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Nama
November 12th, 2013, 04:35 AM
Hi all,
I'm back. I'm sure many of you recall the epic thread where we fixed both my 2001 motherboard, and the 2001 motherboard for my PET in a suitcase.
Well, I went to power on the PET in a suitcase the other day and boot screen appeared, although after a few seconds strange things stared to happen. The cursor became unresponsive, and some of the characters became corrupt. I did a power cycle and the problem was worse. after a few power cycles the screen looked like this:

15915

The suitcase PET has a Nick Welte RAM/ROM board (I also have one in my full PET 2001). At first I thought maybe it was an issue with this, but I swapped the RAM/ROM units between the machines and that both work fine.

I replaced the two video RAM but that didn't change anything.

As a simple test I then pulled the video RAM expecting to see the typical checkerboard, but instead I saw this.

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...it's constantly moving busy mess of pixels.

Well, that's where I'm at now, Just wondering if anyone had any thoughts or ideas on what I could test next.

Thanks (again)

Philip

MikeS
November 12th, 2013, 05:57 AM
Well, the obvious candidate would be the character generator and associated shift register, but that wouldn't explain the dead cursor.
I'd start by checking the +5V power (the connector is a notorious trouble spot) and replacing the character generator IC anyway.

barythrin
November 12th, 2013, 08:48 AM
I say we start from ground zero here. What kind of suitcase is it? Obviously we'll need some pictures ;-) Kidding aside, I think Mike's on to something with the voltage issues.

MikeS
November 12th, 2013, 10:05 AM
I say we start from ground zero here. What kind of suitcase is it? Obviously we'll need some pictures ;-) You mean you missed this epic PET saga where Philip earned his stripes?

http://www.vintage-computer.com/vcforum/showthread.php?25264-Another-PET-2001-motherboard-needs-your-help&highlight=suitcase

See post #199; unfortunately it looks like you have to log into the iCloud to see the pics.

Compgeke
November 12th, 2013, 10:41 AM
Is it possible for anyone to upload those images elsewhere? Just having an account won't let you view them unless you also have iCloud setup on a device - something I don't have.

http://www.vintage-computer.com/vcforum/showthread.php?25264&p=182729#post182729

Edit: It seems they're dead links, as I was told by someone else who has a device setup with iCloud.

vbriel
November 12th, 2013, 01:29 PM
It's not the character generator because you can see characters. The character generator is simply a ROM and if corrupt the characters wouldn't look correct. The video circuit appears to be working correctly as it is normal to have garbage on the screen before the system clears the video memory. I would focus on the CPU logic. I see you swapped RAM/ROM which contains the CPU, did you swap CPU's with it? I had an issue with mine in my 2001 where it wasn't seated correctly and made the same screen. Check your voltages but I'd look more closely at the RAM/ROM board even though you swapped it. Check the rest of the board for rising chips, simple enough to just give a good push to them all to make sure they are seated correctly.

MikeS
November 12th, 2013, 02:45 PM
It's not the character generator because you can see characters. The character generator is simply a ROM and if corrupt the characters wouldn't look correct. The video circuit appears to be working correctly as it is normal to have garbage on the screen before the system clears the video memory. I did say that although it might be the first thing one thinks of since just because the @ is OK the rest could be corrupted, and the second screen certainly doesn't look like "normal garbage" or correct characters, it probably isn't the problem and wouldn't explain the dead cursor; still, since it's socketed and he has a spare it couldn't hurt. A screen of @s usually indicates a stuck bit in the video RAM section, but that doesn't explain everything either.

AFAIK you can remove all the RAM and ROM and still get the 'normal' garbage screen; mine isn't accessible right now or I'd see just how many ICs you can remove and still get a checkerboard display.

Check the rest of the board for rising chips, simple enough to just give a good push to them all to make sure they are seated correctly.Good advice if he hasn't already tried that.

Nama
November 12th, 2013, 02:57 PM
Thanks for all the replies. Let me try to answer as many questions as possible.

