is actually got something going on.. The tube is lit when the power is on, just not enough to show something.

I have this weird feeling that opening and closing the case continuously (during my ram and rom testing) may have jarred something. Part of me thinks that whatever is driving the tube doesn't have enough power to bring it up.

I have my notes at http://wbochar.com which has more pics of the MB etc.

Thanks for reading,

--Wolfgang Bochar

giobbi
August 9th, 2014, 06:58 AM
Hi,

just a thought about that.

I don't remember if the 2001/8 CRT assy is similar to the 8032 one, but: I had a similar problem with one of my 8032s: motherboard ok, tube ok, tube neck lit (like the one in your photo), etc. --> after some hours spent testing everything, I discovered there are some resistors made by wire that are mounted raised, with a pin far from the pcb. They act like fuses, in someway. One of them was broken and the CRT was dead. Replacing that resistor solved the issue.

As told, I don't remember if the 2001/8 uses the same tecnique in its CRT assy (sorry, too lazy to open mine and verify ;-) )

cheers,
Giovi

dave_m
August 9th, 2014, 08:46 AM
If the horizontal drive signal is not running at about 16 KHz from the main board, there will be no high voltage for the tube.

wbochar
August 9th, 2014, 10:02 AM
If the horizontal drive signal is not running at about 16 KHz from the main board, there will be no high voltage for the tube.

The main board, the connector going to the Display system:

1 O Video
2 O GND
3 O Vertical Drive
4 O GND
5 O Horizontal Dive
6 - (No Pin)
7 O GND

I get 4.08V off Pin's 5/4 but the scope just flatlines. Other mainboard components show activity, but the horizontal and vertical drives don't. Any suggestions where to go next?

dave_m
August 10th, 2014, 10:44 PM
The main board, the connector going to the Display system:

1 O Video
2 O GND
3 O Vertical Drive
4 O GND
5 O Horizontal Dive
6 - (No Pin)
7 O GND

I get 4.08V off Pin's 5/4 but the scope just flatlines. Other mainboard components show activity, but the horizontal and vertical drives don't. Any suggestions where to go next?
what does pins 5/4 mean? Pin 5 with respect to pin 4?

anyway this will be easy to fix as these signals are generated with a few flip flops. Get the correct schematic from zimmers.com. We'll start with sheet 6 at the lower right and trace back the horizontal signal. Can you find chip H7?
-Dave

wbochar
August 11th, 2014, 08:53 AM
This is the Logic Assembly Version (320312) for me:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320132-3.gif

The horizontal Drive connects up to 74L500 (pin2) just over from the video connector. Also shown on the schematics.

H7 is a 6540 on my board, is this what you wanted?

dave_m
August 12th, 2014, 10:13 AM
No, I had wrong schematic for your machine. Look at top right and follow horizontal Drive back to flip flop C5 pin 2. If not a 16 KHz waveform, look at J and K inputs for 16 KHz pulses. They are the Display ON and OFF signals.

wbochar
August 12th, 2014, 02:40 PM
I've got to head out for work, i'll be back to try this out in 2 weeks. Thanks for the tip, I'll et back to you :)

MikeS
August 12th, 2014, 02:56 PM
I've got to head out for work, i'll be back to try this out in 2 weeks. Thanks for the tip, I'll et back to you :)Give me a shout when you get back; maybe we can figure this out together.

BTW, those are pretty short pulses and easy to miss if your trigger an sweep aren't appropriately set.

m

wbochar
August 27th, 2014, 09:13 AM
20137
This is what I get on C5 Pin 2, similar results appear on Pin 4. I powered up the machine and monitored the pin's with each power cycle. I attached the monitor cable and removed it to see if that affected the results. Same.

I also noticed something weird, I hooked up my C2N to a C64, recorded and a test program. I then mounted the C2N on the PET's rear tape connector and powered it up. I typed load and hit play and then enter. A few different combo's.. I also notice that I could not rewind the tape or fast forward it. the transport controls work on the C64...

I moved the C2N to the internal cassette port, which if the transport controls are engaged (FF,REW,PLAY) when the PET powers up, they work for about 3 seconds then stop.

Maybe that helps with the diagnostics.

dave_m
August 27th, 2014, 01:21 PM
This is what I get on C5 Pin 2, similar results appear on Pin 4. I powered up the machine and monitored the pin's with each power cycle. I attached the monitor cable and removed it to see if that affected the results. Same.


Maybe you are lucky and the C5 JK flipflop is bad. To make sure, see if there are 15.625 KHz waveforms on JK inputs pins 1 and 4. And a good 125 KHz square wave clock on pin 12. If so, or unless the the C5-2 output is shorted to ground by something else, then the C5 74LS107 flip flop is bad.

wbochar
August 27th, 2014, 03:27 PM
20143
C5 PIN 1
20144
C5 PIN 4
20145
C5 PIN 12

Can I test the flip flop with the pet off, using some external power and pulse generator?

dave_m
August 27th, 2014, 04:23 PM
It looks like something in the basic clock countdown chain is out. This should be easy to spot. Before you start make sure the scope is working and also set the time base to around 1 microsecond not milliseconds although it may not be critical at this point as all you are looking for right now is a clock signal or no clock signal.

chip/pin SIGNAL
E2-pin 6 8 MHz
C9-pin11 1 MHz
C9-pin12 500 KHz
B5-pin14 500 KHz
B5-pin12 250 KHz
B5-pin 9 125 KHz

Maybe you should check the source of the CPU Phase 0 clock input at C8-pin 6. It should be 1 MHz. Without that the CPU will not run at all.

wbochar
August 28th, 2014, 08:36 AM
E2-pin 6 8 MHz
20150
C9-pin11 1 MHz
20151
C9-pin12 500 KHz
20152
B5-pin14 500 KHz
20153
B5-pin12 250 KHz
20154

wbochar
August 28th, 2014, 08:38 AM
B5-pin 9 125 KHz
20155

C8-pin 6
20156

dave_m
August 28th, 2014, 09:04 AM
C9-pin12 500 KHz
20152


The C9 74LS93 counter is definitely a suspect. We need to see if it has a good input clock at C9-pin14. It should have the good 1MHz signal you saw at C9-pin11. If it is there then the C9 chip may be bad. I can't tell, is the flatline at C9-pin 12 low (zero volts), high (+4 V) or open (1.6 V)?
-Dave

wbochar
August 28th, 2014, 11:01 AM
C9-PIN14
20157

Zero Volts for C9 PIN 12

BTW / Thanks for the help in all this :)

--Wolf

dave_m
August 28th, 2014, 11:46 AM
C9-PIN14
20157




Wolf,
OK, the clock to generate the C9-pin12 (0.5MHz) signal is fine so it's starting to look like like C9-pin12 is dead. Since it is low, there is a small chance that something is holding it down (short). Take a quick look at it loads at B5-pin14 and C5-pin9 for solder splash, etc. If nothing obvious then you will have to replace the 74LS93 counter (C9).

If you are not experienced with replacing a 14 pin IC, it will be easier if you use a small diagonal cutter to cut off all the legs as close to the IC body as you can to leave legs accessible at solder joints. Then using solder wick, remove solder and leg using needle nose, etc., one at a time using minimum heat. This will allow removal and cleanup of solder feedthroughs with minimum chance of overheating the printed circuit board and associated traces.

You might consider installing a socket first rather than installing the new chip directly in.

If this fixes the clocks, you will have good video horizontal & vertical drive signals. The horizontal drive will also be used as the switching signal to generate the high voltage for the CRT. You should then have an operating CRT. We can then go on to whatever the next issue is with the PET if any.
-Dave

wbochar
August 28th, 2014, 11:59 AM
Thanks a lot for the hand holding :)

I'm decent with a Solder gun. But before soldering I'll check the board again with a magnifier. I have to say I've learned a lot from this and its amazing to learn new things :) I'll get back to you with my findings :)

dave_m
August 28th, 2014, 02:13 PM
Take your time; you will do fine. As crude as they are, the Commodore printed wiring boards are pretty tough and can handle some heat. Use a soldering iron rather than a gun if you have one. The old solder guns put out a lot of watts.

MikeS
August 28th, 2014, 02:52 PM
Thanks a lot for the hand holding :) Don't forget the folks following along in the background, keeping Dave honest ;-)


I'm decent with a Solder gun. But before soldering I'll check the board again with a magnifier. I have to say I've learned a lot from this and its amazing to learn new things :) I'll get back to you with my findings :)I hope you didn't literally mean a solder gun ;-)

I assume you have a source for parts. If you need anything, let me know; if I've got it I can drop it off.

Good luck!

wbochar
August 29th, 2014, 05:54 AM
Hehe, Soldering Gun is a mental holdover from older days. I use a wand type that's low powered. My Father was a HAM (which we built a lot of things together), so his was a gun type so I got used to calling soldering irons "Guns".

My Board etiquette is rusty, I did mean to everyone -- community is the true power behind this -- thanks everyone :)

I found a trace on the back that was damaged, so I jumpered to the points and tested that segment. It didn't fix the video issue, but it will likely help something else from causing problems.

I'll check the regular hangouts for parts, if I can't get them -- Mike, I'll take you up on your offer.

It's been fun working on this, thanks again guys :)

wbochar
August 29th, 2014, 02:44 PM
Wolf,
OK, the clock to generate the C9-pin12 (0.5MHz) signal is fine so it's starting to look like like C9-pin12 is dead. Since it is low, there is a small chance that something is holding it down (short). Take a quick look at it loads at B5-pin14 and C5-pin9 for solder splash, etc. If nothing obvious then you will have to replace the 74LS93 counter (C9).

If you are not experienced with replacing a 14 pin IC, it will be easier if you use a small diagonal cutter to cut off all the legs as close to the IC body as you can to leave legs accessible at solder joints. Then using solder wick, remove solder and leg using needle nose, etc., one at a time using minimum heat. This will allow removal and cleanup of solder feedthroughs with minimum chance of overheating the printed circuit board and associated traces.

You might consider installing a socket first rather than installing the new chip directly in.

If this fixes the clocks, you will have good video horizontal & vertical drive signals. The horizontal drive will also be used as the switching signal to generate the high voltage for the CRT. You should then have an operating CRT. We can then go on to whatever the next issue is with the PET if any.
-Dave

I replaced chip and installed a socket with a new chip. it's not working still. C9-PIN12 is still flat lined. Maybe we should start from the begining -- as something else in the chain might have died.

MikeS
August 29th, 2014, 02:50 PM
I replaced chip and installed a socket with a new chip. it's not working still. C9-PIN12 is still flat lined.What about the rest (1,2,11,14)?

wbochar
August 29th, 2014, 02:57 PM
What about the rest (1,2,11,14)?

1 1mhrz
2 flat
11 1mhrz
14 1mhrz

dave_m
August 29th, 2014, 04:12 PM
1 1mhrz
2 flat
11 1mhrz
14 1mhrz

OK, pin 1 must actually be 8 MHz so they are OK.

Remove C9 for its socket and check to see if the C9 socket pin 12 is still at ground. If so, there is a short. Do you have a DVM or some kind ohmmeter?

wbochar
August 29th, 2014, 05:12 PM
OK, pin 1 must actually be 8 MHz so they are OK.

Remove C9 for its socket and check to see if the C9 socket pin 12 is still at ground. If so, there is a short. Do you have a DVM or some kind ohmmeter?

pin 12 is now 15-21khrz without the chip
after pulling the chip out and in again I am getting pin 12 31khrz

dave_m
August 29th, 2014, 07:03 PM
pin 12 is now 15-21khrz without the chip
after pulling the chip out and in again I am getting pin 12 31khrz

We may have some various intermittent shorts going on. C9-12 should be 500 KHz. You are going to have to power off and do some ohm checking.
15.625 KHz is at C5-pin 8 and 11, while the problem 500 KHz output at C9-12 goes to C5-pin9. Maybe a short between C5-pin9 and C5-pin8?

31.25 KHz is at B5-11. So start an ohm check between C9-12 and all those points. If using an old VOM, keep the selector on 1K ohm or less.

Take your time and continue taking good notes. You are getting close. Remember you are being Sherlock Holmes.

wbochar
August 31st, 2014, 03:46 PM
Take your time; you will do fine. As crude as they are, the Commodore printed wiring boards are pretty tough and can handle some heat. Use a soldering iron rather than a gun if you have one. The old solder guns put out a lot of watts.

Well, I am getting all kinds of random things. I think I might just measure everything all over again and post the details. I am getting lost in this... my scope seems to crash now with 500k - 1 mghrz signals which is making things go slow. Arg. I opened a ticket with the manufacturer.

dave_m
August 31st, 2014, 04:54 PM
Well, I am getting all kinds of random things. I think I might just measure everything all over again and post the details. I am getting lost in this... my scope seems to crash now with 500k - 1 mghrz signals which is making things go slow. Arg. I opened a ticket with the manufacturer.

OK, take your time. What is the brand of scope. Is it a gadget that plugs into a PC? Those are sampling scopes which usually have very limited bandwidth and are subject to 'aliasing' errors which can give wrong frequency information. Do you happen to have the specifications that came with the unit?

Don't worry, despite the apparent chaos, I think you are getting close to finding the offending chip.

wbochar
September 2nd, 2014, 04:26 PM
OK, take your time. What is the brand of scope. Is it a gadget that plugs into a PC? Those are sampling scopes which usually have very limited bandwidth and are subject to 'aliasing' errors which can give wrong frequency information. Do you happen to have the specifications that came with the unit?

Don't worry, despite the apparent chaos, I think you are getting close to finding the offending chip.

I've got a DSO QUAD, it's a standalone unit. I've got some firmware updates and another app that's a lot more stable.

Lets begin with, I'm a complete *Idiot* hehe. Mid testing, my Magnifier burnt out, which was LED based. I borrowed a friends, which ended up being tube based. So all my measurements were getting messed up, things that didn't have a signal started getting signals. I totally forgot about the noise tubes make. Which after banging my head against the wall I noticed I was getting different values at night.. Geez.

So here are the measurements again...

(No signal is --)

H Drive
--

C5
1: --
4: --
10: 234 khz
13: 527 khz

E2
6: 8 mhz (sometimes as high as 9.96)

C9
1: 9.96 mhrz
2: --
11: 1.17 mhrz
12: --
14: 2 mhrz

B5
9: --
12: --
14: 468 hrz

KC9UDX
September 2nd, 2014, 05:40 PM
Tubes don't 'make noise' any more than LEDs do...

wbochar
September 2nd, 2014, 09:08 PM
Tubes don't 'make noise' any more than LEDs do...

