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Commodore Pet 4032 Problems

C

cavemanweb

New Member
Joined
Oct 9, 2015
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5
Hi All,

A friend of mine have a problem with a commodore pet 4032. This pet was in pretty bad shape, it has been complete restored but no boots up.

Symptoms: The Pet first show a screen full of letters and characters but then it just clear the screen and stay in blank.

For those who have repaired one, there is some questions that we like to ask.

On the system memory, the pins 1 and 9 have 5 and -5 volts but the pin 8 have only 0.04 and we think that this need to be 12v right?. So we replace the 7812 transistor (the big one) but still there is not 12v on the pin 8 (and it heats up quickly). ¿Maybe there is a short circuit that makes the 7812 heats up but i don't know. My friend has the board and i just can help him via email.

¿Any tip on what can be the problem ? This pet has a 3032 board.

This one:

p0dQPM7m.jpg

Replaced components:
p0dQPM7m.jpg
 
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cavemanweb said:
On the system memory, the pins 1 and 9 have 5 and -5 volts but the pin 8 have only 0.04 and we think that this need to be 12v right?. So we replace the 7812 transistor (the big one) but still there is not 12v on the pin 8 (and it heats up quickly). ¿Maybe there is a short circuit that makes the 7812 heats up but i don't know. My friend has the board and i just can help him via email.

¿Any tip on what can be the problem ? This pet has a 3032 board.
Click to expand...

Hi Cavemanweb, Yes the 4116 Ram chips require 12v on pin 8.

I noticed the board has two 4116's missing. If I were you I would pull all the memory chips from the sockets and then check pin 8 for 12v.
It could be a case of a shorted ram chip pulling down the 12v line, you could then put each chip back retesting pin 8 each time to find the defective chip.
If you pull the chips but still can't get the 12v you will need to check for shorted/broken traces or shorted capacitors!
Hope that helps for now!
 
An ohmmeter is your best friend chasing a short. The 7812 is typically rated 1 amp which translates 12 ohm load to ground. I'm guessing you shouldn't read less than 100 ohms normal. If you read lower ohms, follow the 12v circuit trace. You will need to pull ram chips or lift leads on tantalum caps to find the short. Many small caps filter the 12v line.

PS - of course if you don't read a short then it could be an open trace (crack) in the circuit

Larry G
 
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And of course make sure the chips are inserted the right direction and no folded pins. The 12v short could have been created
by other two-handed gremlins.
 
Thanks for the reply's we will check all the tips mentioned here. Thanks for the support.
 
Hi again! We have made some progress on the PET by replacing the tantalum caps, two sn74ls244 and all the ram (including sockets) ;). But.. now, when we start the PET, there is no basic, only shows some kind of code on the machine language monitor.

First symptom:


Sin título.jpg

Error code in case that the image is too small:

C
PC IRQ SR AC XR YR SP
0EDF E455 E0 1F E0 1E 01

Then turn off and on again and the screen show almost all the characters but after that, the message from the first capture appears again:

s2.jpg

Any help is highly appreciated, we don't think that the cpu is damaged, but we don't have another PET to try parts :)
Regards.
 
I think quite a few things can cause a Pet to fall into the monitor program like this. In the case of mine just recently it was a faulty ROM.
 
Yes, since you replaced the RAM, it may be a bad ROM. I see that all are soldered except the Edit ROM. If your PIAs (6520) and VIA (6522) chips were on sockets, removing them and seeing if your got the initial Commodore display would have been the next easy thing to try in case one of them were bad. But unsoldering 40 or 24 pin chips is not easy unless you have experience. Are you able to replace big chips?
 
dave_m said:
Yes, since you replaced the RAM, it may be a bad ROM. I see that all are soldered except the Edit ROM. If your PIAs (6520) and VIA (6522) chips were on sockets, removing them and seeing if your got the initial Commodore display would have been the next easy thing to try in case one of them were bad. But unsoldering 40 or 24 pin chips is not easy unless you have experience. Are you able to replace big chips?
Click to expand...

Good to see you are still keeping the PET vet clinic alive Dave :) I'm sure quite a few PETs are here today because of your input.

I've recently had my PET out for testing, and found one of the ROMS had suffered bitrot as mentioned above. Actually I have 2 PET 3032 boards and BOTH had developed faults in storage. Still haven't fixed the spare board (boots into a blank screen) but it's on the "to do later" list.

These PETS certainly seem to need loving. They always have a fault or two when retrieved from the kennels (.i.e. storage).

Tez
 
dave_m said:
Yes, since you replaced the RAM, it may be a bad ROM. I see that all are soldered except the Edit ROM. If your PIAs (6520) and VIA (6522) chips were on sockets, removing them and seeing if your got the initial Commodore display would have been the next easy thing to try in case one of them were bad. But unsoldering 40 or 24 pin chips is not easy unless you have experience. Are you able to replace big chips?
Click to expand...

Yes , of course , by taking extremely care, yes we can ;) . Also we are thinking to replace all the roms using new EPROM (with adapters ) to discard any issues with the original roms.
 
tezza said:
I think quite a few things can cause a Pet to fall into the monitor program like this. In the case of mine just recently it was a faulty ROM.
Click to expand...

Thanks Dave, we are gonna try that too using new eproms. I will let you guys know if getting better or worst.
 
cavemanweb said:
Also we are thinking to replace all the roms using new EPROM (with adapters ) to discard any issues with the original roms.
Click to expand...

If you use 2532 EPROMs for the 4K ROMs and a 2716 for the Edit ROM, you will not need any adapters. Another thing to consider would be to leave the ROMs in place, and use a RAM/ROM Replacement board that fits in the CPU socket. This avoids any unsoldering. Checkout PetVet in California or Tynemouth in Great Britain.
-Dave
 
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