New Project - Commodore Pet 2001

[al00000]

Hi,

I've just taken delivery of a non-working Commodore Pet 2001, for a new 'restoration' project.

The original plan was to use the carcass for building a retro PC.

I thought I would have a go a reviving it first...

When I switch it on, there is a gentle hum.

After a while, i seem to get some lines on the screen; which comes and goes.

I can best describe the lines as looking a bit like a vertical heart rate monitor. The screen makes a fizzing sound as it displays.

Any pointers to try a simple fix would be appreciated.

Al.

 

[Eudimorphodon]

You've come to a pretty good place to get advice on such things. To help people get started, one quick question/clarification: there was the original PET 2001 with the calculator style keyboard and a tape deck built in, and then there were "2001-Ns" which had a more normal looking keyboard on them but no internal tape deck. Which of the two are you looking at here? They use different motherboards that use static and dynamic RAM respectively. (And that the "calculator ones" have several sub-revisions using a different mix of non-directly-swappable parts.)

The screen making a "fizzing" sound sounds ominously to me like you might have some bad capacitors (or similar) on the analog board, which might need to be resolved before moving onto the digital portions. The "Standard Dead Pet" for 2001s is a screen full of garbage characters where the BASIC prompt should be; not seeing that implies gremlins harder to catch, but certainly not hopeless.

And... not to be a vulture, because I'd personally prefer this PET gets repaired, but if you do end up gutting it I may have some interest in the motherboard if it's one of the static RAM varieties. (I have a "calculator PET" that was refurb/repaired during its service life and has a non-original dynamic replacement.)

 

[al00000]

Thanks for the reply.

It must be the NS as it has no tape deck and a 'proper' keyboard and numeric keypad.

There's certainly no characters being displayed. Sometimes a small square in the centre of the screen and sometimes just white noise.

I've lifted the lid and there is certainly some rust and corrosion to the motherboard. They keyboard looks as new as the day it was made. I have not yet ventured in to the monitor.

I will certainly liberate any useful parts if I end up repurposing the case.

 

[jimdinunzio]

Greetings. I too have been spending the last couple of weeks attempting to repair a PET to working order, although this time an original 2001-8 (chiclet keyboard with built in tape drive). I'm a software engineer with a semester or so experience working on digital hardware and basic soldering skills, but a strong desire to learn and stubbornness to bring back a childhood first computer experience.
I've made some good progress with the help of resources on the web to get me started, especially Matthew's article at http://www.dasarodesigns.com/projects/troubleshooting-common-problems-with-the-commodore-pet-2001/
Also, I bought a composite video adapter board from him so I could remove the CRT and boot many times without harm to it. The CRT itself seemed ok.

My progress so far:
Computer initially booted up to basic with vertical lines through all columns of the screen or stuck at initial screen full of random chars, not all RAM recognized, keyboard nearly non-functioning. I found a picture of the exact vertical line issue on the web link above and explanation which pointed to a Video ROM problem!

While removing and cleaning all socketed chips, I found a bent pin on one, 3 broken pins on the video ROM and very weak pins that broke off on a system ROM when I lightly fine sand papered it (I was told don't use sand paper again, use an eraser) (all gold colored pins interestingly). I painstakingly soldered pins on the chips (really hard for me to keep 'em on), good for one insertion only I found out, and the video problem was fixed and system boot more stable!

For the keyboard I applied a pencil eraser to the conductive key plungers, used contact cleaner, swapped around working plungers to more important keys, and finally used product called Wire Glue to dab on the worst ones. Keyboard fixed, all buttons working!

Then while checking the power supply I found a bad voltage regulator (0 v). I replaced it, which not easy for me, having trouble desoldering, but struggled through it. After replacing the regulator all existing RAM was recognized (only 4K is installed) and attaching a cassette deck and using it didn't crash the computer anymore! Perhaps the additional current draw of the cassette deck made the regulator give up its ghost.

I started going forward using the tape deck and loading the breakout program and it worked! However then I had a setback. The computer boots up fine when completely cold. After a while the computer would hang with a print out of illegal quantity error and then will not boot passed the blank screen unless it is left off for a couple of hours. Using the tape drive to load a program seems to hasten this condition.

I decided to try removing the 6520 PIA chips during this failure state and found that with the keyboard one out the system would always boot to basic even albeit with no cursor and no keyboard function as expected. I bought a logic Probe and started checking lines. With this chip in during the hang the ram select lines are all high and the CPU address lines high as well. With the chip out select lines on ram show pulse waves and CPU address lines are pulsing rapidly more like normal. I swapped 6520 with the other one but no difference was observed so the chips seem good. I checked The integrity of the socket pins with an ohm meter to the back side of the board and found no issues.

Latest theory: Looks like I may have a thermally sensitive component or solder joint, although not sure where, but I suspect the I/O circuity down stream from the 6520. I'm planning to get some Super Cold to cool down areas when this happens. I don't have a heat gun which I heard is another option.

I'm open to advice. Please help if you have any suggestions. I'd like to get this thing back together and move on from this OCD episode!

Thanks a bunch!
Jim

 

[jimdinunzio]

Update:
So this is how it's going down. After several hours off it turns on and boots to basic fine and you can use the machine for 15 minutes or less if you load something from tape after which it hangs and when rebooting only it makes it to a black screen. I tried spray freeze first in the area around the 6520 chip and then systematically over all areas of the board and corresponding back but it did not boot. I sprayed enough each time to make the section cold to the touch. If I remove the keyboard 6520 PIA chip at that point then it boots to basic. Doesn't seem like a heat issue anymore unless I somehow missed it. Could it be a capacitor issue around the 6520 chip area? All I can think of is check resistance across the caps. I suppose both 6520 chips could be bad because I don't think you need a good one in the "second" PIA socket to run the system. I'll examine the traces and the bottom with a magnifier and see if I can see anything. This is a frustrating problem.

Jim