So why does this florescent lamp when connected and powered up cause noise on my scope? When I use the LED lamp there is no noise. I used and an induction wand and I can hear it. Maybe something to do with the ballast? I've always noticed weird things with Florescent based tube lights.

dave_m
September 2nd, 2014, 09:33 PM
So why does this florescent lamp when connected and powered up cause noise on my scope? When I use the LED lamp there is no noise. What about a tv tube? Use and an induction wand and you can hear it.

Wolf,
We have something basically wrong with the scope setup. You may have the vertical sensitivity set for measuring millivolts. If you are using a 1X problem set the level for 2 V/division. You reported C5 pins 10 and 13 measuring over 200 KHz when in reality they are the reset inputs of the flip flops and are held at +5 VDC through a pull-up resistor. So I'm thinking you are measuring noise. Make sure the scope input channel is set to DC not AC. You do not want to AC couple the input signal. Also make sure the scope is grounded properly. Is there a black alligator clip on the probe that you can tie to signal ground?

I checked the analog specification of your DSO and it has a effective bandwidth of 5 MHz. That means you will get erratic results with any signal over about 1MHz.

Also do you have a voltmeter or DVM? Can you check the places that you are not getting a signal. If the DC voltage reads 0V, the signal is a good low, > 4V, it is a good high. 2.5V means a good square wave, and 1.6V means the point is open.
-Dave

KC9UDX
September 3rd, 2014, 06:03 AM
So why does this florescent lamp when connected and powered up cause noise on my scope? When I use the LED lamp there is no noise. I used and an induction wand and I can hear it. Maybe something to do with the ballast? I've always noticed weird things with Florescent based tube lights.

You may have issues with certain ballasts. "Electronic" ballasts have a potential to be extremely RF noisy, "magnetic" ballasts less so.

I find the opposite that you do: I generally find modern LED light fixtures very noisy, most likely due to the integral switching power supplies.

wbochar
September 3rd, 2014, 06:34 AM
Wolf,
We have something basically wrong with the scope setup. You may have the vertical sensitivity set for measuring millivolts. If you are using a 1X problem set the level for 2 V/division. You reported C5 pins 10 and 13 measuring over 200 KHz when in reality they are the reset inputs of the flip flops and are held at +5 VDC through a pull-up resistor. So I'm thinking you are measuring noise. Make sure the scope input channel is set to DC not AC. You do not want to AC couple the input signal. Also make sure the scope is grounded properly. Is there a black alligator clip on the probe that you can tie to signal ground?

I checked the analog specification of your DSO and it has a effective bandwidth of 5 MHz. That means you will get erratic results with any signal over about 1MHz.

Also do you have a voltmeter or DVM? Can you check the places that you are not getting a signal. If the DC voltage reads 0V, the signal is a good low, > 4V, it is a good high. 2.5V means a good square wave, and 1.6V means the point is open.
-Dave

I have DVM, I'll try that out. I'll get back to you.

wbochar
September 3rd, 2014, 06:43 AM
You may have issues with certain ballasts. "Electronic" ballasts have a potential to be extremely RF noisy, "magnetic" ballasts less so.

I find the opposite that you do: I generally find modern LED light fixtures very noisy, most likely due to the integral switching power supplies.

Thanks for the insight, the LED light I have has power brick while the florescent doesn't. More likely the power circuitry is right beside the tube (attached to the magnifier), so when I use the magnifier (my eyes are going a little wonky as I get older :)) and bring it in close the effect is stronger. The difference between light on and off is crazy.

KC9UDX
September 3rd, 2014, 08:13 AM
Thanks for the insight, the LED light I have has power brick while the florescent doesn't. More likely the power circuitry is right beside the tube (attached to the magnifier), so when I use the magnifier (my eyes are going a little wonky as I get older :)) and bring it in close the effect is stronger. The difference between light on and off is crazy.

Unfortunately, things are getting more and more RF noisy every day. There is a push for smaller, lighter, faster, more efficient; and not enough people care about the RF part anymore. It is possible to achieve all those things without the extra noise, but no one seems to care enough.

wbochar
September 3rd, 2014, 02:33 PM
Values are in Volts


ChipPinSample 1Sample 2-ChipPinSample 1Sample 2
C510.710.71-E214.914.91
C523.13.09-E220.220.22
C531.151.15-E233.413.41
C543.543.54-E244.914.91
C550.710.71-E253.43.4
C563.533.54-E264.914.91
C5700-E2700
C584.914.91-E284.914.91
C594.914.91-E293.43.4
C5102.062.07-E2104.914.91
C5114.284.28-E2113.43.4
C5124.914.91-E2124.914.91
C5132.062.05-E2133.43.4
C5141.711.71-E2144.914.91
ChipPinSample 1Sample 2-ChipPinSample 1Sample 2
C911.421.34-B511.981.98
C920.170.17-B520.170.17
C930.170.17-B530.170.17
C9400-B5400
C954.924.92-B554.914.91
C962.152.1-B5600
C9700-B5700
C981.931.92-B582.022.02
C9900-B5900
C9102.052.05-B5101.991.99
C9111.931.92-B5111.941.94
C91200-B51200
C9131.871.84-B5132.062.06
C9141.941.93-B5142.022.02

MikeS
September 3rd, 2014, 04:55 PM
Values are in Volts...Wow!
There's gotta be an easier way ;-)

dave_m
September 3rd, 2014, 04:56 PM
OK, before we get too far, let's review things: you are using sheet 3 of the schematic from PET assembly # 320132?

C5 is a 14 pin 74107 flip flop so pin 7 is ground which you correctly measured, but pin 14 is Vcc but you get 1.71 V ?
C5 pin 10 and 13 are tied to +5V through a 1K Ohm pull up resistor but you measure 2.05V on both of them.
C9 is a 14 pin 74LS93 counter. Pin 10 should be ground but you get 2.05 V and pin 5 should be Vcc and correctly you get 4.91V.
Can you verify this data? Here is the link to the assembly drawing 32008. I don't have the 320132, but I think they are similar except for memory chips. Note that pin 1 of the chips is to the left of the notch on the top of the chip.

EDIT: deleted link to 320008 Assembly drawing as drawing must be old and does not agree with schematics.

dave_m
September 3rd, 2014, 05:00 PM
Wow!
There's gotta be an easier way ;-)

The scope photo data is questionable so I wanted some DVM data on a few pins, but Wolf gave us a lot of data!

wbochar
September 3rd, 2014, 06:47 PM
OK, before we get too far, let's review things: you are using sheet 3 of the schematic from PET assembly # 320132?

C5 is a 14 pin 74107 flip flop so pin 7 is ground which you correctly measured, but pin 14 is Vcc but you get 1.71 V ?
C5 pin 10 and 13 are tied to +5V through a 1K Ohm pull up resistor but you measure 2.05V on both of them.
C9 is a 14 pin 74LS93 counter. Pin 10 should be ground but you get 2.05 V and pin 5 should be Vcc and correctly you get 4.91V.
Can you verify this data? Here is the link to the 320008 Assembly drawing (http://generalthomas.com/PET/PET320008.jpg). I don't have the 320132, but I think they are similar except for memory chips. Note that pin 1 of the chips is to the left of the notch on the top of the chip.

Using:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320132-3.gif

Ok, something I didn't consider. On my mainboard the letters (A-J) and numbers (1-9) are a grid, I was locating chips based on the grid position. So in cases of the Letters J, K and C they have 9 chips -- which make sense. So when I was told to check E2 I was a little confused at first as the E block of chips start in grid slot 2. So I shifted over one for that. The schematic that you included for 320008 works the same way, which is good. I was a little worried that it was pure grid reference (Ex: no E1 on the board). I am right in assuming this correct?

wbochar
September 3rd, 2014, 07:21 PM
OK, before we get too far, let's review things: you are using sheet 3 of the schematic from PET assembly # 320132?

C5 is a 14 pin 74107 flip flop so pin 7 is ground which you correctly measured, but pin 14 is Vcc but you get 1.71 V ?
C5 pin 10 and 13 are tied to +5V through a 1K Ohm pull up resistor but you measure 2.05V on both of them.
C9 is a 14 pin 74LS93 counter. Pin 10 should be ground but you get 2.05 V and pin 5 should be Vcc and correctly you get 4.91V.
Can you verify this data? Here is the link to the 320008 Assembly drawing (http://generalthomas.com/PET/PET320008.jpg). I don't have the 320132, but I think they are similar except for memory chips. Note that pin 1 of the chips is to the left of the notch on the top of the chip.

Yes I've double checked it,
Pin 14 on C5 is 1.7V
Pins 10 and 13 on C5 are 2.05V
Pin 10 on C9 is 2.07 / 2.05V

I downloaded the schematics for the chips as well, to verify things. I am measuring based on the notch and PIN 1, I doubled checked that at the beginning in case some chips didn't follow the standard convention. The Image you provided and the orientation is consistent with my measurements as well. I've begun also writing up a schematic of the board (chip/component wise), from images I have taken and using the example you provided.

dave_m
September 3rd, 2014, 07:30 PM
I was a little worried that it was pure grid reference (Ex: no E1 on the board). I am right in assuming this correct?

Yes you are correct; it is not a true geometrical grid. Looking at the assembly drawing, A4 is the 4th chip in the A column, but is not physically next to B4 which is the 4th chip in B column.

MikeS
September 3rd, 2014, 07:42 PM
Yes I've double checked it,
Pin 14 on C5 is 1.7VDefinitely not cool; considering that the +5V and GND busses run right beside their respective pins and they're pretty heavy traces, it sounds like a bad regulator or possibly a bad solder joint, either at C5-14 or the corresponding 7805 regulator.

Did we really forget the first step of any trouble-shooting and not measure the power supply voltages? There are four separate regulators and the respective busses run very clearly across the board from each; just measure from ground (the left side of the 3rd power diode from the front, closest to the connector) and the right pin (looking from the front) of each of the four regulators. If they all read +5V (and the left pins all read around +8 to +9V or so) then check across the thick power bus traces going across the board.

MikeS
September 3rd, 2014, 07:52 PM
Yes you are correct; it is not a true geometrical grid. Looking at the assembly drawing, A4 is the 4th chip in the A column, but is not physically next to B4 which is the 4th chip in B column.Uh, oh; am I looking at the wrong drawing/board? There's an assembly drawing? Got a number?

I've got a hand-drawn one here and A4, 5 & 6 are the 6522...

Maybe we'd better specify chip numbers along with the location number...

dave_m
September 3rd, 2014, 08:01 PM
Yes I've double checked it,
Pin 14 on C5 is 1.7V
Pins 10 and 13 on C5 are 2.05V
Pin 10 on C9 is 2.07 / 2.05V



With the DVM, what did you hook up to the black lead? It should be connected to signal ground for best results, but chassis ground will do in a pinch as long as you get to bare metal.

If C5 pin 14 is at 1.7V, we are in trouble. Check other pin 14s at C6 and C7. If low, we have a bad +5 V regulator VR3 as it powers the C column of chips. That would explain all the bad clock outputs! Is VR3 hotter than the other regulators?

wbochar
September 3rd, 2014, 08:01 PM
Definitely not cool; considering that the +5V and GND busses run right beside their respective pins and they're pretty heavy traces, it sounds like a bad regulator or possibly a bad solder joint, either at C5-14 or the corresponding 7805 regulator.

Did we really forget the first step of any trouble-shooting and not measure the power supply voltages? There are four separate regulators and the respective busses run very clearly across the board from each; just measure from ground (the left side of the 3rd power diode from the front, closest to the connector) and the right pin (looking from the front) of each of the four regulators. If they all read +5V (and the left pins all read around +8 to +9V or so) then check across the thick power bus traces going across the board.

4.95 V for each regulator, now checking the traces.

MikeS
September 3rd, 2014, 08:04 PM
I doubled checked that at the beginning in case some chips didn't follow the standard convention.You're absolutely right, they do not all follow the 'standard convention' of diagonally opposite power pins; the 74LS93 (B5) is a good example.

MikeS
September 3rd, 2014, 08:19 PM
I'm outta here; hope to find everything resolved in the morning! ;-)

wbochar
September 3rd, 2014, 08:20 PM
With the DVM, what did you hook up to the black lead? It should be connected to signal ground for best results, but chassis ground will do in a pinch as long as you get to bare metal.

If C5 pin 14 is at 1.7V, we are in trouble. Check other pin 14s at C6 and C7. If low, we have a bad +5 V regulator VR3 as it powers the C column of chips. That would explain all the bad clock outputs! Is VR3 hotter than the other regulators?

I was using Chassis Ground, The Internal Tape Connector Ground as well as a secondary check.

Pin 14 on C6: 0.21 V
Pin 14 on C7: 4.91 V

Left side of regulators are in 8-9V range.

wbochar
September 3rd, 2014, 08:21 PM
I'm outta here; hope to find everything resolved in the morning! ;-)
Thanks :)

dave_m
September 3rd, 2014, 08:24 PM
Uh, oh; am I looking at the wrong drawing/board? There's an assembly drawing? Got a number?

I've got a hand-drawn one here and A4, 5 & 6 are the 6522...

Maybe we'd better specify chip numbers along with the location number...

Mike,
You are right something is wrong with my assembly drawing. It looks like the assembly drawing I posted of 320008 is very old and incorrect. We should not use it for locating parts as it does not agree with the schematics shown on Zimmers or with the schematics I have attached to the assembly drawing which are the same as Zimmers.

I got them from a Rockwell engineer who got them from Commodore in 1977. Maybe it was a prototype board. We (Rockwell) ordered many dozens of PETs from Commodore in 1977/1978 when we were deciding to second source the 6502.

wbochar
September 3rd, 2014, 08:25 PM
The regulator at the back of the machine closest to the IEEE interface is the hottest. But the one that's in Row C is different the rest, the metal is another colour. It's a TI 7805C where the others are Motorola's 7805CT's

dave_m
September 3rd, 2014, 08:37 PM
I was using Chassis Ground, The Internal Tape Connector Ground as well as a secondary check.

Pin 14 on C6: 0.21 V
Pin 14 on C7: 4.91 V

Left side of regulators are in 8-9V range.

OK, now we are getting somewhere. We may have bad +5V power distribution. Maybe bad solder joints or broken traces?
-Dave

dave_m
September 3rd, 2014, 08:44 PM
The regulator at the back of the machine closest to the IEEE interface is the hottest. But the one that's in Row C is different the rest, the metal is another colour. It's a TI 7805C where the others are Motorola's 7805CT's

The TI and Motorola regulators are equivalent. The are good for outputting +5V at 1 Amp. We must have bad traces or cold solder joints. Power off the PET and start visual inspection and use the DVM to check continuity on the C row power pins. Bad power connections explains a lot of the problems you have been having.

wbochar
September 3rd, 2014, 09:42 PM
The TI and Motorola regulators are equivalent. The are good for outputting +5V at 1 Amp. We must have bad traces or cold solder joints. Power off the PET and start visual inspection and use the DVM to check continuity on the C row power pins. Bad power connections explains a lot of the problems you have been having.