- Firstly, all those pictures were uploaded to Apple's .mac service via iWeb which is now dead and replaced by iCloud. It seems the link is just redirected to iCloud.
fortunately I still have iWeb as I still use it to build my neoncluster.com page, and I still have archived the data for the now broken link. I have re-uploaded everything here:
http://www.neoncluster.com/projects-pet/2001-Fix.html

and you can read a review of the fix here:
http://www.neoncluster.com/projects-pet/pet-2001repair.html

- Yes, when I moved the RAM/ROM board I also moved the CPU with it. In fact I've even replaced the CPU with another spare just to be sure. Although one issue maybe that the RAM/ROM board is not making good contact with the socket. These sockets are very dodgy, to say the least. I've wiggled things around and reseated every IC that is socketed. Most IC's however are not socketed.

- The character EPROM is a replacement 27xx mounted on an adaptor, I'm pretty sure this is ok. I've pulled it, and I get mostly an all white screen, but with some weird artifacting, missing pixels etc.

Next I'll check out the 5V power.

Thanks again

Phil

Nama
November 12th, 2013, 03:05 PM
Oh...and here is a pic of the suitcase:

15937

barythrin
November 12th, 2013, 03:09 PM
Woohoo!! Thanks for the pic ;-) I would have thought the dead CPU would result in properly rendered but random graphics if the reset was failing. I suppose I'm not 100% familiar with PETSCII though but the pic above sorta looked like malformed characters? You have just enough room in that suitcase for a sony watchman or something for video output ;-p

Nama
November 12th, 2013, 03:40 PM
Yes, I have a couple of small 7" monitors that I got on a trip to China a few years back. I was actually just looking at mounting one internally when I discovered the PET in a suitcase (PETiaS) didn't work anymore. With the 7" monitor It'll make a great portable machine :-/

I'll hook up the PETiaS to a better monitor and if I can't see the character forms a little better. My guess is that are fine, but it's something I should certainly check.

I just tested the 5V line on the motherboard in various places. I'm reading anywhere from 5.01 to 5.17Volts. So the 5V line looks ok to me.

Phil

Phil

vbriel
November 12th, 2013, 03:40 PM
Just curious, what are you using for composite output on the suitcase computer? I know there are some schematics out there, just never saw one done. All this playing makes me want to pull mine out of the box.

MikeS
November 12th, 2013, 03:48 PM
I just tested the 5V line on the motherboard in various places. I'm reading anywhere from 5.01 to 5.17Volts. So the 5V line looks ok to me.All four of them? There's a convenient test point row beside the regulator in Row F.

Nama
November 12th, 2013, 04:03 PM
I'll check the 5V again. I was just testing a few random points around the board.

The composite adaptor was one I purchased off someone who made a run of them. It was a few years ago now, so I can't remember the details.

Phil

Nama
November 12th, 2013, 04:09 PM
Ok...all 4 regulators are now tested and are roughly producing about 5V each.

Phil

Nama
November 13th, 2013, 10:23 PM
Vince, here is the composite adaptor I have:

http://home.comcast.net/~medasaro/6540rom.com/petvideo.html

Nama
November 14th, 2013, 05:22 AM
Hello again,
Tonight I did a little poking around with my logic probe.
One of the first things I looked at was the reset line on the 6502. Well it doesn't seem to be getting any reset pulse, and is always stuck low.
I traced the line back through the 74LS04 at D9 (and this was indeed inverting the signal), then to the LM555. Pins 7 & 6 of the LM555 are tied together and seem to pulse briefly low, then no discernible signal for a very short moment, then they jump high. Pins 4 & 8 both seem to be tied to 5V and read high all the time. The output at pin 3 is constantly high which certainly seems wrong. I would expect this to be high for about half a second then jump low.

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I tried to jumper the 6502 reset line to 5V, but I couldn't seem to get the CPU to reset this way. I'm not sure if this is actually possible anyway.
I also tried piggybacking a spare NE555 on top of the LM555, with no change. Again, I'm not sure if this is possible, but I thought it was worth a try.

