Help wanted to Repiar Commodore CBM PET 4032

[KLund1]

I have a dead CBM 4032.
I do not know much about these. I have read several web pages about repairs, but alot is over my head.
First thing I did was disconnect the PSU and test with a multimeter. It shows about 10.3v on 2 lines, and 40v dropping to 0 on some others. Though I'm not sure sure what pins to test with eachother. I just touched red to blacks, then browns, then blues.
From my reading, I think the 10.3v should read 12v. So I assume the big green cap needs to be replaced. Y/N.
If have read my other help wanted posts, (thank you all!) you know I do not have a EE degree, nor a O-scope, just my cheap Multi-meter, and a logic probe.
If the main cap is bad, could you provided a link to the correct digikey item to order. (When I order, I get the specs usually right, but get the wrong size, diameter, heat wrong and then blow something up after soldering it in) (but my soldering skills are getting better

The main board looks to be in good condition, as dose the case. Little dust, no corrosion. So I think it has been stored in a good place. Things I should check?

Thanks

[iz8dwf]

I have a dead CBM 4032.
I do not know much about these. I have read several web pages about repairs, but alot is over my head.
First thing I did was disconnect the PSU and test with a multimeter. It shows about 10.3v on 2 lines, and 40v dropping to 0 on some others. Though I'm not sure sure what pins to test with eachother. I just touched red to blacks, then browns, then blues.
From my reading, I think the 10.3v should read 12v. So I assume the big green cap needs to be replaced. Y/N.
If have read my other help wanted posts, (thank you all!) you know I do not have a EE degree, nor a O-scope, just my cheap Multi-meter, and a logic probe.
If the main cap is bad, could you provided a link to the correct digikey item to order. (When I order, I get the specs usually right, but get the wrong size, diameter, heat wrong and then blow something up after soldering it in) (but my soldering skills are getting better

The main board looks to be in good condition, as dose the case. Little dust, no corrosion. So I think it has been stored in a good place. Things I should check?

Thanks

First of all, always refer to the schematics, yours appear to be an "universal" board:
ftp://www.zimmers.net/pub/cbm/schema...pet/index.html
Find the PCB assy. number and get the correct schematic.
You should check the supply voltages on one of the 4116 RAM chips.
If any supply on the 4116 isn't correct (and hoping it isn't the -5V missing) then you go back on the input of the regulators and measure there.
The green cap is probably ok. I wouldn't start substituting random parts if I were you.

Frank IZ8DWF

[MikeS]

+1 What Frank said.

The "power supply" is actually on the main board and you can't test it with J8 disconnected; that disconnects the transformer and the big filter cap and all you'll be able to check that way are the fuse and raw AC voltages from the transformer. See here for the transformer and cap wiring:
ftp://www.zimmers.net/pub/cbm/schema.../8032051-3.gif

Full schematics are in the univ2 folder at the link Frank provided; the only difference between the 8032 shown and your 4032 are some jumper settings and four or five chips related to the extra screen memory.

Power supply issues are actually pretty rare but it is still a good idea to check them first; as Frank suggest, check the DC voltages on any of the RAM chips, being careful not to accidentally short any pins.

You'll find lots of good advice here but you should be more specific when describing what you see; for example, what you describe is useless without the actual terminal numbers you're checking and the meter setting.

Good luck!

[KLund1]

Thanks everyone!
Very much appreciated.
Yes that all makes seance to me. Will do. That Zimmers site has a lot of info. But I'm not the best at schematics. Is there a pic like this in post #3 for a 4032?
http://classic-computers.org.nz/blog...connectors.jpg

I found this Pet repair site/posting, and it looks helpful. But I do not know where things are on a PET MB like 'userport'. or 'Phi2', or where the PLA IC's are, etc


PET's dead
This section describes situations where the PET seems to be "dead".
Apparently the new "CRTC" PET are more difficult than the older ones, as the CRTC must be programmed to generate a valid video signal. Older PET would show at least garbage even if the CPU was dead.

Initial Check: Before you check anything else, check that there is power on the board e.g. by measuring 5V on the userport, then check Phi2. If both are ok, then check what the CPU interrupt lines (IRQ, NMI, RES) do, and then the address lines. This should give you an idea if it is a very basic problem or maybe something with the ROMs for example.
CRTC PET: In the CRTC PET a "dead" PET may not be completely dead. The CPU must program the CRTC to actually generate a valid video signal. So if the CPU is running and the PET still does not work, check the ROM contents and the address decoding (either with the CPU emulator or by monitoring the ROM select lines, esp. after reset.
8296(D): The 8296 and 8296-D (that use the same board, so I'll use 8296 only) are notoriously known for broken PLA chips. To check that measure the output pins of UE5 and UE6 with the scope. If they are constant around 1-3V, but not clean TTL signals (0V or 5V) they are broken. Build a replacement e.g. as described on this page (in German, but should be easy to understand in english as well). Or use the info on this page, including a GAL replacement of the UE5 PLA.

[MikeS]

Don't know if it'll help, but here's the board layout:
http://www.zimmers.net/anonftp/pub/c.../8032090-7.gif

I'm afraid what you posted isn't very helpful without an oscilloscope; the user port is not a good place to check voltages, and the 4032 does not use any PLA's.