I took a look at both sides of the board, couldn't find any broken traces. Time for bed, I'll check +5 power pins in Row C at lunch :)

MikeS
September 4th, 2014, 07:48 AM
I was using Chassis Ground, The Internal Tape Connector Ground as well as a secondary check.Chassis ground isn't always the best place; I find that third diode between rows H and I a nice convenient place to put a clip on.


Pin 14 on C6: 0.21 V
Pin 14 on C7: 4.91 VHard to see how a thick trace like the 5V bus could be broken, but anything's possible; could just be a bad solder joint to the C6-14 pin. What do you measure on pin 14 of the chips past C6 (C5, C2 and C1)? Note that C9 is a non-standard 'LS93.

dave_m
September 4th, 2014, 08:30 AM
Hard to see how a thick trace like the 5V bus could be broken, but anything's possible; could just be a bad solder joint to the C6-14 pin.

So far there is bad Vcc on C5 and C6 as well as the pull-up resistor R39. It is probably a break in a power trace, but as you say, the trace should be wider than a signal trace. Didn't Commodore know about ground and power planes for their boards? Or don't tell me this is not a multilayer board??
-Dave

MikeS
September 4th, 2014, 10:59 AM
So far there is bad Vcc on C5 and C6 as well as the pull-up resistor R39. It is probably a break in a power trace, but as you say, the trace should be wider than a signal trace. Didn't Commodore know about ground and power planes for their boards? Or don't tell me this is not a multilayer board??
-DaveJust plain double-sided. Sorry for the poor quality but you'll get the idea; Vcc and a somewhat thicker GND bus running adjacent to the respective IC corners, with surprisingly few bypass caps:

20283

wbochar
September 4th, 2014, 11:31 AM
I decided to pull the C9 chip and its socket. I put the C9 Chip directly into the spot (no socket)

Something happened after I did that. The monitor tube started making noise, like when it starts to power up. Then I looked at the back of the tube and it was charged. I powered the machine off and the display showed the fade out dot.

Now, when I power up, I can hear a really high pitched whine that changes tone slightly. It's not loud but I can hear it. When I shut the machine off, I get the fading white dot.

dave_m
September 4th, 2014, 03:33 PM
I decided to pull the C9 chip and its socket. I put the C9 Chip directly into the spot (no socket)

Something happened after I did that. The monitor tube started making noise, like when it starts to power up. Then I looked at the back of the tube and it was charged. I powered the machine off and the display showed the fade out dot.

Now, when I power up, I can hear a really high pitched whine that changes tone slightly. It's not loud but I can hear it. When I shut the machine off, I get the fading white dot.

I think this is good news. It appears a problem was induced on the +5V by the soldering of the socket. Now with that resolved, and a good counter, you appear to have good Horizontal Drive which will drive the switching high voltage supply for the CRT. However you should have a screen with at least garbage characters.

Do a quick check on the +5V power pins, and then see if you have a good Horizontal drive of 15.625 KHz and a Vertical Drive of 60 Hz. If so, you should be getting video; make sure the brightness is up.

wbochar
September 4th, 2014, 05:12 PM
I think this is good news. It appears a problem was induced on the +5V by the soldering of the socket. Now with that resolved, and a good counter, you appear to have good Horizontal Drive which will drive the switching high voltage supply for the CRT. However you should have a screen with at least garbage characters.

Do a quick check on the +5V power pins, and then see if you have a good Horizontal drive of 15.625 KHz and a Vertical Drive of 60 Hz. If so, you should be getting video; make sure the brightness is up.

I started smelling burning plastic from the tube section. I powered down the machine and noticed one of the wires going into the back of the tube had begun to melt its plastic coating off.

Here is a pic:
20290


What to do now? Beyond killing myself with the tube. :P

MikeS
September 4th, 2014, 05:18 PM
What to do now? Beyond killing myself with the tube. :PNot a very reliable or efficient method; colour monitors are better, especially big ones.

Maybe you should leave the monitor disconnected until we get what looks like reasonable signals...

dave_m
September 4th, 2014, 05:48 PM
Not a very reliable or efficient method; colour monitors are better, especially big ones.

Maybe you should leave the monitor disconnected until we get what looks like reasonable signals...

YES, disconnect the J7 cable going to the CRT. Check the Horizontal Drive on the J7 header at pin 5. Pin 6 is the pin missing. Is the signal at 15.625 KHz?

wbochar
September 4th, 2014, 07:02 PM
Do a quick check on the +5V power pins, and then see if you have a good Horizontal drive of 15.625 KHz and a Vertical Drive of 60 Hz. If so, you should be getting video; make sure the brightness is up.

Power is around 4.92 V
the Horizontal Drive is not showing anything, I'm getting some background noise pattern thats erratic and the voltage is 3.91 . With Vertical drive has nothing on it and I am getting 4.2 V

dave_m
September 4th, 2014, 09:04 PM
Power is around 4.92 V
the Horizontal Drive is not showing anything, I'm getting some background noise pattern thats erratic and the voltage is 3.91 . With Vertical drive has nothing on it and I am getting 4.2 V

Boy that is disappointing. We must have had some horizontal drive there for a moment to generate the high voltage before something happened. What is the color of the burnt wire going to the CRT?

OK, when you are ready, let's start from the top.

Once you are sure we have good +5V, then we start at the top left of the Sheet 3 schematic and check the entire clock countdown chain to see what happened to the horizontal drive. Apparently we never had a vertical drive as there was only a blank screen, but that's something to fix after we get the timing signals working. Keep your spirits up.

After the main board looks OK, and before we hook up to the video board, we need to check that there is not a fault on the horizontal drive load in the video board. Lots to do.
-Dave

MikeS
September 5th, 2014, 07:10 AM
I know you're having a lot of fun and learning a lot and I can't guarantee that I'd be able to do any better than you, but would you like me to have a look at the board, just in case?

m

dave_m
September 5th, 2014, 10:47 AM
I know you're having a lot of fun and learning a lot and I can't guarantee that I'd be able to do any better than you, but would you like me to have a look at the board, just in case?

m

Mike,
I think there is a lurking problem of a high impedance short on the power buss in that area of the C row. It is causing the timing signals including Horizontal Drive to be erratic. Possibly it was just a solder splash under the new socket, but the counter was misbehaving before the socket was installed.

I wish I had Wolf disconnect the CRT when we first discovered a potential problem with the H drive. It would have been prudent. The problem of troubleshooting by remote control is that you can't quite tell what is really going on; too much filter. I remember my first reaction to 'magic smoke'. It is very traumatic. Wolf seems pretty tough and I'm sure he will get back on the saddle again.
-Dave

MikeS
September 5th, 2014, 11:39 AM
As I mentioned in our other current PET thread, I wonder how effective remote diagnostics would be using webcams and voice with something like Skype or Google... Should try it some time, tackle old time problems with modern technology. Hmm, ethernet interface to the oscilloscope perhaps...

As it is, Wolf is only 5 minutes away and Santo less than 1/2 hour along a road I travel frequently, so that's always a last resort for whatever it might or might not be worth...

dave_m
September 5th, 2014, 12:52 PM
As I mentioned in our other current PET thread, I wonder how effective remote diagnostics would be using webcams and voice with something like Skype or Google... Should try it some time, tackle old time problems with modern technology.

I'm game. No one can call me "Totally Obsolete". They can only say 'mostly obsolete'. ;)



Hmm, ethernet interface to the oscilloscope perhaps...

You are right. Here is a blurb I found on the Tektronix site.

"INSTRUMENT CONTROL


Utilizing the built-in ethernet port, e*Scope web-based remote control allows you to a control TDS3000C series oscilloscope from anywhere, using the internet and your PC."


I would guess one could see the display remotely too. Of course these scopes cost a pretty penny.
-Dave

MikeS
September 5th, 2014, 01:42 PM
I would guess one could see the display remotely too. Of course these scopes cost a pretty penny.
-DaveYeah, but what (if anything) could you do remotely with those cheap PC-based 'scopes? Not much bandwidth, true, but might be enough to get a rough idea of what's happening...

wbochar
September 5th, 2014, 02:30 PM
I'm sure a lot of what's happening is my lack of understanding (electronics wise) coupled with a few possible bad assumptions on my part at the beginning.

If you want to come by, I'm all for it. I'll just have to man up and accept the ridicule of my horrendous soldering job. Breadboards killed the soldering star.

I do have a Skype account, web camera and few other cameras to act as remotes. I could strap a camera to the scope output.

I could record a video of the process as well and post it.

Until my shame is public, what shall I measure next?

dave_m
September 6th, 2014, 12:47 PM
what shall I measure next?

Wolf,
Until Mike and you get a new plan, first makes sure the +5V is OK on the C row.

Then we'll start checking the clocks. First the 8MHZ buffered output at E2-pin 6. Your scope will have a little trouble with that frequency but you can tell that a high frequency clock is running.

Then work down the schematic with the outputs of counters C9 and B5. At B5-pin 11 you should have 31.25 KHz. Next the Q and Q BAR of C7 should be at 15.625 KHz, the horizontal drive frequency. All the above clocks should be square waves. Those following will not be square waves.

Next check C5 pins 5 and 6 are pulsing at 15.625 KHz. These are the horizontal display ON and OFF signals.

Lastly if all this is working, you should have Horizontal Drive at C5-pin 2 at 15.625 KHz.

wbochar
September 7th, 2014, 09:18 PM
All the +5v in the C section are fine.

Next is a Scope question. My little mini scope is totally messed up. Do you have a suggestion for a decent scope? Rigol DS1000E Series or OWON SDS5032E is what keep seeing on the net.

dave_m
September 7th, 2014, 10:25 PM
Next is a Scope question. My little mini scope is totally messed up. Do you have a suggestion for a decent scope? Rigol DS1000E Series or OWON SDS5032E is what keep seeing on the net.

I would be very patient and look for a local Tektronix analog scope like a 465 or 454. Make sure you see it working or photos of both channels working with right to return if not working, and if it has to be shipped, make sure vendor will package it properly (foam in place, or double boxed, etc). Keep looking until you find one in your price range. They can range from $125 to $500 plus shipping. A local pick up would be best so you know its arrives in one piece. They are fairly small and perfect for vintage computers.

I am not too familiar with the little DSO gadgets but if you have your heart set on one, I could do a little research on their specifications. You would want an equivalent analog bandwidth of at least 30 MHz on two channels for looking at 8 MHz TTL signals.

wbochar
September 12th, 2014, 09:03 PM
I'll try to scrape up a 465, I've a 475 availabel recently for around 200$.

On other news. I was looking at other pics of boards, similar to mine. I noticed a difference. Whoever had it before wired extra lines to the mainboard..
20437

Next, I spent some time looking at all the power supply voltages to everything. The majority of the chips measure 4.92V, but anything in the area of: Slots H-G rows 5-9 measure 5.04V. Might have something to do with the extra wiring? Should I remove that extra power wiring?

Until I get a scope that works, I guess I have to wait on what to do next.

dave_m
September 12th, 2014, 11:45 PM
I'll try to scrape up a 465, I've a 475 availabel recently for around 200$.

The 475 is is good scope also with a bandwidth of 200 MHz which is better than the 465.



On other news. I was looking at other pics of boards, similar to mine. I noticed a difference. Whoever had it before wired extra lines to the mainboard..


Next, I spent some time looking at all the power supply voltages to everything. The majority of the chips measure 4.92V, but anything in the area of: Slots H-G rows 5-9 measure 5.04V. Might have something to do with the extra wiring? Should I remove that extra power wiring?



It looks like a previous owner needed to beef up the +8V unregulated power and the ground from the big capacitor to the board for some reason. Perhaps the power connector is worn and not making good contact. We should find out why the C row has only 4.92 Vcc rather than the proper +5V. There may be a bad trace or a chip is drawing too much current. Did you determine that all the +5 V regulator outputs were at the proper +5V?

wbochar
September 13th, 2014, 03:44 PM
20442

Well I thought the power was acting kind of weird. So I mapped out the transformer up. I removed the extra power connections as well. If they are beefing something, it might mean there was a problem with something else. The plastic connector for the cable going into J8 has seen some heat, there is some head damage on the two middle pins.

Is this wired correctly?

wbochar
September 15th, 2014, 08:18 PM
I was reading the pins off of J8 right out of the connector, using the middle black pin and measuring out to each of the pins. But as I read it today, the inner pins measured against ground (middle) is 4.6VAC.

dave_m
September 15th, 2014, 09:10 PM
I was reading the pins off of J8 right out of the connector, using the middle black pin and measuring out to each of the pins. But as I read it today, the inner pins measured against ground (middle) is 4.6VAC.

I am confused. By "inner pins" do you mean the red wires from the big capacitor? If J8 is connected to the board, they pick up the 1/2 wave rectifier diodes and should measure at least 8VDC. Remember to use the DC scale for the red wires.

MikeS
September 16th, 2014, 12:18 PM
Is this wired correctly?Yes, it's wired correctly but your readings don't sound right; as Dave points out, we're looking for around 8VDC between pin3 and pins 2 and 4 of the connector, and around 8VAC between pin 3 and pins 1 and 5. (VA means Volt-Amps, which is Volts times Amps, often equivalent to Watts and not relevant here). If you really are only reading around 7.5V that might indicate something drawing excessive power somewhere, especially with that one VR outputting <5V. If you really are reading 4.6VAC on the red wires that might indicate a bad diode and also explain the low voltage; what does the scope show on pins 2 and 4 of J8?

I would have left the green and white wires for the time being; the power connector is really inadequate for the current going through it and is a common failure point.

Nevertheless, we are getting close to 5V on the four Vcc lines...

The jumper between J9-6 and H9-1 is interesting; it's presumably replacing a trace that is missing, broken or corroded. Can you see what the reason for it is, and are there any other jumpers or signs of corrosion?

wbochar
September 16th, 2014, 12:28 PM
I am confused. By "inner pins" do you mean the red wires from the big capacitor? If J8 is connected to the board, they pick up the 1/2 wave rectifier diodes and should measure at least 8VDC. Remember to use the DC scale for the red wires.

This is really weird, I keep getting different signals.When I powered up without anything connected, I was getting half the voltage at 4.2VDC. I popped the J8 male connector and adjusted pin holders, checked/tugged the cables then powered the system and now its outputting 8.96VDC. I guess the connections out of the big capacitor were not stable (or the capacitor is wonky).

