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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 19th Sep 2022, 6:11 pm   #1
BrackenFix
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Default 1978 Commodore PET project

Hi everyone

Just wanted to drop by and say hi. I recently came into possession of a 1978 PET 2001-8 BS that I am going to try and revive for my channel.

Looking through the posts, I think I will be here a lot as there is so much good information!!

I have spent the last 25 years working in IT but this is a completely different animal to what I am used to
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Old 19th Sep 2022, 7:11 pm   #2
ScottishColin
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Default Re: 1978 Commodore PET project

Plenty of very helpful people here - they helped me with my PET which was a bit of a saga.

https://colinjhaynes.wordpress.com/

Good luck and I look forward to watching the thread.

Colin.
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Old 19th Sep 2022, 7:29 pm   #3
SiriusHardware
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Default Re: 1978 Commodore PET project

Hello BrackenFix, welcome. As a new forum member the first few posts you make will be subject to new user moderation, so why don't you tell us a little bit about what your current interests in retro computing are, what you feel your current level of ability / experience is with electronics and what you have in the way of test gear.

Owning a multimeter and knowing how to use it, perhaps also knowing how to solder and unsolder will have you off to a good head start.

Unless your PET is just basically dead due to power / power supply problems there may come a point where you will also need an oscilloscope, but we would ask you just to check the basics with a meter first so don't worry about that for now.
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Old 19th Sep 2022, 7:39 pm   #4
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Default Re: 1978 Commodore PET project

Hi everyone

I do indeed own a multimeter and am able to solder. Amazingly, they were the tools needed for the original concept of my channel (building electronics).

Although I had a commodore 64 and an Amiga 1200+ as a kid, I never really did anything other than play games. I am now an IT manager but, obviously, my focus there is on modern hardware.

My love and interest in retro computers came from stumbling upon and binge watching a guy on YouTube called Adrian's Digital Basement. Watching him gave me the confidence to try this and do it on camera
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Old 19th Sep 2022, 7:39 pm   #5
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Default Re: 1978 Commodore PET project

Very keen to get an oscilloscope as think it will be a handy tool to add to my bench either way
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Old 19th Sep 2022, 7:44 pm   #6
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Default Re: 1978 Commodore PET project

While not wanting to discourage you, making component level repairs to computers is very challenging, particularly to pre-PC era hardware.
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Old 19th Sep 2022, 7:49 pm   #7
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Default Re: 1978 Commodore PET project

On the other hand, we have a reasonable track record of guiding people who initially have quite basic knowledge through some fairly complex repairs, as long as the OP is enthusiastic and committed.

Let's see what we are up against first.
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Old 19th Sep 2022, 7:49 pm   #8
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by paulsherwin View Post
While not wanting to discourage you, making component level repairs to computers is very challenging, particularly to pre-PC era hardware.
Fear not, I am under no illusions that my chances of success are very limited. I feel that getting this to work will be equal parts luck and the knowledge of others.

I like a challenge though and am eger to learn
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Old 19th Sep 2022, 7:52 pm   #9
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Default Re: 1978 Commodore PET project

So, as it stands the computer did turn on.

I'm told it showed a cursor for 5 minutes then the screen filled with characters. Now when you turn it on, the screen full of characters is all you see.

Only the number 1 and : keys seem to work at present.

The motherboard as VERY dusty and dirty so have washed and dried it (water then IPA).
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Old 19th Sep 2022, 7:59 pm   #10
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Default Re: 1978 Commodore PET project

Just a couple of pictures. You can see the screen full of text

I think the text issue is a video RAM problem maybe
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Last edited by BrackenFix; 19th Sep 2022 at 8:20 pm.
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Old 19th Sep 2022, 10:09 pm   #11
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Default Re: 1978 Commodore PET project

Are any of the ICs on the board in sockets, or any other signs of previous repair attempts?

Its quite promising as it looks like the 6502 is running and just the display circuits need attention, the keyboard is probably a separate issue.

It seems the display is showing 0 where it should show space, that would only be a single bit position wrong. 0x30 for 0 instead of 0x20 for space. Also probably explains display of 333instead of *** in the boot screen.