Can anyone confirm that the LM555 may be a possible culprit for my problems, and have any suggestions for how to confirm that it has failed before I try to remove it. I really dislike working on these old 2001 boards as the traces seem to be easily damaged, and unlike some other hardier boards, I'd want to be 100% sure the LM555 is dead before attempting a replacement. I know you all understand :-/

Thanks
Phil

dave_m
November 14th, 2013, 07:09 AM
Can anyone confirm that the LM555 may be a possible culprit for my problems, and have any suggestions for how to confirm that it has failed before I try to remove it.

Hi Phil,
So you have a constant reset (low) on the system? This line should go low for about 1/4 second on power up and then stay at high. Before you replace the 555 timer, check to see if C6 or C5 are shorted. I think that is a common failure.
-Dave

Nama
November 14th, 2013, 03:12 PM
Thanks Dave,
Using the continuity setting on my digital multimeter I tested across both C5 and C6. I couldn't see any reading of continuity, meaning that there is likely no short on these capacitors.

What next? Replace the 555?

Phil

vbriel
November 14th, 2013, 04:11 PM
Hey Phil, yes I agree, check the caps, that is the auto reset circuit. The circuit I'm looking at has an inverter D9 74LS04 inverting the signal out of the 555. Check that is working as well. Make sure the signals are correct there. Worse case, you can remove the 555, and make a manual reset. The way I would do that is to remove the 555, put a 3.3K resistor between pin 3 of the 555 socket and ground. It then gets inverted and is pulled high and should be tested at pin 40 of the CPU. Then you can manually reset the circuit by touching pin 40 of the CPU to ground and releasing. Normally I would run a pull up resistor off of RESET pin 40 but with that inverter connected to RESET I was thinking you should just use the socket of the 555 for ease of use. Touching pin 40 RESET of the CPU to ground will do the same thing the 555 does. This will tell you if the 74LS04 or 555 is the problem.

Nama
November 14th, 2013, 05:33 PM
Hi Vince, thanks for the reply.
As far as I can tell the caps look fine to me. No shorts, although I've only tested them for continuity. The 74LS07 also seems ok too, as the (constant high) signal from the 555 is inverted (to a constant low signal).

I may try taking out the 555 and putting in a socket. Then it's easy to do the things you mentioned, and to also just drop in a new 555.

Phil

vbriel
November 16th, 2013, 05:30 AM
Sorry, I was thinking it was in a socket. 100 percent positive the 6502 will not function with reset pulled low. If you can't get reset to come high, the PET won't work.

Nama
November 16th, 2013, 09:37 PM
Some small progress today.
I took out the old 555 and dropped in a socket and a new 555. I was then able to get a reset on the CPU, however things are still not right.
15999

(Please excuse the photo because the screen is constantly rolling on the screen I'm using, although that's more an issue of the composite adaptor)

So as you can see the screen is visually divided into 10 columns, each 4 characters wide. You can see parts of the splash screen displayed. The 'E BA' and '***' and 'FRE' etc.
I can even type, but the characters appear in one area on the 10 columns, then as I keep typing they over-write themselves, then they jump 4 characters over (2 columns) and the cycle repeats.

I have 2148 video RAM installed instead of the 2114, but that are supposed to be compatible, I've changed the video RAM out for some other 2148 and then for some reason the CPU reset stopped working!!!! Then I put in some 2114 that I stole from one of my OSI RAM boards and the characters are different (more random) but still the computer seems to do basically the same thing and you can see the same parts of the splash screen being displayed, so right now I don't think the issue is the video RAM.

16000
(again, please excuse the bad photos. I'll try to find a setup with a screen that doesn't constantly roll)

So thats it for now. I'll do some more experimenting this evening.
Any thoughts?

Phil

Nama
November 16th, 2013, 10:27 PM
I hooked up one of my small 7" monitors that doesn't roll, so here is a slightly better picture:

16002

Seems that the 555 is not always sending the reset signal. Maybe the caps in the reset circuit are indeed dying as suggested.