Measuring the voltages across the board again, without the added power lines. Any chip around J8 has 5VDC, as it expands out in a semi circle the voltage drops to 4.91VDC at the corners (Tape Connector/Expansion and last bank of Ram). I've noticed the ROM Bank closer to J8 is warmer than the rest.

The voltage regulators:

Row A : 4.93 VDC
Row C: 5.0 VDC
Row D: 4.93 VDC
Row F: 4.93 VDC

But reading Chip voltages in Rail C, everything hovers around 4.91 VDC.

MikeS
September 16th, 2014, 12:35 PM
This is really weird, I keep getting different signals.When I powered up without anything connected, I was getting half the voltage at 4.2VDC.If J8 is disconnected you won't get any DC voltage at all, so you might just have been measuring residual voltage on the capacitor.

The voltages sound close enough, so we may well be off on an irrelevant tangent...

wbochar
September 16th, 2014, 01:29 PM
The jumper between J9-6 and H9-1 is interesting; it's presumably replacing a trace that is missing, broken or corroded. Can you see what the reason for it is, and are there any other jumpers or signs of corrosion?

I assumed that jumper was part of the original board. This is first time for me seeing the inside of this model.

I noticed a broken/corroded trace going to H8 PIN 1, so I jumpered that a while back. Should I get rid of the other jumper?

MikeS
September 16th, 2014, 05:30 PM
I noticed a broken/corroded trace going to H8 PIN 1, so I jumpered that a while back. Should I get rid of the other jumper?As long as you have a good connection between J9-6, pin 1 of both H8 and H9, and pin 10 of all the 2114 RAM chips it should be OK.

But I'd inspect the board carefully for any other broken/corroded traces. This could indeed be a challenging board... ;-)

Also, can you confirm whether or not you really have an AC voltage on the +8VDC line(s) (the red wires on J8-2 and J8-4) ?

And are the screws on the capacitor nice and tight?

wbochar
September 16th, 2014, 08:49 PM
As long as you have a good connection between J9-6, pin 1 of both H8 and H9, and pin 10 of all the 2114 RAM chips it should be OK.

But I'd inspect the board carefully for any other broken/corroded traces. This could indeed be a challenging board... ;-)

Also, can you confirm whether or not you really have an AC voltage on the +8VDC line(s) (the red wires on J8-2 and J8-4) ?

And are the screws on the capacitor nice and tight?

Set my DVM for VAC,
j8-2 + j8-4 0VAC.
J8-2 + J8-3 7.1VAC
j8-4 + J8-3 7.1VAC

MikeS
September 17th, 2014, 09:31 AM
Set my DVM for VAC,
j8-2 + j8-4 0VAC.
J8-2 + J8-3 7.1VAC
j8-4 + J8-3 7.1VACAgain,
what does the scope show on pins 2 and 4 of J8?and
are the screws on the capacitor nice and tight?

wbochar
September 17th, 2014, 10:09 AM
Screws tight.

No Scope yet, my current one is dead. Trying to locate one to use.

wbochar
September 22nd, 2014, 07:46 PM
I replaced the J8 Connector and Socket to the board. Things look better.

My DSO Scope is working after a small fix to the tuning section and a re calibration.

There is no AC on the DC lines J8 PIN2&4.

Voltage Regulator Outputs:

Row A : 4.93 VDC
Row C: 5.0 VDC
Row D: 4.93 VDC
Row F: 4.93 VDC

5V off the internal tape connector
and 4.91V off the external tape connector

The Power output from 5-4.91V depending on how far out you are from the J8 Connector.

E2-PIN6 is Flatlined, no pattern with the scope.

Horizontal Drive is getting this in short bursts:
20581

I checked all the traces and couldn't find anything broken.

dave_m
September 22nd, 2014, 09:42 PM
E2-PIN6 is Flatlined, no pattern with the scope.



No buffered 8 MHz at pin 6? Try the input at E2-pin 5. If this is not a 8 MHZ square wave then the oscillator circuit may be bad. Your DSO does not sample well enough to see 8 MHZ cleanly but you should see a high frequency signal.

wbochar
September 23rd, 2014, 09:03 AM
No buffered 8 MHz at pin 6? Try the input at E2-pin 5. If this is not a 8 MHZ square wave then the oscillator circuit may be bad. Your DSO does not sample well enough to see 8 MHZ cleanly but you should see a high frequency signal.

It's flat with the occasional very slight dip into the negative. If I switch to DC0.5V increments it looks like a -.2V to 0.

dave_m
September 23rd, 2014, 01:12 PM
It's flat with the occasional very slight dip into the negative. If I switch to DC0.5V increments it looks like a -.2V to 0.

Let's look at the inverter amplifiers in the oscillator circuit: D9-pins 8,9,10 and 11.

wbochar
September 23rd, 2014, 02:39 PM
Let's look at the inverter amplifiers in the oscillator circuit: D9-pins 8,9,10 and 11.

D9- 8,9,10 Slight positive value but flat
D9- 11 Tight square wave around 1v

dave_m
September 23rd, 2014, 03:34 PM
D9- 8,9,10 Slight positive value but flat
D9- 11 Tight square wave around 1v

To me, it seems the crystal is trying to do its thing, but the inverters are bad. Piggyback a 74LS04 on D9 to see if things get better. If so, replace the D9 inverter chip (74LS04). There is supposed to to a test point called TP2-1 that monitors the 8 MHZ. See if you can find it, or just look at D9-pin8.

wbochar
September 23rd, 2014, 05:33 PM
Replaced,

Getting signals on all pins.

D9-PIN8
20596
D9-PIN9
20597
D9-PIN10
20598
D9-PIN11
20599

MikeS
September 23rd, 2014, 05:41 PM
Makin' progress; excellent!

dave_m
September 23rd, 2014, 09:27 PM
Replaced,

Getting signals on all pins.



That just looks like a little noise on a static zero to me. What is the vertical scale in volts per division and are you using a 10X probe or not? What is the zero volt line? Remember anything less than 0.4V is a logic zero. Anything above 2.4V is a logic one regardless of a little noise on it.
-Dave

wbochar
September 24th, 2014, 05:40 AM
That just looks like a little noise on a static zero to me. What is the vertical scale in volts per division and are you using a 10X probe or not? What is the zero volt line? Remember anything less than 0.4V is a logic zero. Anything above 2.4V is a logic one regardless of a little noise on it.
-Dave

2V/Div x1 probes, zero line is the middle of the grid vertically.

dave_m
September 24th, 2014, 07:13 AM
2V/Div x1 probes, zero line is the middle of the grid vertically.

Wolf,
This is curious. If your DSO scope is working correctly, it says you have zero on the input and output of both inverters.

Check that you have good Vcc on pin 14 and ground on pin 7. Double check with voltmeter.
-Dave

wbochar
September 24th, 2014, 07:57 AM
Wolf,
This is curious. If your DSO scope is working correctly, it says you have zero on the input and output of both inverters.

Check that you have good Vcc on pin 14 and ground on pin 7. Double check with voltmeter.
-Dave

Pin 7 has good ground and 15 has 4.92V using a DVM.

I guess I will have to just dump that mini scope, its causing more headaches than help.

dave_m
September 24th, 2014, 08:10 AM
Pin 7 has good ground and 15 has 4.92V using a DVM.

I guess I will have to just dump that mini scope, its causing more headaches than help.

What does the voltmeter read on pin 8,9,10 and 11 of the inverter D9?

wbochar
September 24th, 2014, 09:35 AM
what does the voltmeter read on pin 8,9,10 and 11 of the inverter d9?

8: 1.6v
9: 1.36v
10: 1.36v
11: 1.28v

dave_m
September 24th, 2014, 12:08 PM
The oscillator circuit on the top right of the schematic (http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320132-3.gif) is having a hard time. Maybe a bad crystal (Y1) or shorted capacitors C3 or C4. Let's see what others think. Make sure there are no solder splashes between pins on the D9 inverter chip. Of course pins 9 and 10 should be connected. Let's see if others think it good to cause a quick short to ground on pin 9 to try to start the oscillation.

KC9UDX
September 24th, 2014, 12:19 PM
Let's see if others think it good to cause a quick short to ground on pin 9 to try to start the oscillation.

It won't likely hurt, and might be worth a shot. I've forced an oscillator to start with a signal generator, and that works too but isn't really the way to go either.

I would replace the capacitors, and if it still doesn't oscillate, replace the crystal. Either one is as likely as the other to fail at this point, but the capacitors are easier to obtain.

dave_m
September 24th, 2014, 12:40 PM
I would replace the capacitors, and if it still doesn't oscillate, replace the crystal. Either one is as likely as the other to fail at this point, but the capacitors are easier to obtain.

I agree. First, with power off, ohm across each capacitor looking for a low reading and replace that one first.

wbochar
September 26th, 2014, 06:54 AM
I agree. First, with power off, ohm across each capacitor looking for a low reading and replace that one first.

I looked at the capacitors, but I have a few questions:

1. most of them have no markings, but the few that do have "Z .1 10V" and are disc shaped ceramic (the majority that seem to isolate the power rails). The smaller ones that are around the NPN power transistors are marked 103 which are 10000pf. So the flat Ceramic ones marked Z .1 10V are they .1 Micro Farad? I also assume these ones have no polarity, correct?

2. I don't have a capacitor mode on my DVM, I've seen some writing on the net about using Ohm measurement around set around 2k. But I am not sure, what a successful measurement is. The DVM picks up a few spikes from the capacitors then stops. I assume this is the discharge from them, if there is no discharge then that's bad. Correct?

dave_m
September 26th, 2014, 07:31 AM
1. most of them have no markings, but the few that do have "Z .1 10V" and are disc shaped ceramic (the majority that seem to isolate the power rails). The smaller ones that are around the NPN power transistors are marked 103 which are 10000pf. So the flat Ceramic ones marked Z .1 10V are they .1 Micro Farad? I also assume these ones have no polarity, correct?
Did you check the schematic? C3 is shown as 22 pF and C4 at 10 pF. Oddly the voltage is listed at 500 V, but anything over 15V should do.


2. I don't have a capacitor mode on my DVM, I've seen some writing on the net about using Ohm measurement around set around 2k. But I am not sure, what a successful measurement is. The DVM picks up a few spikes from the capacitors then stops. I assume this is the discharge from them, if there is no discharge then that's bad. Correct?

After it stops, replace the lowest Ohms first. a good capacitor should show a high reading. We are just hoping it is one of the caps as they are easier to find than an 8 MHz crystal.

EDIT: I just realized that C4 has a 470 Ohm resistor in parallel so it will read less than 470 so the test is not valid. Just replace them both and hope that they are the problem.
If the problem is the crystal we will need something like this: http://www.mouser.com/ProductDetail/ECS/ECS-80-18-4XDN/?qs=sGAEpiMZZMsBj6bBr9Q9aQ6SZfIlWoiSivervqkPV0g%3d

We will need a little advice on the correct type of crystal to use.

MikeS
September 26th, 2014, 08:45 AM
Boy, we may be on the way to setting a record for number of posts in a thread, at least in the Commodore area ;-)

Are we positive that there is no clock signal at E2-6 ? Sometimes sticking a scope probe into an oscillator like this can affect its operation.

Also, can that scope even handle an 8MHz signal? I thought we'd seen signals elsewhere, which would suggest that the master clock is running, wouldn't it?

dave_m
September 26th, 2014, 09:36 AM
Boy, we may be on the way to setting a record for number of posts in a thread, at least in the Commodore area ;-)
Not really, I checked and the record for the Commodore Forum is 354 messages from October 2010:
http://www.vintage-computer.com/vcforum/showthread.php?22305-Random-Characters-Issue-on-a-COMMOODRE-PET-2001



Are we positive that there is no clock signal at E2-6 ? Sometimes sticking a scope probe into an oscillator like this can affect its operation.

Also, can that scope even handle an 8MHz signal? I thought we'd seen signals elsewhere, which would suggest that the master clock is running, wouldn't it?

The signal was dead even at the buffered output (E2-6). I hope the OP is using a probe. The DSO gadget is only good for 5 MHz bandwidth, but I had hoped he could catch some high frequency signal to indicate a clock. The DVM readings were about 1.3 V on the inputs/outputs of the two inverters in the circuit. The 'clock' photos he was posting were noise level signals. What would be a good thing to check? I'd hate to have him replace a good oscillator crystal.

MikeS
September 26th, 2014, 09:48 AM
Not really, I checked and the record for the Commodore Forum is 354 messages from October 2010:
http://www.vintage-computer.com/vcforum/showthread.php?22305-Random-Characters-Issue-on-a-COMMOODRE-PET-2001Pshaw! I'm sure we can beat that!


The signal was dead even at the buffered output (E2-6). I hope the OP is using a probe. The DSO gadget is only good for 5 MHz bandwidth, but I had hoped he could catch some high frequency signal to indicate a clock. The DVM readings were about 1.3 V on the inputs/outputs of the two inverters in the circuit. The 'clock' photos he was posting were noise level signals. What would be a good thing to check? I'd hate to have him replace a good oscillator crystal.Sorry, I haven't been following in detail for a while; do we have a clean TTL-level signal anywhere on the board? Wherever it is, it's presumably derived from the 8MHz clock.

I offered to have a look but Wolf understandably prefers to work through it himself and learn along the way; what I might be able to offer is a working board that he could use to compare (and see if his scope can actually display these signals), but that'd be next week at the earliest.

dave_m
September 26th, 2014, 01:04 PM
Sorry, I haven't been following in detail for a while; do we have a clean TTL-level signal anywhere on the board? Wherever it is, it's presumably derived from the 8MHz clock.



Apparently not at the moment, but for a short while after he replaced a counter, there was some clock on the horizontal Drive as the CRT lit up for a moment with some high voltage sounds and then smoke. We had him disconnect the CRT connector (belatedly).

If you recall there was some +5V issues and clock issues we were troubleshooting. Now the power seems stable (although I don't like that there is only 4.93 on some rows, but no clocks at all. Curiouser and curiouser...

That's a nice offer of the loaner main board. And he is looking for a good scope, but it takes time to find the right one at the right price.
-Dave

wbochar
October 8th, 2014, 04:25 PM
I'm back with a tektronix 465 in tow :)

I've redone the scans and double checked things. Before I started I ran the scope with function generator to make sure everything is crisp/sharp.