Can we assume that it doesn't make any tweedle-dee sound when you turn it on?

I wonder if there's someone else familiar with this sort of thing in or near Livermore who could help.

m

[KLund1]

Thanks
I have a 80320 90 univ2 board. With power on, testing on various ram ic's , the 12v line showed 9.9v for a couple minutes with went to near zero. the 5v line show constant 2.2v, The -5v line was about -1.2v. Some ram IC's show nothing, I assume those ram ic's are bad?
There is no beep on power up.
How to test the power regulators? Are they those large items in the black metal track on side of the MB? Where is to touch meter probes?

[daver2]

Just because the voltages are wrong across the devices you are measuring - don't assume the device is faulty...

Let us assume that the transformer is working correctly (it may not be though). We will have a look at that later if the voltages we are about to measure are not correct...

Set your multimeter to read d.c. voltage. We are going to measure the +5V regulator line first - so set your meter to read something like 10V d.c. full scale.

Measure the voltage across C10 (1uF) and let us know what the voltage is. This is the input to the +5V regulator- so it should be somewhere between +7.5V and +9V d.c. Note the polarity of your meter.

Measure the d.c. voltage across C11 (47uF). This is the output from the +5V regulator. It should be +5V +/- 0.25V.

In a similar manner, measure the voltages across the following capacitors:

C5 (4,700uF) input to the +12V regulator. C17 (10uF) output from the +12V regulator. Set you multimeter to read something like 15-20 Volts full scale.

C2 (470uF) input to the the -5V regulator. C16 (10uF) output from the -5V regulator. Again, go back to something like a 10 Volts full scale setting on your multimeter.

I would also measure the voltage across the very large, external, green capacitor. This voltage should be something like +9V d.c.

Let us know what all the readings are - and we can take it from there.

If the input voltages are low to the voltage regulators, then this could indicate a problem with the bridge rectifier diodes or the transformer. If the input voltages are OK - but the output voltages are either wrong or start off correct and then fall over time - this could indicate faulty voltage regulator(s) or a fault somewhere on the main board. However, we will address these issues once we know what the voltages are and what we are dealing with.

Incidentally, the schematic diagram I am using is to be found here ftp://www.zimmers.net/pub/cbm/schema...8032087-11.gif. I would suggest printing it out, identifying the components that I have referenced and writing the voltages down for future reference.

You will also find the parts layout useful at ftp://www.zimmers.net/pub/cbm/schema.../8032090-7.gif.

Note that all of the capacitors I am describing are polarised. The positive / red lead of the multimeter should be connected to the lead identified as '+' on the component layout. On the capacitor itself, it is usual to mark the negative lead '-' just to be awkward!!!

Dave

[daver2]

I have just read your first post in a little more detail.

To be clear - the output from the transformer is a.c. voltages and the external green capacitor has nothing to do with any of the voltages that come directly from the transformer - so your assumption regarding 12V is not quite correct. The a.c. voltage from the transformer is converted into d.c. on the main board via a bridge rectifier and then smoothed by the external large capacitor. This voltage (as I have stated above) should be nearer +9V d.c. The only reason the large green capacitor is not on the mainboard is that it is too large...

Most people have successfully managed to get their PETs working again with a cheap multimeter and a logic probe - so you stand a pretty good chance !

Dave

[daver2]

You may find some of the capacitors awkward to measure the voltage on from the top of the main board. If so, proceed as follows:

Meter negative (black lead) to the negative end of capacitor C7.

Use the multimeter positive (red lead), to then measure the voltages on either side of diode CR7 (input and output of the +5V regulator), either side of diode CR5 (input and output of the +12V regulator).

Measure either side of diode CR1 (input and output of the -5V regulator). This should be OK if you have a digital multimeter - it will indicate negatively.

The higher voltage reading is obviously the input to the regulator...

I will measure the a.c. Voltages from my transformer and get back to you shortly.

Dave

[daver2]

So, if you look at J8 (the power connector), you will see it is a 9 pin connector with pin 7 missing (the key way).

The pins are numbered: 1, 2, 3, 4, 5, 6, 7 is missing - the keyway, 8, 9.

Disconnect J8 from the main board and set your multimeter to read something like 20 Volts full scale A.C.

Between pins 1 and 3 I get 8 Volts a.c. (Brown to black on the wire colours on my PET).

Between pins 5 and 3 I also get 8 Volts a.c. (Brown to black on the wire colours on my PET).

Between pins 1 and 5 I get approximately 16.3 Volts a.c. (Brown to brown on the wire colours on my PET).

Between pins 8 and 9 I get approximately 14.8 Volts a.c. (Blue to blue on the wire colours on my PET).

The wire colours are (I think) a bit arbitrary on the PET...

If you open up the monitor and disconnect the twisted pair power leads to the monitor, I measure approximately 19.8 Volts a.c. For my 12" screen.

Be careful about opening the monitor - high voltages are present. Turn off the PET, have a cup of tea, coffee or whatever to allow the high voltages to discharge, then perform this test...

I will now wait for you to come back with some measurements.

I hope this sort of detail is helpful to you?

Dave