Phil

Nama
November 16th, 2013, 11:03 PM
Here is a video showing the CPU reset, then me pressing some number keys, and finally the 'return' key:


http://www.neoncluster.com/projects-pet/PiaS_fix_files/Media/IMG_1515/IMG_1515.MOV

Nama
November 17th, 2013, 04:16 AM
Hi,
Me again.
Well I'm really struggling to understand what is happening with the reset line.
I replaced the two caps C5 and C14, and it then fail to get a reset about 95% of the time. Pins 6 and 7 of the 555 just didn't seem to drop low.
After playing around for a wile I felt that maybe the issue was actually the resistor at R16. I could piggyback another resistor (didn't seem too important what the value was) in parallel over R16, and reset seemed to work all the time. So I pulled the resistor out. It a 1M, but on my multimeter it reads at about 0.9M. and in circuit it was reading about 0.2M. Seems I can replace it with pretty much any resistor and I still get a solid reset. Only when I put the old resistor back in does reset fail. Strange, but hopefully that is that problem solved.
So I now have a new 555 and two new capacitors in the reset circuit and I'll get a new resistor of the right spec soon and solder it in.

Still haven't made any progress the funky screen issues. Any thoughts would be greatly appreciated.

Phil

dave_m
November 17th, 2013, 09:06 AM
Still haven't made any progress the funky screen issues.


Phil,
It seems to be a video RAM addressing problem with the SA lines. I think the address counters may be OK because you get the correct 1000 character screen and vertical timing signal. So I would take a look at the D2, D3, and D4 multiplexers (74LS157) that switch between the CPU addresses and the screen scanning addresses. Start with the middle address mux D3. Look for something struck on the outputs. I am looking at this schematic (http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320008-3.gif) (I hope it is the correct one).
-Dave

Nama
November 17th, 2013, 03:07 PM
Hi Dave,
Had a few spare mins this morning to quickly probe the 74LS157's.
I marked what pins seem to be constantly high.

16010

Will have some time a little later to do some more probing.

Phil

P.S. I think my board is 320132 variety here:
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320132-3.gif

Nama
November 17th, 2013, 03:25 PM
*** Some small corrections to the image I posted:
- D2 pin 6 is actually pulsing but was marked as stuck high.
- D2 pin 7 is pulsing high, but very slowly...seems to be in time with the flashing cursor

Sorry for the confusion.

Nama
November 18th, 2013, 03:32 PM
This morning I had some time to look over the schematic. As I mentioned before I was seeing stuck high pins on the inputs B2 and B3 of D3 (74LS157), and this seemed very suspect to me. These pins are usually fed from the 2114 RAM but as I have a RAM/ROM board installed I figured that the signals must now originate from A2 and A3 (pins 11 and 12) of the 6502. I tested these pins on the 6502 and I was seeing pulses. These pins are fed through a 7417 and then onto the 74LS157 where I took the readings before. I decided to test the inputs and outputs of the 7417. It was a little tricky to get at these IC's as that are covered when the RAM/ROM board is installed, but I attached some jumper wires to the pins and took readings from that...here seemed to be my problem. Even though the inputs pins (pins 5 and 9) were pulsing, the outputs (pins 6 and 8) seems stuck high.

I have some spare 74LS07's which are 7417 equivalents, and decided to piggy back one over the 7417. The results can be seen below:

16028

Yay!
Just need to remove the 7417, drop in a socket, and replace that funky resistor in the reset circuit too.
Done!

A big thanks to Dave, Vince, MikeS, barythrin and compgeke for your time and help.
Thanks

Phil

Nama
November 19th, 2013, 03:33 AM
I've written everything up in a repair-log here:

http://www.neoncluster.com/projects-pet/pet-suitcaserepair.html

MikeS
November 19th, 2013, 04:38 AM
I've written everything up in a repair-log here:

http://www.neoncluster.com/projects-pet/pet-suitcaserepair.htmlExcellent! Glad to read another success story.

This forum does seem to have become the place to go to when you have an ailing PET...