E2 - PIN 06 - Flat @ 5.9V
C9 - PIN 11 - 1 mhrz
C9 - PIN 12 - 500 khrz
B5 - PIN 14 - 500 khrz
B5 - PIN 12 - 250 khrz
B5 - PIN 09 - 125 khrz
C9 - PIN 05 - Flat @ 5.9V
C8 - PIN 06 - 1 mhrz
C9 - PIN 14 - 1 mhrz
C9 - PIN 10 - Flat 0V
C5 - PIN 09 - 500 khrz
C5 - PIN 08 - 15 khrz
C5 - PIN 11 - Flat 4.5V
B5 - PIN 11 - 31 khrz (Weird Looking)
C5 - PIN 13 - Flat @ 5.91V
C5 - PIN 10 - Flat @ 5.91V
C5 - PIN 14 - Flat @ 5.91V
C6 - PIN 14 - Flat @ 5.91V
C7 - PIN 14 - Flat @ 5.91V
C5 - PIN 05 - Strange Waveform (not even pattern)
C5 - PIN 06 - Strange Waveform (not even pattern)
C5 - PIN 02 - Strange Waveform (not even pattern)
E2 - PIN 05 - Flat @ 4V
D9 - PIN 08 - 8 mhrz (top of wave goes 4v)
D9 - PIN 09 - 8 mhrz (top of wave goes 4v)
D9 - PIN 10 - 8 mhrz (top of wave goes around 3.5v)
D9 - PIN 11 - 8 mhrz (2v width of waveform, but starts 1v up)
D9 - PIN 14 - Flat @ 4.91V
D9 - PIN 07 - Flat @ 0V GRND

HORIZ LINE - Has some Pulses on it
VERT LINE - Flat @ 4.5V

I have some pics for the weird pulses (the 31/15khrz ones)

MikeS
October 9th, 2014, 08:31 AM
Weird; if there's no clock signal at E2-6 how can we have any signals anywhere else? 5.9V ???

Did I miss something?

dave_m
October 9th, 2014, 09:00 AM
Weird; if there's no clock signal at E2-6 how can we have any signals anywhere else? 5.9V ???

Did I miss something?

Yes, he has made a number of wrong measurements. He was probably on the wrong pins on some of them and I hope we do not have 5.9V on anything. On two connected pins, he has the correct 8 MHZ (D9-8 ) and +4V (E2-5)?? But overall not bad for the first time with a new scope. When you see a waveform with skinny negative pulses, use a negative edge trigger and turn the timebase to see about two or three periods. It may help to brighten the persistence. The strange waveforms are probable OK. They will not be square waves. A period is one high time followed by the low time. They may not be symmetrical. It should then repeat. Also the Vertical Drive will be a very skinny negative pulse every 16.67 mS (60 Hz). Without careful syncing on the negative edge and setting the time base to 5 mS, it might appear to be just a high signal. Try some of those again. The Tek 465 is the perfect tool for these measurements.

wbochar
October 9th, 2014, 09:39 AM
Weird; if there's no clock signal at E2-6 how can we have any signals anywhere else? 5.9V ???

Did I miss something?

OK. In a previous part of the thread -- When I asked about the positioning of the chips and the grid reference. I read over it again and now I can see me thinking one thing and you pointing at another.

ROW E has seven chips, starting in slot #2 through to slot #9 in the middle of this row there is a SN74L00N taking two slots horizontally.
So is the first chip at the bottom of ROW E (Near Slot 2)(which is a 74LS08N) is E1 or E2?, I have been treating it as E1. When I re-read the schematic, it has E2 as 74LS08, so E1 Doesn't exist. The schematic skips over it.
Like the rest of the Row E8 and E9 are listed in the schematic as 74LS20's and there is no E7 as E6 is listed at the SN74L00N in the schematic.

Sorry for the mess up, I totally screwed that up.

Here are the E2 Values:
E2 - PIN05 - 8Mhz
E2 - PIN06 - 8Mhz

wbochar
October 9th, 2014, 10:18 AM
I checked the VERT pin, which I listed as flat -- it does a have a waveform. I switch to .5V/Div and then I could see a waveform (after messing with the the trigger).

dave_m
October 9th, 2014, 12:03 PM
I checked the VERT pin, which I listed as flat -- it does a have a waveform. I switch to .5V/Div and then I could see a waveform (after messing with the the trigger).

This is good. Tell me that the period is 16.67 mS which would be 60 Hz. Measure the period of the Horizontal Drive signal. Is it about 64 uS? If so, we are getting close to hooking up the CRT. We need to also check the video data line (J7-1) for high frequency pulses. Each pulse will become a green dot and gets shifted out at 8 MHz.

wbochar
October 9th, 2014, 07:21 PM
The Horizontal is around 64uS.
The Vertical is around 17
J7-1 is one of the RAM chips, which has .5V wide waveform. Sawtooth like.

dave_m
October 9th, 2014, 08:53 PM
The Horizontal is around 64uS.
The Vertical is around 17
J7-1 is one of the RAM chips, which has .5V wide waveform. Sawtooth like.

OK, the Vertical & Horizontal signals seem good.

As far as the 3rd item, I meant to say the pin 1 of the CRT connector J7 (on the main board) to look at the video data signal.

I sorry to find that IC J7-pin1 looks like a sawtooth. It is the Buffered Address line BA6. Perhaps it is just noise. We'll look at that later.

We are doing good.
-Dave

wbochar
October 10th, 2014, 05:21 AM
This is good. Tell me that the period is 16.67 mS which would be 60 Hz. Measure the period of the Horizontal Drive signal. Is it about 64 uS? If so, we are getting close to hooking up the CRT. We need to also check the video data line (J7-1) for high frequency pulses. Each pulse will become a green dot and gets shifted out at 8 MHz.

I'm sorry, was asleep. J7-1 has some .25V waveform -- it just continues no change in pattern.

dave_m
October 10th, 2014, 08:14 AM
I'm sorry, was asleep. J7-1 has some .25V waveform -- it just continues no change in pattern.

Are you using a 10X probe? If not, we have something to look into there. If you are using a 10X, then the signal is low at 2.5V unless you hooked up the cable to the CRT which has a transistor circuit load. You should have the CRT connector unconnected for now.
-Dave

wbochar
October 10th, 2014, 08:32 AM
Are you using a 10X probe? If not, we have something to look into there. If you are using a 10X, then the signal is low at 2.5V unless you hooked up the cable to the CRT which has a transistor circuit load. You should have the CRT connector unconnected for now.
-Dave

Nothing is connected (Keyboard or Video)

I do have a 1/10x probe, but its on 1x at the moment.

dave_m
October 10th, 2014, 09:21 AM
Nothing is connected (Keyboard or Video)

I do have a 1/10x probe, but its on 1x at the moment.

Then we must have a dead video line. Let's look at gate output E9-6, the video output and input at E9-1 at the top right of schematic. Any pulses at those points?

http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320137-3.gif

wbochar
October 10th, 2014, 01:31 PM
Then we must have a dead video line. Let's look at gate output E9-6, the video output and input at E9-1 at the top right of schematic. Any pulses at those points?

http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320137-3.gif

E9-1 has a complex waveform which seems to overlap on itself, E9-6 is flat at 2.25V

dave_m
October 10th, 2014, 02:22 PM
E9-1 has a complex waveform which seems to overlap on itself, E9-6 is flat at 2.25V

Probably the E9 NAND gate is bad but first check the other inputs at E9 pins 5, 2, and 4 for enabling signals that are mostly or always in the high state. If so, replace the 74LS20 Dual 4 input NAND chip.

dave_m
October 10th, 2014, 02:35 PM
Nothing is connected (Keyboard or Video)



Keep the video connecter disconnected until we get a chance to check the loads at the video board end for the Horizontal, Vertical and Video data signals for shorts or other issues.

wbochar
October 10th, 2014, 03:48 PM
Probably the E9 NAND gate is bad but first check the other inputs at E9 pins 5, 2, and 4 for enabling signals that are mostly or always in the high state. If so, replace the 74LS20 Dual 4 input NAND chip.

Pin 2 has a flat signal @ just above 0 less than .2V
Pin 4 has a flat signal @ 2V
Pin 5 has a Pulse on it. Irregular pattern, @2V 1 pulse 2uS, 2uS 0 then another 2uS pulse and then 10uS 0.

dave_m
October 10th, 2014, 04:16 PM
Pin 2 has a flat signal @ just above 0 less than .2V
Pin 4 has a flat signal @ 2V
Pin 5 has a Pulse on it. Irregular pattern, @2V 1 pulse 2uS, 2uS 0 then another 2uS pulse and then 10uS 0.

OK we still have a few problems.
Pin 2 is 'Video On' and should be mostly on and you have it as ground.
Pin 4 is TV Blank which should be usually at a high and you have at 2V. Is the scope vertical channel on 'CAL' (all the way clockwise) so the vertical divisions are accurate?
Pin 5 is 'Display On" which should be high during each horizontal line and quite a regular waveform.

We have some work ahead of us.

wbochar
October 10th, 2014, 06:32 PM
OK we still have a few problems.
Pin 2 is 'Video On' and should be mostly on and you have it as ground.
Pin 4 is TV Blank which should be usually at a high and you have at 2V. Is the scope vertical channel on 'CAL' (all the way clockwise) so the vertical divisions are accurate?
Pin 5 is 'Display On" which should be high during each horizontal line and quite a regular waveform.

We have some work ahead of us.

The divisions are accurate, the uncal light is not on and the center knob is all the way over to the right. I used the 465's internal calibration and the external waveform generator to double check the values.

dave_m
October 10th, 2014, 09:38 PM
The divisions are accurate, the uncal light is not on and the center knob is all the way over to the right. I used the 465's internal calibration and the external waveform generator to double check the values.

OK then pin 4 (TV Blank) must be reading closer to 1.5V or an input to a gate. The output is from PIA G8-pin 39 and is probably not being set up by the computer (program is not running).

We'll worry about that output later when we know the CPU is running in program.

Let's find out what wrong with Video On (Sync) at C6-pin 8. Probe C6-pins 8, 9 & 10. They should all have long positive pulses that repeat every 16.67 mS.

wbochar
March 15th, 2015, 09:05 PM
Cough Cough, I'm back.

I've been out on a job in Europe for the last couple of months and back at home in the lab.
I'm ready to get back to the check list, but its been so long that I am not sure where to restart.

I picked up a http://www.dasarodesigns.com/product/commodore-pet-video-mixer/, which should help with taking the screen out of the loop.

dave_m
March 15th, 2015, 09:14 PM
I'm ready to get back to the check list, but its been so long that I am not sure where to restart.

I picked up a http://www.dasarodesigns.com/product/commodore-pet-video-mixer/, which should help with taking the screen out of the loop.

This looks like a nice device to generate composite video to use with a monitor. However you need a valid video signal first. Review all the messages and pick up from there.

wbochar
March 16th, 2015, 07:03 PM
Starting at
J7 P1: 5V (Flat)
E9 P6: 5V (Flat)
E9 P2: .2V (Flat)
C6 P8: .2V (Flat)

C6 P9: 5V (Flat)
B6 P3: 4.85V (Flat)
B6 P8: 4.8V (Flat)

C6 P10: 5V (Flat)
B6 P5: 5V (Flat)
B6 P4: 5V (Flat)

23249
http://wbochar.com/downloads/320132-3-WB-VIDEO2.jpg Here is the high res one.. linked from my server.

dave_m
March 16th, 2015, 08:37 PM
Starting at
J7 P1: 5V (Flat)
E9 P6: 5V (Flat)
E9 P2: .2V (Flat)
C6 P8: .2V (Flat)

C6 P9: 5V (Flat)
B6 P3: 4.85V (Flat)
B6 P8: 4.8V (Flat)

C6 P10: 5V (Flat)
B6 P5: 5V (Flat)
B6 P4: 5V (Flat)


C6-pin8 is suspicious. It should be > 4 V (high most of the time), but is 0.2V. The C6 AND gate two inputs are both close to 5V so the output should be high. We may have to replace the C8 AND gate (7408 ).

Good detective work so far. I forget, are you using a DVM or do you have a scope?

wbochar
March 16th, 2015, 08:44 PM
Tektronix 465 and DVM.

dave_m
March 16th, 2015, 09:06 PM
Tektronix 465 and DVM.

So when you say 4.85V (flat), do you mean looking with the scope and seeing a DC level with no pulses at all? If so that means our video timing circuit is stuck. It is a 'state sequencer' and some term is bad so we are stuck in one state instead of sequencing to the next state. You should be seeing a lot of pulses on those lines. We should start by replacing C6.

wbochar
March 17th, 2015, 06:48 AM
So when you say 4.85V (flat), do you mean looking with the scope and seeing a DC level with no pulses at all? If so that means our video timing circuit is stuck. It is a 'state sequencer' and some term is bad so we are stuck in one state instead of sequencing to the next state. You should be seeing a lot of pulses on those lines. We should start by replacing C6.

Yeah, no Pulses. I'll replace C6 today, then re-test.

wbochar
March 17th, 2015, 11:16 AM
I replaced C6, and we have some complex waveforms appearing on J7 pin 1. They are pretty dense.

Here is an image:

23258
2V/Div 1us

Looks like Video is up (from the converter)
23259

dave_m
March 17th, 2015, 11:28 AM
I replaced C6, and we have some complex waveforms appearing on J7 pin 1. They are pretty dense.



Good! We have Video data. Now look for a 15 KHz Horizontal Drive signal on J7-pin 5 and a skinny negative going 60 Hz pulse on Vertical Drive J7- pin 3, and if so, you are ready for the composite video gadget.
-Dave

EDIT: I just noticed you have the gadget working. Good deal! I was expecting garbage screen due to the busy video signal. That probably means the system is not running in program or you have bad video RAM. Is there any response on screen to key strokes? Also, are you still not getting a display on the PET? That would be a bad sign for the PET video board.

wbochar
March 17th, 2015, 12:18 PM
Good! We have Video data. Now look for a 15 KHz Horizontal Drive signal on J7-pin 5 and a skinny negative going 60 Hz pulse on Vertical Drive J7- pin 3, and if so, you are ready for the composite video gadget.
-Dave

EDIT: I just noticed you have the gadget working. Good deal! I was expecting garbage screen due to the busy video signal. That probably means the system is not running in program or you have bad video RAM. Is there any response on screen to key strokes? Also, are you still not getting a display on the PET? That would be a bad sign for the PET video board.

The PET Display is black. Doesn't even give me any tube activity. I may have nuked it a while back after all the off and on's during initial testing.

I swapped out the Video Ram for some newer chips, same result. On 1 of 4 powerups it seems the character garbage gets scrambled even more, with noise that shifts around. Power cycling again will bring the character garbage up but it remains static.

Keyboard doesn't do anything.

dave_m
March 17th, 2015, 03:06 PM
The PET Display is black. Doesn't even give me any tube activity. I may have nuked it a while back after all the off and on's during initial testing.

Later you may have to find an old (the older the better) TV repair guy to fix the monitor. If no high voltage sounds, there may be a problem in the horizontal drive circuit. The HD signal drives the high voltage transformer among other things. It should be fixable.


