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Commodore PET 4032 Problem

E

emptymind

Member
Joined
Jun 22, 2010
Messages
28
Location
Jersey, Channel Islands, UK
Hi, this was actually my first computer many years ago! I finally found one for sale but I need help to revive it. At the moment when I turn it on the screen is completely filled with a checkerboard pattern. I reseated all socketed chips and got the same result. I then swapped around the two 2114 video chips in UF7 and UF8 and managed to get the screen to fill completely with question marks. This would suggest that there is a problem with the RAM so I have ordered some replacement 2114s. I am a little concerned that there are no different characters in the top left of the screen where the BASIC prompt should be. Perhaps there is an additional problem? Any advice on how to proceed would be greatly appreciated. (Of course I should probably just wait until the 2114s arrive, right?)
 
The 'question mark' character is a hex 3F while the 'space' character is a hex 20, so I don't see much correlation there. So it may not be a stuck bit in the video RAM, but replacing is an easy first thing to try.

The system may not be getting far enough into the boot sequence to initialize the video RAM anyway. If that is the case, the problem could be about anything, but often it is one of the five system ROM chips. Do you have the capability to program a 2532 EPROMs to replace some ROMs? If not, there may be vendors that sell a big EPROM and adapter that replace all the ROMs at once.

Right now you only know that the video timing and analog circuits are working which is a good thing as they are fairly difficult to troubleshoot and fix.

Good luck with this project. There are several of us here that like to help getting PETs back on the air. Let us know about your troubleshooting capability. Do you have a voltmeter or scope, etc.
 
Interrrestinggg... ;-)

Have you swapped those 2114s several times to see if the symptoms are consistent? Those characters might just be random power-up bytes and leading you down the wrong diagnostic path, as it were; it does sound like the problem lies elsewhere, before the VRAM.
 
4032 project

4032 project

Thanks for the encouragement. I'm glad there are people who want to see these working as much as me! I have a little knowledge when it comes to electronics but I'm definitely going to need help from you guys! I have a voltmeter and I've ordered a scope.

Regarding the 2114s, I swapped them four or five times with the same results. In the original positions they give either checkerboard or completlely green characters which soon change to checkerboard. The other way around I was getting question marks every time. I think maybe one of them is dead and the other is not - but there must be a problem with one of the ROMS or the 6502?

I am more than willing to purchase an EPROM programmer and replace some ROMS if I can find a way of identifying where the problem lies. I don't have any other PET boards or chips but I do have a BBC model B micro, Commodore 64 and a few others I might be able to borrow chips from if necessary.

I will post some pictures of the PET when I can.
 
Interestingly, I just noticed that the 'Checkerboard' pattern seems to be made up from the character painted on the front of the question mark key. I think MikeS is right about the problem lying before the VRAM since I get the checkerboard pattern with no VRAM installed at all...
 
The question mark is 3F (0011 1111), the checkerboard is 7F (0111 1111) or even FF (1111 1111) depending on which way it is facing.
 
FF it is then. Not sure how that helps you in troubleshooting, but good luck. If you get an EPROM programmer, make sure to get one of the perhaps slightly more expensive variants that reads and writes to 2532 chips as well as the more common ones. It will let you read the ROMs and you can compare them with known good dumps.
 
carlsson said:
FF it is then. Not sure how that helps you in troubleshooting, but good luck. If you get an EPROM programmer, make sure to get one of the perhaps slightly more expensive variants that reads and writes to 2532 chips as well as the more common ones. It will let you read the ROMs and you can compare them with known good dumps.
Click to expand...

Thank you - I will start looking for one now!
 
If I remove any one of the ROMS or Processors and switch on I get the same result, except for the Chip in UF10 which is 901447-10 - next to the VRAM. (This is the character generator, right?) With this removed I just get blank screen (which would make sense, right?).

The weird thing is if I put just one VRAM in UF8 I get the checkerboard pattern but if I put it in UF7 I get diagonal lines !

Not too sure how to proceed now :(
 
emptymind said:
Thank you - I will start looking for one now!
Click to expand...
I was thinking that we don't need to see any pictures, but good thing you posted them; over here a 4032 is almost always a "FAT40", i.e. the 12" model with the "universal" 80/40 column board, which is considerably different from yours. I keep forgetting about CBM's inconsistent model numbers, especially NA vs. Europe; where are you, BTW?

