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Commodore PET 2001 Hangs on Startup

M

MeanDumpsterCat

Member
Joined
Mar 3, 2015
Messages
21
Hello!
Here is every vintage computer collectors nightmare: a working system suddenly decides to completely give up. This precisely what happened to me. I was playing a game on my Commodore PET 2001 16N and for whatever reason I adjusted the brightness dial on the back. When that had happened, the computer displayed
?ILLEGAL QUANTITY ERROR

I restarted the computer and the usual random character screen came up but did not go away. A few moments later I disconnected the tape drive and disk drive and then booted it up. Same thing.
A while later, I tried again but this time it did not display the random data screen but booted up into BASIC displaying the normal screen however it was completely frozen.

It now appears to malfunction in one of four ways.
1: Permanent garbled screen
2: Blank screen
3: Erroneous BASIC screen (hangs)
4: boots up but hangs when basic loads.

Here are some pictures. (The first one is interesting)
WP_20150303_21_28_31_Pro.jpgWP_20150303_21_30_24_Pro.jpgWP_20150303_21_32_11_Pro.jpgWP_20150303_21_34_48_Pro.jpg
 
Though not always guilty, RAM is an usual suspect. I see your RAM ICs are all socketed so you may try swapping lower and higher bank and see if the situation improves.
Also ensure DC voltages are stable.
 
MeanDumpsterCat said:
It now appears to malfunction in one of four ways.
1: Permanent garbled screen
2: Blank screen
3: Erroneous BASIC screen (hangs)
4: boots up but hangs when basic loads.

/QUOTE]

These are the standard symptoms with bad zero page RAM or a bad ROM. Item 3 could also mean no 60 Hz interrupt. You are so lucky that you have all the big chips on sockets. This is fixable. what kind of test equipment do you have? DVM, logic probe, or scope?

Remember to swap RAM chips one at a time so you know which one may be bad. Also reseat all big chips by gently rocking the ends into socket for better connections.
Click to expand...
 
No word from the OP, it seems. In case you're still out there, figured I'd expand on what I said above a bit: I had the experience in a PET I was repairing where after leaving it on overnight running a BASIC loop (this after getting it to work "at all" from the garbage screen condition, issue there being a bunch of bad traces) I found it hung and not starting... most of the time, anyway. (I think I got a BASIC prompt once out of about 10 "flipping the power switch in anger" attempts, but it hung shortly thereafter.) Long story short I found that the RESET signal sourced from the 555 timer at A2 was sticking, and the problem resolved itself after replacing the three capacitors at C66, C67, and C68. Here's the schematic page, you can probe the reset signal at pin 40 on the 6502.

Certainly not saying this is definitively the problem, but it's an easy thing to check.
 
dave_m said:
MeanDumpsterCat said:
It now appears to malfunction in one of four ways.
1: Permanent garbled screen
2: Blank screen
3: Erroneous BASIC screen (hangs)
4: boots up but hangs when basic loads.

/QUOTE]

These are the standard symptoms with bad zero page RAM or a bad ROM. Item 3 could also mean no 60 Hz interrupt. You are so lucky that you have all the big chips on sockets. This is fixable. what kind of test equipment do you have? DVM, logic probe, or scope?

Remember to swap RAM chips one at a time so you know which one may be bad. Also reseat all big chips by gently rocking the ends into socket for better connections.
Click to expand...

Unfortunately the only equipment I have is a multimeter. I should also point out that the machine now functions normally but only for about 30 seconds before freezing.
One of my guesses was a bad capacitor.
Click to expand...
 
Eudimorphodon said:
No word from the OP, it seems. In case you're still out there, figured I'd expand on what I said above a bit: I had the experience in a PET I was repairing where after leaving it on overnight running a BASIC loop (this after getting it to work "at all" from the garbage screen condition, issue there being a bunch of bad traces) I found it hung and not starting... most of the time, anyway. (I think I got a BASIC prompt once out of about 10 "flipping the power switch in anger" attempts, but it hung shortly thereafter.) Long story short I found that the RESET signal sourced from the 555 timer at A2 was sticking, and the problem resolved itself after replacing the three capacitors at C66, C67, and C68. Here's the schematic page, you can probe the reset signal at pin 40 on the 6502.

Certainly not saying this is definitively the problem, but it's an easy thing to check.
Click to expand...

Sorry for the inactivity. The caps was the first thing that came to mind since it seems like a problem they would cause. I really do need to get myself a logic probe from Maplin some when too.
 
MeanDumpsterCat said:
dave_m said:
Unfortunately the only equipment I have is a multimeter. I should also point out that the machine now functions normally but only for about 30 seconds before freezing.
One of my guesses was a bad capacitor.
Click to expand...

An IC may be heat sensitive (probably not a capacitor). Try Gab72's idea of swapping one RAM chip at a time because if it is a RAM problem, it is in the lower 16K. Swap J2 with I2 then power up and see. If no change, swap J3 with I3 and try again. Keep going until all 8 chips of the lower bank (J2 thru J9) are swapped. See RAM schematic for reference.

So you don't bend the pins, get a cheap IC puller and IC insertion tool. If that doesn't fix the problem, get a can of circuit freeze to test the ROMs and other chips.

4618389.jpg mos1416.jpg



403a285g.jpg
Click to expand...
 
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