I swapped out the Video Ram for some newer chips, same result. On 1 of 4 powerups it seems the character garbage gets scrambled even more, with noise that shifts around. Power cycling again will bring the character garbage up but it remains static.

Keyboard doesn't do anything.

If the two PIAs and VIA are on sockets, take them out and see if you get the sign-on screen. The system will not work, but will bring up the Commodore message. Are the ROMs on sockets? How about the RAM chips? What kind of RAM in your system? The old 6550s (22 pins) or the 2114 (18 pins).

wbochar
March 17th, 2015, 03:30 PM
Later you may have to find an old (the older the better) TV repair guy to fix the monitor. If no high voltage sounds, there may be a problem in the horizontal drive circuit. The HD signal drives the high voltage transformer among other things. It should be fixable.



If the two PIAs and VIA are on sockets, take them out and see if you get the sign-on screen. The system will not work, but will bring up the Commodore message. Are the ROMs on sockets? How about the RAM chips? What kind of RAM in your system? The old 6550s (22 pins) or the 2114 (18 pins).

Everything is socketed, 2114 RAM.

PIA's and VIA removed -- same result.

dave_m
March 17th, 2015, 04:08 PM
Everything is socketed, 2114 RAM.

PIA's and VIA removed -- same result.

You are lucky to have the 2112 RAMs. What kind of ROMs? I hope not the 28 pin 6540's. The 24 pin 2316's are easier to deal with. I think you can substitute the 2532 EPROM for every two 2316s.

Replace the 2114s one at a time and see if one of them fixes the problem. Bad RAM in the area of 0000 - 0100 (page zero) will not let the PET come up.
-Dave

wbochar
March 17th, 2015, 05:31 PM
You are lucky to have the 2112 RAMs. What kind of ROMs? I hope not the 28 pin 6540's. The 24 pin 2316's are easier to deal with. I think you can substitute the 2532 EPROM for every two 2316s.

Replace the 2114s one at a time and see if one of them fixes the problem. Bad RAM in the area of 0000 - 0100 (page zero) will not let the PET come up.
-Dave

Switching out the RAM didn't help.

6540's are the ROM's I have. But I have a few ROM replacements (same place the video converter) just in case the ROM's went south.

I swapped out the 6502 with a spare I had, it didn't help either.

dave_m
March 17th, 2015, 06:18 PM
Switching out the RAM didn't help.

6540's are the ROM's I have. But I have a few ROM replacements (same place the video converter) just in case the ROM's went south.

I swapped out the 6502 with a spare I had, it didn't help either.

Which ROMs do you have? A complete set includes H1 through H7. I would start by replacing the two kernel ROMs, H4 and H7.

wbochar
March 17th, 2015, 06:36 PM
Which ROMs do you have? A complete set includes H1 through H7. I would start by replacing the two kernel ROMs, H4 and H7.

The Rom's get really warm when the system is on for a period of time.

I have ROM's in banks H1-H7, I think they are original roms. I guess its time to break out the EEPROM burner..

dave_m
March 17th, 2015, 10:18 PM
The Rom's get really warm when the system is on for a period of time.



The ROMs are NMOS and do run hot. And of course Commodore never bothered with a cooling fan. I find it amazing these type of devices have functioned for 40 years.

wbochar
March 19th, 2015, 11:18 AM
The ROMs are NMOS and do run hot. And of course Commodore never bothered with a cooling fan. I find it amazing these type of devices have functioned for 40 years.

Hey, I'm having some weird issues reading the ROM's from my PET. I have an adapter, that will allow me to configure the EEPROM burner to read 6540's. It's not complaining about a problem, but I am getting strange reads from it (alternating patters of bytes for 100h then another patern for 100h then starts over till the end of the ROM). Everything else works fine with it (made a few c64 carts as a test yesterday).

Anyways, my PET started with *** COMMODORE BASIC *** so that's original ROM's.

H1 MPS6540 019 2978A
H2 MPS6540 013 2578A
H3 MPS6540 015 2478A
H4 MPS6540 016 2878A
H5 MPS6540 012 1878A
H6 MPS6540 014 2878A
H7 MPS6540 018 2478A
A2 MPS6540 010 1978A

From Zimmers ROM Readme...

I have the following from PBE (Pet Benelux Exchange) issue 1980-2
(including apparent inconsistencies):

When the PET 2001 went into production for the first time in 1977 there
were two ROM sets available for the system. The firt ROM set is of
the type 6540. This is a 28 pins ROM produced by MOS Technology. These
ROMs are placed in the following locations of the PET 2001-4 and 2001-8
motherboard:

Location ROM Part Number
------------------------------
H1 6540-019 901439-09
H2 6540-013 901439-02
H3 6540-015 901439-03
H4 6540-016 901439-04
H5 6540-012 901439-05
H6 6540-014 901439-06
H7 6540-018 901439-07
A2 6540-010 901439-08 [character generator, I think -rhialto]

Note: There is a 019-ROM at location H1. On earlier motherboards there is
a 6540-11 at H1. This ROM has been replaced by a 019 because there was
an intermittent bug in the 'edit software'. We call this ROM set Basic
level I.

So, 901439-04 (H4) and 901439-09 (H7) is what I need. I'll burn these and see if that helps.

dave_m
March 19th, 2015, 12:31 PM
Hey, I'm having some weird issues reading the ROM's from my PET. I have an adapter, that will allow me to configure the EEPROM burner to read 6540's. It's not complaining about a problem, but I am getting strange reads from it (alternating patters of bytes for 100h then another patern for 100h then starts over till the end of the ROM). Everything else works fine with it (made a few c64 carts as a test yesterday).


The 6540 is a different kind of ROM that requires clock on pin 16 for every READ. In the PET, it uses Phase 2 clock (1 MHz). Does your programmer properly provide this clock? If not, don't worry about reading the ROMs, just start swapping them until it start working. I have an adapter I made from a 8051 uP board that will read 6540s if you want me to check your chips.
-Dave

wbochar
March 19th, 2015, 01:11 PM
The 6540 is a different kind of ROM that requires clock on pin 16 for every READ. In the PET, it uses Phase 2 clock (1 MHz). Does your programmer properly provide this clock? If not, don't worry about reading the ROMs, just start swapping them until it start working. I have an adapter I made from a 8051 uP board that will read 6540s if you want me to check your chips.
-Dave

Yeah, i've got the clock off the programmer patched in. I'm double checking everything (it's spaghetti patching :)) But I'll get back to you about the new ROM's.

wbochar
March 19th, 2015, 09:08 PM
The 6540 is a different kind of ROM that requires clock on pin 16 for every READ. In the PET, it uses Phase 2 clock (1 MHz). Does your programmer properly provide this clock? If not, don't worry about reading the ROMs, just start swapping them until it start working. I have an adapter I made from a 8051 uP board that will read 6540s if you want me to check your chips.
-Dave

I got the ROM's to read in my Burner, I read H1-7 and A2 they all match ROM's from Zimmer.

When you mentioned clocks, thought I'd double check the clock on all the ROM's.. they are good.

I popped every chip that I could. Cleaned them, the socket and put them back in.

I replaced H4 and H7 with alternate ROM's (same version) and still the same.

What is the power on sequence? The video is working, we get characters on the screen (though some wicked upper frame warping, see the pic in previous articles).

I swapped RAM chips around. Only populated the first two slots in j9/i9. Even without Ram it does the exact same thing. Same image too.

dave_m
March 19th, 2015, 10:15 PM
What is the power on sequence?
The code disables interrupts, sets up the stack pointer, initializes the editor and screen, sets up the interrupt and break vectors, enables interrupt, checks for diagnostics on user port, then then initializes the BASIC RAM and tests RAM.


The video is working, we get characters on the screen (though some wicked upper frame warping, see the pic in previous articles).

The video timing is hard wired logic in old PETS and does not depend on the processor running. The PET does not quite meet the NTSC TV requirements, but close. So you must fiddle with the TV vertical controls.




I swapped RAM chips around. Only populated the first two slots in j9/i9. Even without Ram it does the exact same thing. Same image too.

I hope that is a typo (J9/I9). One needs at a minimum, good RAM in J8 and I8 to give one $0400 bytes of RAM (1KB) with boards that have 2114 RAM chips.

You have changed all the key parts to no avail. This is starting to look like a bad address line or data line (short or open). It may be time to break out the NOP Generator to check all address paths. You are doing all the right things that should have fixed most PETs. Your PET is a 3 Sigma hard case. Hang in there.
-Dave

wbochar
March 20th, 2015, 06:49 AM
The code disables interrupts, sets up the stack pointer, initializes the editor and screen, sets up the interrupt and break vectors, enables interrupt, checks for diagnostics on user port, then then initializes the BASIC RAM and tests RAM.



The video timing is hard wired logic in old PETS and does not depend on the processor running. The PET does not quite meet the NTSC TV requirements, but close. So you must fiddle with the TV vertical controls.





I hope that is a typo (J9/I9). One needs at a minimum, good RAM in J8 and I8 to give one $0400 bytes of RAM (1KB) with boards that have 2114 RAM chips.

You have changed all the key parts to no avail. This is starting to look like a bad address line or data line (short or open). It may be time to break out the NOP Generator to check all address paths. You are doing all the right things that should have fixed most PETs. Your PET is a 3 Sigma hard case. Hang in there.
-Dave

Yes Typo :) j8/i8 is what is populated.
NOP Generator.. Hmm, I guess I'll need a couple sockets and jumpers. I have never used one, do you have a suggestion for reading up on the topic?

wbochar
March 20th, 2015, 12:35 PM
Ok NOP is running. Not sure where to go from here..
Checked ROMS H1-7

Address Lines are match across Roms H1-7

from H5-7 PIN4(CS3) is High, while the others show waves around 1ms

H1-7 PIN24(D2) doesn't match the D Series (d0,2,3,4,5,6,7) they have complex waveforms and D2 is flat high 5V.
CHAR ROm in A2 doesn't match ROM's H1-7. Very different reads PIN1-4 is LOW, address lines all seem to have stepped squarewaves. And are not in the timescale. Very different looking than H1-7.

alxnexus
November 15th, 2015, 05:11 PM
I know this is an old thread but after following it now for 156 post as I try to fix my own little blue PET 2001-8 with chicklets I am left wondering how this movie ends? I have gone on your blog. Good work on the Amiga stuff. Did you ever get this working? Did you find a TV repair for the monitor? (MY monitor has the same issues as yours)

Alex

wbochar
November 16th, 2015, 02:56 AM
I know this is an old thread but after following it now for 156 post as I try to fix my own little blue PET 2001-8 with chicklets I am left wondering how this movie ends? I have gone on your blog. Good work on the Amiga stuff. Did you ever get this working? Did you find a TV repair for the monitor? (MY monitor has the same issues as yours)

Alex

Well, it's been on hold for a while. It's going to a be an Xmas project when I get back to Canada. The Pet takes up so much room on the bench; so when I get a chance to pack up enough projects I'll have room to get back to him.

alxnexus
November 16th, 2015, 07:42 AM
Well thank you for replying. And I truly understand since mine is scattered around as well. i will keep this thread marked and if i have anything positive on my PET to report that might help i will post. I purchased the composite out attachment you discussed and am waiting for that to arrive to see what my video looks like since my monitor is not doing anything.

The good news i think is that I have a nice Video, HSYNC, and VSYNC signals. The bad news is that the voltage on the video signal goes as high as 3 volts and i have read they need to be closer to 0-1 volt.

dave_m
November 16th, 2015, 08:11 AM
The bad news is that the voltage on the video signal goes as high as 3 volts and i have read they need to be closer to 0-1 volt.

That my not be the problem. The video signal leaving the PET to the video board is TTL not analog. It should switch from about 0.5V (low) to about 4 V (high).

dave_m
November 16th, 2015, 08:49 AM
Ok NOP is running. Not sure where to go from here..
Checked ROMS H1-7

Address Lines are match across Roms H1-7
And are they all they all pulsing at half the frequency of the next lower address bit? That is what you are looking for in addition to matching across ROMs.


from H5-7 PIN4(CS3) is High, while the others show waves around 1ms

On the 32132 boards, I think those pins should be tied to ground. Re-check. Check schematic sheet 2 to see if I am on the right schematic for your system. (http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320132-2.gif)


H1-7 PIN24(D2) doesn't match the D Series (d0,2,3,4,5,6,7) they have complex waveforms and D2 is flat high 5V.

If the NOP Generator is wired correctly and have the correct open connections, those ROM signals are not connected to the CPU. They should show outputs of all ROMs in turn as address lines are incremented just like the other data lines. Something is wrong there. You may have shorted the line to +5V in the NOP Generator or you may have found the real problem. You are doing good.
-Dave

alxnexus
November 16th, 2015, 09:34 AM
Dave,

First let me say i was impressed with the help you have given on this forum. I read this thread and almost every other PET 2001 thread i could trying to repair this PET. And even though i don't even have characters on a screen yet i have learned a bunch from the posts.

Well that is good to hear. I created a circuit to combine VIDEO, HSYNC, and VSYNC. As i said it has a great pattern on the scope that looks like it should for composite (front porch, back porch, blanking, etc.). It also has the right frequencies (60hz and 15khz). What i was concerned of was how high the voltage levels where and that it was not driving the PET monitor. After testing the signal I am looking in my area for a TV repair. I am also waiting for the composite crt monitor (15khz - Commodore 1702) since i cant seem to get the signal to work on my LCD with composite (which is weird since the output from my C64 with S-video and Composite cable does work). Anyway in my research i found the video adapter made at 6550ROM and he said it needs to work on a 15khz monitor. i hope to receive that in a couple days and see what my PET is outputting so i can continue with the rest of the troubleshooting. Thank you!

Alex

KC9UDX
November 16th, 2015, 10:20 AM
Most non-CRT TVs/monitors these days (read LCD), are extremely picky about the input signal and will display nothing if the signal isn't exactly the way it expects it to be.

I've got one that will not work for any source except an Apple ][. I had it for over a year and thought it just did not work at all. Then one day I was testing an Apple ][ and plugged in the wrong cable, to find the 'dead' monitor displaying the Apple video just fine.

dave_m
November 16th, 2015, 11:28 AM
I created a circuit to combine VIDEO, HSYNC, and VSYNC. As i said it has a great pattern on the scope that looks like it should for composite (front porch, back porch, blanking, etc.). It also has the right frequencies (60hz and 15khz). What i was concerned of was how high the voltage levels where and that it was not driving the PET monitor.