The fact that the symptoms are consistent depending on the 2114 order does kinda suggest that maybe one or both has a problem, perhaps not responding to the address lines. Let's see what happens when you replace them; they were one of the more notoriously failure-prone chips of the day.

What else have you got? Any old PCs or XTs by any chance?
 
I'm in the UK in the Channel Islands.

I have a BBC Micro, a Commodore 64, some Atari STs, Spectrum +3 and ZX, some Amiga 1200s and a couple of Amiga 500s, an Apple Performa, an old Vectra console and quite a lot of PC stuff. I have been collecting for a while but have never gotten around to looking through what I've got. I just started getting all the stuff out of the attic at the weekend and then yesterday the PET turned up. The lady got it from a car boot sale and said she was planning on turning it into a goldfish bowl !!

So I guess now is the time to clean it all up and get as much working as possible. Especially the PET ! I have a lot of learning to do, but it's worth it.

Hopefully the 2114s I ordered will arrive in the next couple of days and I'll know more...
 
MikeS said:
over here a 4032 is almost always a "FAT40", i.e. the 12" model with the "universal" 80/40 column board, which is considerably different from yours.
Click to expand...

Mike,
If I am understanding this, the type of PET we are looking at is listed in zimmer's as the 2001N/3000/4000 type with a discrete logic CRT controller rather than the 6545 CRT chip?

From the schematics, it has the 6332 ROMs, 4116 dynamic RAMs and 2114 video RAMs.

For emptymind, here is the link to the zimmer schematics, if this is the right computer: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html

For some reason with these files, I have to download the file first and then click on it to view the schematics.
-Dave
 
Cool. Another PET awaiting ressurection.

emptymind, you are in good hands with the guys who have responded so far. Keep at it and I'm sure you'll be able to nail down the problem.

I'll be monitoring this thread with interest. I'm not a hard-core techie but if I think of anything I'll contribute to the discussion.

Tez
 
emptymind said:
Thank you - I will start looking for one now!
Click to expand...

Many of the newer PROM programmers available say they can program the 2532 EPROM (which has the same pinouts as the PET 6332 ROM), but it will turn out they need a special extra-cost adapter and well as a 25 volt power source. Check the specs carefully.

If you are handy with soldering, you can make your own 2532 programmer fairly easily. The hardest part is generating a precision 50 mS pulse. Some of us here can burn the part for you if you are willing to suffer the shipping delays.
 
I'm wondering what else could read the ROM chips; I don't suppose you know anybody in your neighbourhood who has a similar PET?
 
dave_m said:
Many of the newer PROM programmers available say they can program the 2532 EPROM (which has the same pinouts as the PET 6332 ROM), but it will turn out they need a special extra-cost adapter and well as a 25 volt power source. Check the specs carefully.

If you are handy with soldering, you can make your own 2532 programmer fairly easily. The hardest part is generating a precision 50 mS pulse. Some of us here can burn the part for you if you are willing to suffer the shipping delays.
Click to expand...

Thanks for that - I will suffer shipping costs if that's what it takes ! The PET must live ! It seems awfully tricky to find a programmer that does 2532 out of the box without spending a fortune. Soldering is not a problem so I might make something if I can find some instructions somewhere. I only really need to read the ROMS in order to verify them against the bin files on the net, right ?

MikeS said:
I'm wondering what else could read the ROM chips; I don't suppose you know anybody in your neighbourhood who has a similar PET?
Click to expand...

Unfortunatley not. That would be ideal - unfortunately I live on a small island where I haven't seen one of these since I was about ten !

tezza said:
Cool. Another PET awaiting ressurection.
emptymind, you are in good hands with the guys who have responded so far. Keep at it and I'm sure you'll be able to nail down the problem.
Tez
Click to expand...

Don't worry - I'm not going to give up ! Thanks to everyone who wants to help !
 
The 2114s arrived

The 2114s arrived

Wow - that was quick ! I ordered 4 of the 2114s so I have spares. These are the chips I ordered:

IMG_3735.jpg

It looks like both the old chips must have been dead because I now get this with either pair of the new chips:

IMG_3734.jpg

The characters are a little different each time you turn it on but stay steady until you turn it off. Progress. I guess now I have a bad ROM or processor, right? Does anyone know if I can try the processor out of the BBC Model B?
 
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