Alex,
OK, I see that you have created a composite video signal for a TV type monitor (NTSC). The signals from the older PETs should be OK for this. The 4032/8032 PETs with the 6545 CRT Controllers have a Horizontal rate of 20 KHz and usually do not work with most old monitors except maybe MultiSyncs.

I'm no expert on video, but I think most composite video signals are zero to -1 Volt. If the signal is AC coupled, polarity may not made much difference although maybe the signal will be inverted? And since the signal is on or off, switching only between minimum and maximum(no shades of grey), amplitude may not make too much difference, but I would choose to limit the signal to 1 or 2 volts max.

alxnexus
November 16th, 2015, 12:21 PM
KC9UDX, Dave - Thanks for the quick replies. When i get home a Commodore 1702 monitor is waiting for me so I will be able to test.

Further to those looking to replace the pet video monitor that can't be fixed I found this 9" CRT replacement that also comes in green as either a kit or open frame. It seems to except both TTL with Video, hsync, and vsync, as well as Analog Composite.

http://www.omnivisionusa.com/monitors/replacement-crt/crt-monitors/9-inch-monitor-.html

KC9UDX
November 16th, 2015, 01:49 PM
KC9UDX, Dave - Thanks for the quick replies. When i get home a Commodore 1702 monitor is waiting for me so I will be able to test.

Further to those looking to replace the pet video monitor that can't be fixed I found this 9" CRT replacement that also comes in green as either a kit or open frame. It seems to except both TTL with Video, hsync, and vsync, as well as Analog Composite.

http://www.omnivisionusa.com/monitors/replacement-crt/crt-monitors/9-inch-monitor-.html

Does it fit? Did you get a price? I've seen things like that go for $900. Maybe even more now that everything is LCD.

alxnexus
November 17th, 2015, 07:10 AM
Have not had a chance to call them and check with them. BUt you are right it might cost too much to make sense. Have not checked the measurements either.

I hot the video working and hooked up to my 1702 CRT (makes a big difference). Here is what I got with the best 2 video rams i could find and it stopped here. Any ideas where to move along from here? I think all of my RAM is bad or mostly bad. I looked for the best video ram and i think this is Random but might not be random enough.

27869

KC9UDX
November 17th, 2015, 07:22 AM
Nice work!

That looks to me like a program gone haywire. Does it ever change?

That is, does it have one pattern when you first turn it on (monitor already warmed up) and then change to something different?

dave_m
November 17th, 2015, 08:25 AM
Any ideas where to move along from here? I think all of my RAM is bad or mostly bad.



Is your PET using the original 320008 board with the unobtainable 6540 ROM and 6550 RAM chips? If so, I would replace them with a small RAM/ROM Replacement board. There are boards from bitfixer.com (http://bitfixer.com)(PETVet) in California and Tynemouth in Great Britain (link (http://blog.tynemouthsoftware.co.uk/2015/05/commodore-pet-romram-replacement-boards.html)) that I use and both work very well. There are others also. The cost is about $75.

The PET 320137 board has 2316 ROM and 2114 RAM and you can still find replacement parts for them such as the 2532 EPROM.

Eudimorphodon
November 17th, 2015, 11:29 AM
That is, does it have one pattern when you first turn it on (monitor already warmed up) and then change to something different?

That looks like the typical "dead PET" screen you get if the video hardware's running but the CPU is getting lost and confused before it can clear the screen... which is good, really, it means you're down to troubleshooting the less scary non-analog parts. ;)

A ROM/RAM replacement board would certainly be the "easy" way to go. The other option would be to cobble together whatever socket adapter is needed to put a conventional EPROM into the F800 socket and burn a copy of PETTESTER and work from there.

KC9UDX
November 17th, 2015, 01:20 PM
It looks like the screen failed to clear, and something (corrupted BASIC probably) is writing to screen memory.

Perhaps /RESET isn't held long enough. If it is, I would expect to see the pattern change at least once during power-up.

At least the character generator, and display circuits are working properly. It's too bad the CRT isn't working.

alxnexus
November 17th, 2015, 01:35 PM
wow guys all good ideas.

1) I have the original 32008 motherboard with the unobtainable parts b/c i like a challenge and am an idiot
2) I have ordered PETVet and spoken with him and he said it would be a couple of days so I think that will be a good diagnostic tool.
3) I would like to get as much of the original motherboard working so i am not just using PETVet b/c as i said i am an idiot
4) it comes on with a different set of characters and then changes to these but only a few not the whole screen. However on each power cycle it is a different set of characters.

I will check reset and some other measurements as discussed in other threads. I have ordered at least 2 working RAM kits as discribed earlier to make sure i know that my Video RAM is good. I know my Video ROM is good so that is a start. Now maybe check for a dead 6502.

Best way to look for that?

KC9UDX
November 17th, 2015, 01:44 PM
Swap 6502s.

I wouldn't expect to find a bad one though.

Have you rung out all the address and data lines?

Eudimorphodon
November 17th, 2015, 01:49 PM
4) it comes on with a different set of characters and then changes to these but only a few not the whole screen. However on each power cycle it is a different set of characters.

So... does it just change a few characters at random scattered around the screen a little while after power-up but the general pattern remains the same? My theory: are the 6520s and 6522 present on the board? One of those (I forget offhand which) is connected to a line which chooses whether lowercase letters or PET graphics symbols are active. Perhaps the line controlling that is being toggled, perhaps by the state of the RESET signal. If those chips are socketed pull them and see if it still changes.

KC9UDX
November 18th, 2015, 12:37 PM
Here's what mine did when I got it:

https://www.youtube.com/watch?v=LH7vtDTZD_k

It was mostly bad RAM. I think the Character Generator was not making good contact in the socket, too. But I don't really recall too well.

alxnexus
November 18th, 2015, 06:23 PM
Here's what mine did when I got it:

It was mostly bad RAM. I think the Character Generator was not making good contact in the socket, too. But I don't really recall too well.

I have ordered a couple of RAM replacement boards and am stuck until they get here. I that point once I know I have good RAM I can move along with the rest of the troubleshooting. As I said I also ordered PETVet so I can run other diagnostics. I will hopefully get all that before Thanksgiving break and I can play around over the holiday and report back.

Thanks for your video. As you all have said I think I am on the right track.

alxnexus
December 8th, 2015, 03:18 PM
Hello again!

So it has been a bit but in the intervening weeks I received a video adapter and 1072 Monitor for my PET 2001 so I could continuing testing. I followed much of the testing steps in this thread and others I did the following:

1) replaced the 6502 processor (my was not working)
2) My Video RAM was bad so replaced the video ram with the replacements at:
3) A large amount of my system RAM is bad but as you will see in the picture I scraped up enough to boot and have 4096 left for basic.
4) Keyboard works

I am thrilled that I am at least to this point. The machine still does not always work on first boot. It almost takes a second or 3rd try. But I know that even the system RAM I have in it now is not all good. I tried building the Donni design for a System RAM replacement using the 6264 SRAM but it does not work and I am sure my electronic skills are not strong enough to know why.

I also still have the none working PET monitor with the back glowing but no picture appearing. I don't know where to go from here on troubleshooting the monitor. I know that is where this thread ended up at some point. Hope all have good holidays and I will check back in if anyone has advice. I have attached where I was to where I am.

Alex2823028231

retrogear
December 8th, 2015, 05:34 PM
Wow, quite the thread. As far as your non-working monitor, were the last signs of life when the red wire was burning? If I'm looking at the right schematic that is a wire to your horizontal deflection yoke. There is a non-polarized capacitor C715 (on the schematic I'm looking at) that could be leaky or shorted.

Larry G

dave_m
December 9th, 2015, 09:45 AM
Wow, quite the thread. As far as your non-working monitor, were the last signs of life when the red wire was burning? If I'm looking at the right schematic that is a wire to your horizontal deflection yoke. There is a non-polarized capacitor C715 (on the schematic I'm looking at) that could be leaky or shorted.

Larry G

Larry,
Alex has the oldest PET configuration so check those video board schematics. There are three versions with this one being the oldest: video schematic (http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-1.gif).

I think you are one the right track with something in the horizontal circuit as it is also used as the source of the switching signal to create the 10 KV high voltage. The small glow on the neck of the tube that he sees is probably the 12V filament.
-Dave

retrogear
December 9th, 2015, 11:31 AM
>There are three versions with this one being the oldest: video schematic

Hmmm - if that is the correct schematic then red wire is to the vertical yoke.
Both horizontal and vertical deflection are driven by signal from the logic board so waking up the board would have created the smoke signal :p
IIRC the vertical drive provides more current so more likely to burn a wire.
OP needs to determine which schematic applies to his monitor before we proceed but I am more than glad to help troubleshoot.
(More pictures of main board top and bottom might help identify)
As I've said in other posts, I did CRT service for over 20 years as a shop tech.

Larry G

alxnexus
December 9th, 2015, 12:14 PM
Larry, Dave -

Thank you. I do have the oldest of the video boards and I believe that is the correct schema. Also zimmers has the board layout and it looks just like mine 28241. When i get home i will send actual pictures also.

With the main board almost fixed (minus a few 6550s), I look forward to getting what to me is the hardest part the CRT working. I have both a oscope and multimeter so can use as needed with guidance. as i said the most i get when connected is the faint glow at the back of the tube. What i have checked is that there is enough voltage coming from transformer (14V) and that i have a proper video signal coming from the PET. It was actually working at one time recently (year).

retrogear
December 9th, 2015, 01:48 PM
Ok and be sure not to power it on until some ohmmeter checks are done first. I'll wait for some pictures.
Take a picture of the yoke coil on the CRT itself. Also if R28 or R32 look darkened. Also Q9 or Q10, C15 shorted.
If we're dealing with a vertical circuit failure with high current to burn wires, the current is going thru those somewhere.

dave_m
December 9th, 2015, 02:40 PM
Hmmm - if that is the correct schematic then red wire is to the vertical yoke.
Both horizontal and vertical deflection are driven by signal from the logic board so waking up the board would have created the smoke signal :p


Larry,
I think the smoking monitor belongs to wbochar. He is to continue troubleshooting during XMAS holidays. Currently Alex is working on his monitor which has good timing signals from the main board but no display.

retrogear
December 9th, 2015, 03:38 PM
Oh ok, my bad. didn't catch the change. Let's start over. If we're thinking there is no horizontal drive, check boost voltage on C27, it should be 85 volts. Without horizontal drive, it will be 0.

alxnexus
December 9th, 2015, 04:55 PM
Larry, Dave -

Good evening. I am uploading the pictures. let me know what to check and I will not turn it on until it makes sense. :-) As Dave said mine is not smoking it just sits with a faint glow in the back (yoke) part of the CRT.

Sorry for the number off pictures was not sure if they had enough detail.

2825128252282532825428255

retrogear
December 9th, 2015, 05:16 PM
Ok let's start with something simple. Power on and measure the DC voltage on your brightness control. One end should be -30V and the middle should vary 0 to -30V as you change it.

alxnexus
December 9th, 2015, 06:39 PM
No voltage there. I have double checked transformer and I have 16v between 8 and 7 brown wires going to the video board.

retrogear
December 9th, 2015, 06:41 PM
so you have no 85V across C27 either?

alxnexus
December 9th, 2015, 06:46 PM
I do have 24.4v there, but nothing at r1.

I had my meter set for AC so here are some measures set to DC.

R1 - 21v
C27 - 11v

VR1 - 21v in 11.5v out

retrogear
December 9th, 2015, 06:59 PM
Do you have -30V across C22 ?

alxnexus
December 9th, 2015, 07:08 PM
No. I seem to be getting none of the higher voltages and nothing to and after r9.

retrogear
December 9th, 2015, 07:10 PM
ok now I see your edited voltages. 11V across C27 indicates no voltage boost is occurring which means you have no horizontal drive on the transformer T2 which also means there will be no voltage on C22.
since you have a scope, walk along Q11, Q12, Q13, Q14 which is waveforms 8, 9, 10, 11, 12 and look for horizontal drive pulse

PS - w/o horizontal drive there will be no voltage on R9

alxnexus
December 9th, 2015, 07:15 PM
Getting the oscope. Just so I am clear it seems like I should be getting 16 out of VR1 and through C3 and I am only getting 11v DC. Is that also an issue? Should I replace VR 1? I am getting 21v across C1.

retrogear
December 9th, 2015, 07:20 PM
VR1 only outputting 11v could be an issue but I would think you would get some horizontal drive. An excessive load could be pulling it down. Make sure it's not getting hot. Also with scope check the AC ripple out of VR1.

PS - VR1 is a 12V regulator so 12V output

WARNING - I AM ASSUMING YOU HAVE A COLD GROUND FOR YOUR SCOPE. IF YOU'RE NOT SURE, CHECK AC VOLTAGE BETWEEN SCOPE GROUND AND CHASSIS GROUND !!!
IT SHOULD BE ALMOST ZERO VOLTS

dave_m
December 9th, 2015, 07:37 PM
Getting the oscope. Just so I am clear it seems like I should be getting 16 out of VR1 and through C3 and I am only getting 11v DC. Is that also an issue? Should I replace VR 1? I am getting 21v across C1.

The output of Regulator VR1 should be +12V. So 11.5 is probably OK. See this link for the scope voltage and timing settings of the waveforms. (http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/notes-2.gif)

alxnexus
December 9th, 2015, 07:50 PM
Sorry for delay. Fighting to find all the Qs. I have attached the pictures for what i get with:

Q11
28261

Q12
28262

Q13
28263

Unfortunatly as i was taking Q14 I pulled out the Black ground wire (i assume) from the yoke to the board. After that i dont get a good signal on Q14. And i get small sparks touching the heat sink fins around Q14. Trying to figure out where the black wire goes back to.

Thanks
Alex

retrogear
December 9th, 2015, 08:15 PM
Time for this old man to go to bed to get up for work tomorrow. Your Q13 waveform is suspicious. The transistor is not getting driven to saturation which means the problem is probably in PREVIOUS stage Q12 but first
check the waveform 11 on Q13. It should have a good flat line on the bottom portion indicating that Q13 is getting turned on hard. If not, go back to Q12 stage and check value of R34 , I think it says 10 ohms ??
Put your scope on C17 - there should be no AC ripple. I'll check back in the morning unless Dave or others have more insight. Hope you get the ground wire figured out. Good night.

Larry G

alxnexus
December 9th, 2015, 08:28 PM
Fixed Black wire

Q14
28264

The others look right but this one does not.

Goodnight

retrogear
December 10th, 2015, 02:59 AM
Yes - what I figured. Put your scope on DC coupling with the ground on the bottom line of scope. Show me waveform #11 on collector of Q13. Then show me waveform on collector of Q12 which is junctioned with R38.
Waveform #11 should have a portion fully grounded when Q13 is fully saturated on. I'm assuming not. Q12 collector should have a section of flat line at DC high level which is almost the same potential as emitter maybe 10V DC
so you will need at least 2V per div on your scope and maybe next higher click. I'm also assuming Q12 is not fully saturated on. Also the DC potential of the 12V supply feeding R34 and the potential on C17 is a major factor
in getting Q12 into saturation (fully on). If C17 were open, Q12 will not turn on. In summary, to get sufficient horizontal drive, each transistor must get fully saturated on.

Larry G

geoffm3
December 10th, 2015, 05:54 AM
Fixed Black wire

Q14
28264

The others look right but this one does not.

Goodnight

Looks like bupkus. Seems to me that with a waveform like you are seeing on the input of Q13 that you would have more from the secondary of the transformer at that point if the transistor is working. I agree check collector on Q13 and see what that looks like.

alxnexus
December 10th, 2015, 04:13 PM
First as always let me say thank you. I had to go to work today but have got some readings in. Second let me say that I am a computer scientist by training but a consultant by trade. So i am gifted in sounding a lot more knowledgeable than i really am so forgive me for the silly questions. I am serviceable with the Oscope and am learning everyday. I dont have a professional grade scope either. With that said i started at the beginning of the HORZ drive as suggested. Using the settings Dave provided (thanks) and worked out from pin 7 to R33. And that waveform looks correct and the Ohms where 1K ohms through R33. I then moved to R37 just before Q12 and got a flat high waveform nothing like the pictures.

Picture #8 (.5V - 20 us) Output of R33 (CR15) - Input to Q11
28266

Picture #9 (.5v - 20us) Output of R37 - Input to Q12
28267

There seems to be an issue here in comparison with what the schematic shows on #9. I measured Ohms and i get -1 on the meter so i assume that means something is wrong (consultant remember). Not trying to be all smart now but does this sound like a possible issue that we see further down. Should i follow the rest of the chain?

retrogear
December 10th, 2015, 04:25 PM
A negative ohms reading just means there is still residual voltage there. Powered on, what is DC voltage on collector of Q11 and base of Q12 (both ends of R37) ? Q11 is not functioning.

One more - also what is the DC on emitter of Q12 (or C17) ??

alxnexus
December 10th, 2015, 04:50 PM
Something there is not working.

DC at Collector of Q11 (7.5V); DC at base of Q12 (11.5V). Getting no resistance reading on R37.

DC at emitter of Q12 or C17 (11.8V)

Hope that means something to you.

retrogear
December 10th, 2015, 05:07 PM
Hmmm - R37 must have some resistance because that's the source for the 7.5V at the collector of Q11.
Be sure the power is off and discharge C17 like with the end of a screwdriver and hold on there maybe 10 seconds to discharge the 12V supply.
Then measure resistance of R37 again. If it is 3.6k then power on and check waveform on collector of Q11. You should be getting a full drive signal of 11.5V p-p.
If R37 checks open you will need to desolder one end and measure it's resistance to confirm.

KC9UDX
December 10th, 2015, 05:08 PM
Are you removing R37 from the circuit to measure it?

alxnexus
December 10th, 2015, 05:37 PM
After discharge and off R37 measures 3.6K ohms. The waveform for collector of Q11 is Square Wave 11.5V peak to base. But when measured with a multi-meter i get 7.5V at the Q11 collector (maybe i am not doing it wrong).

After R37 I measure 11.5V at the base of Q12 but there is no waveform. It is just stuck on high at 11.5V (Is that correct?). I also checked the collector of Q12 and I get a Square wave 11.5V peak to base.

retrogear
December 10th, 2015, 05:46 PM
Your DC meter will average the AC signal so that's why only 7.5V DC. So Q11 is ok. Base of Q12 will only be a .6v p-p signal so that is why you might not be seeing it. With 11.5v p-p on collector of Q12, it is ok too.
We are getting back to where we were last night. What is the waveform on collector of Q13 ? It should also be 11.5v p-p. Also what is the DC on that point? It should also be midrange like 6 to 8 volts.

geoffm3
December 10th, 2015, 05:50 PM
Something there is not working.

DC at Collector of Q11 (7.5V); DC at base of Q12 (11.5V). Getting no resistance reading on R37.

DC at emitter of Q12 or C17 (11.8V)

Hope that means something to you.

You'll have to lift one leg of R37 to take it out of circuit to measure the resistance.

alxnexus
December 10th, 2015, 06:44 PM
Collector Q13 Square wave 0 to 11V. Voltage on Q13 collector moves around a little but around 8.5V the first time but then measured again and it is 11.7V and stays there after repeated measurements. (Sorry for delay, other chores)

alxnexus
December 10th, 2015, 07:00 PM
So i moved on to Q14 and I am getting .3V on the outside post of Q14 and getting 12V on the center post. With that said Q14 has only 2 post attached to the board not the center post.

retrogear
December 10th, 2015, 07:04 PM
put your scope on base of Q14. You should have a healthy square wave maybe .6 to .8v p-p

center post of Q14 is the collector so probably just the heatsink metal.

post a picture of the Q14 base waveform

alxnexus
December 10th, 2015, 07:27 PM
Assuming the base is the post closest to T2 then it is not a Square.

28268

retrogear
December 10th, 2015, 07:28 PM
I need to get to bed soon so I'll keep talking. Google for the TIP55A datasheet which gives you the shape and leads of Q14. Notice the shape of the waveform #12 in the schematic.
See how the duty cycle of the base is two graticules on and one graticule off? That is 66% duty cycle. That is important because the on time of the output transistor loads the coil of the flyback transformer so when Q14 turns off the energy is released
to produce the high voltage, etc. I'm guessing the waveform on the collector of Q14 is supposed to be like 100V p-p which it must not be. The problem must be in the drive signal to Q14 or Q14 itself is bad.

alxnexus
December 10th, 2015, 07:35 PM
THank you. Sleep is a good idea. I will do as you said and look at ordering a Q14 maybe run it in Parallel. As always thank you and i will let you know. Goodnight!

retrogear
December 10th, 2015, 07:43 PM
Ok saw your base waveform. That is not good at all. Make sure the emitter of Q14 is grounded. Make sure waveform 11 on Q13 has a good 50% on time duty cycle which I think it is.
I'm thinking Q14 is bad. That curvature of the base waveform happens with a bad base/emitter junction. Put your scope on collector of Q14. There should be a spike present.
If not, Q14 definitely bad. Does it get warm? Of course power off before touching. You would get an awful shock from that pulse. If transistor is stone cold then it is bad. A base waveform
like that would heat a good transistor tremendously.

PS - and goodnight ...

alxnexus
December 11th, 2015, 06:29 AM
Other that buying on ebay from outside the US where there are some TIP55a listed is there any modern alternatives to the TIP55A?

retrogear
December 11th, 2015, 07:22 AM
The schematic shows MJE-13006 and SJE-1685 but maybe those are just as rare ...

geoffm3
December 11th, 2015, 07:55 AM
According to NTE's website, they show a cross reference for TIP55A with NTE2310:

http://nte01.nteinc.com/nte%5CNTExRefSemiProd.nsf/$all/86B36CCF24DC424A852579100082BCED?OpenDocument

retrogear
December 11th, 2015, 08:18 AM
The NTE2310 is probably the way to go.
Since you're ordering parts, I would recommend replacing C17 47mfd 16V just to be sure. If that capacitor is weak, it lowers the power provided to drive the output transistor which causes it run hotter.
If something caused the failure, that is a likely candidate.

alxnexus
December 11th, 2015, 08:57 AM
thank you all. Found some parts. I have not done the heat test but will when i get home this evening.

retrogear
December 11th, 2015, 10:06 AM
>I have not done the heat test but will when i get home this evening

If that's the test I was talking about, it's not really necessary. I think it's pretty conclusive you need that transistor.
I think you said somewhere you measured 12V on that collector. If it was running correctly it would have measured higher.
When you put in the new parts and power on, if you hear any loud squealing noises, don't leave it on.
And if everything seems normal, leave it on about a minute then power off and feel the temp of the transistor.
It should be just warm to the touch and not hot. Can't wait to hear the results. Good luck.

Larry G

alxnexus
December 11th, 2015, 03:02 PM
>I have not done the heat test but will when i get home this evening

If that's the test I was talking about, it's not really necessary. I think it's pretty conclusive you need that transistor.
I think you said somewhere you measured 12V on that collector. If it was running correctly it would have measured higher.
When you put in the new parts and power on, if you hear any loud squealing noises, don't leave it on.
And if everything seems normal, leave it on about a minute then power off and feel the temp of the transistor.
It should be just warm to the touch and not hot. Can't wait to hear the results. Good luck.

Larry G

Larry,

Got it. Yes I think the TIP55a is the problem given what you said. I just checked again and I am getting 11.5V at the collector and .25V at the emitter and base. I have found some online. Last few question so I don't bother you all too much. 1) should I solder in parallel or just remove the old part? 2) I cant understand from the schema what the voltage through that part should be?

From what I can tell there is a high voltage line that meets up at Q14 with the square waveform (11.5V p-p) making its way through Q11, Q12, Q13 to then drives the CRT? I will write back when the parts come in over the next weekend.

Alex

retrogear
December 11th, 2015, 03:41 PM
No don't try parallel, need to replace and make sure properly heatsinked like old one. You said .25V at emitter AND base, did you mean .25V on base and 0 on emitter? Emitter should be grounded 0 ohms and zero volts. Make sure it is ground with an ohmmeter if your are reading any voltage on it.
Do you have a diode check on your multimeter? Do you know how to do junction test on a transistor? After removed, in diode mode, put positive on base and negative on emitter then negative on collector. You should read .6V on each. Then reverse test
with negative on base and each should read open. Also collector to emitter should read open both directions.

As far as drive circuit goes, Q11,12,13 amplify the current of the square wave signal and shape it for the correct on time vs off time. I can't remember the specs for sure but with a standard 15,734 Hz television, if you look at waveform #13 the off time is the width of the flyback pulse which should be 12 microseconds (???) and the entire frame between pulses is 63.5 uS. The timing is crucial.

alxnexus
December 15th, 2015, 03:23 PM
Guys,

First I think we have good news! As the picture shows it lives. I had to wait a couple days to get the part but I replaced the TI55A and after some bad soldering I got the picture. It is not perfect and there may be somethings you can't notice in the picture is that the first line ***COMMODORE BASIC *** is squeezed and seems too high on the screen. Overall there is a lack of focus.

Before I ask for advice let me say THANK YOU!!! Truly you all have been great and I hope this thread is helping others.

With that said any other advice?

2838028381

retrogear
December 15th, 2015, 04:12 PM
be careful. is the temperature of the transistor ok after running a minute or two? the transistor in stress will give the symptoms you're showing. we might be back to where it failed in the first place. did you replace C17 ?
show a waveform of the base of the transistor. it will have some spike type noise now that the deflection is running but should have good flat on and off sections

PS - don't leave it on maybe 30 seconds then progressively longer periods of a minute as you turn off and check transistor temperature. If it only gets warm but not hot after five minutes then we should be ok.

retrogear
December 15th, 2015, 04:32 PM
I'll keep blabbing since my brain is coming up to speed. Measure the DC across C27. It is supposed to be 85V but I assume it is lower. Put your scope on C23 and measure for ripple. It should be clean DC and measure 12V approx

alxnexus
December 15th, 2015, 05:27 PM
Let me add that a month or two ago when I did not know why stuff was not working I turned a little white knob (R10) and messed around with the metal rings around the yoke. this pictures shows that as I moved R10 I was able to adjust some vertical alignment?

28391

I did not replace C17. As refresher we (you guiding) measured and got waveforms through the whole H-sync circuit that looked good until Q14 (transistor TI55A). After replacement of Q14 the screen showed life for the first time. After adjusting R10 I was able to get various screens as I showed here. Focus is off and the alignment still looks high on the screen and the letters are smashed on top. Q14 was not that hot and not cold either like the old one.

I will measure the waveform tonight after I get my son to bed. and finish some other errands.

retrogear
December 15th, 2015, 06:50 PM
ok if the transistor is running normal temp we can skip the drive waveform for now. Still measure the DC across C27 for 85V. That indicates the amount of high voltage drive we are achieving AND I see in the schematic there is a focus jumper option !!
If the 85V is close, you need to find the jumper and try it in the other position to see if it improves focus. Chances are your mis-centered / compressed top is the centering rings and adj control, etc. Also if 85V is low, can check the ripple on C23. If cap is open it will lower the high voltage drive output. Right now the problem is to achieve focus. If it can't be achieved and the 85V is still low then will need to look at the hor drive waveform again because it can affect the HV output (and width) too.

Larry G

alxnexus
December 15th, 2015, 07:12 PM
So after the tinkering as described I was messing with the brightness knob (R9) i noticed it makes no change. Fooling with it and then doing a measures and it is not changing voltage. While messing with R9 it worked momentarily and the screen cleared up. What we are seeing on the picture is just over saturation of Brightness (not sure if that is a technically correct description). But it looked crisp and great for a moment. I think i will order a 100K Ohm potentiometer and replace the one that is there. You had me check the voltage in the middle leg and it was not changing i should have know to replace.

retrogear
December 15th, 2015, 07:24 PM
Good !! Just try some deoxit D5 type cleaning spray or I just bought some CRC QD spray at Walmart that works ok. you will need a straw nozzle and kink the end to force it into the pot. Your board looked pretty "crusty" so good chance the pot has the same gunk in it.

alxnexus
December 26th, 2015, 05:54 AM
All hope everyone had a great holiday. After a short time out of town I used some of my holiday time to get back to the display. I followed Larry's advice to clean my R9 Potentiometer (brightness control knob) but the problem is bigger (it was broken ... seemed to have snapped) and so I replaced it. That was difficult but complete. With a new Q14 and R9 the display now works sort of. The pictures below show to different situations.

When first turned on the display looks like this: 28523

However, if I touch one of the Capacitors, normally C28 the screen clears and I get this:28528

I know that sounds weird but it is true. Any thoughts if that is a sign of a bigger issue? However the good news is how nice the picture is when it is fixed. Thank you Dave and Larry

retrogear
December 26th, 2015, 06:27 AM
Well I saw your post pop in but I'm leaving shortly so I'll comment and might be gone the rest of the day. C28 is crucial to tuning the flyback and yoke circuit so be careful. Notice when your CRT is out of focus the width is greater? That is a tank circuit
getting detuned. With the schematic, check for solder cracks, etc from ground of C28 back thru the yoke to pin 6 of the flyback. Don't leave running with the CRT de-focused. That might be the condition where the output transistor gets hot.
C28 itself could be defective ? Gotta run ...

Larry G