It could be either the circuit reading video ram to the character generator, the character generator itself or one of the 74ls244 buffers, possibly writing incorrect data to video memory.
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Old 19th Sep 2022, 10:26 pm   #12
BrackenFix
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by Mark1960 View Post
Are any of the ICs on the board in sockets, or any other signs of previous repair attempts?

Its quite promising as it looks like the 6502 is running and just the display circuits need attention, the keyboard is probably a separate issue.

It seems the display is showing 0 where it should show space, that would only be a single bit position wrong. 0x30 for 0 instead of 0x20 for space. Also probably explains display of 333instead of *** in the boot screen.

It could be either the circuit reading video ram to the character generator, the character generator itself or one of the 74ls244 buffers, possibly writing incorrect data to video memory.
Hi

Yes, some of the ICs were in sockets, I removed them before washing the board.

I attach a picture before I removed everything

A fair few of the ic have very tarnished looking pins/legs
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Old 19th Sep 2022, 11:04 pm   #13
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Default Re: 1978 Commodore PET project

The circuit diagrams for several different variations of this model are here:-

http://www.zimmers.net/anonftp/pub/c...001/index.html

If you can find a 32...... board number on the mainboard that would be helpful, and also report which sort of RAMs and which sort of ROMs are fitted in the PCB, that will help us to narrow it down to the diagram which best matches your computer.

I agree with Mark that the initial problem seems as if it may be in the display area rather than the overall CPU area - it looks as though the CPU is running to the extent that it fills the screen memory with spaces and then the startup message, but both the spaces and the startup message are being displayed incorrectly.

If you had a scope then you could (in conjunction with a test EPROM) try to narrow down the area of the fault without disturbing the actual hardware too much.

Without a scope you have to resort to cruder methods which may ultimately include replacing the display RAM and its support ICs, starting with the ones which are the cheapest to buy and easiest to replace. (So usually, you would replace the display RAM last).

You may also have a secondary problem in the form of a mostly none working keyboard but that will be easier to diagnose and fix with a fully working display, so fixing the display is the priority.

If you can first let us know the PCB number and the types of ROM and RAM IC fitted, we can then begin to make suggestions about how to proceed.
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Old 20th Sep 2022, 9:31 am   #14
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Default Re: 1978 Commodore PET project

This very much looks like the 8k version of the 2001N dynamic board shown in the first attachment. If so it will have eight 4108 type dynamic RAM chips soldered in the first RAM bank. The second bank is clearly empty and has been subjected to Commodoreís unpleasant practice of drilling holes to prevent upgrading. The second attachment should be the correct circuit diagram I think.

In terms of early stage fault finding itís good practice to verify that the regulators are generating the correct voltages using the meter. However, the PETís behaviour is suggestive of a poor connection and reseating the socketed 6502 CPU would be a useful early step.

Alan
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File Type: pdf PET 2001N 320349.pdf (1.72 MB, 48 views)
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Old 20th Sep 2022, 9:39 am   #15
SiriusHardware
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Default Re: 1978 Commodore PET project

Hi AJ, I was hoping this thread might attract your attention.

BrackenFix did say he has had all of the socketed ICs out and back in already, what you suggest could hold true but it does look as though the CPU is running code OK.

A folded-over pin on the character generator ROM could also possibly account for the display oddness.

I think we'd better let BrackenFix confirm that his machine is the one you have indicated before we go in much deeper.
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Old 20th Sep 2022, 9:44 am   #16
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by SiriusHardware View Post
Hi AJ, I was hoping this thread might attract your attention.

BrackenFix did say he has had all of the socketed ICs out and back in already .....
Missed that - sorry.

Alan
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Old 20th Sep 2022, 11:06 am   #17
BrackenFix
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Default Re: 1978 Commodore PET project

Wow, you are all so helpful, thank you.

I'll reseat all the ICs tonight and report back.

I'll admit to being very naive and hoping it was just a dirty video RAM chip. We can but hope
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Old 20th Sep 2022, 11:13 am   #18
SiriusHardware
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Default Re: 1978 Commodore PET project

If you could just let us know whether your board is (or is not) the one in the first PDF in AJ's post #14, we'll be in a better position to advise.

One of the ICs you had out when cleaning up the PCB would have been the character generator ROM which is one possible cause of your display problem - if you could just make sure none of the ROMs have any of their pins folded over? I have been using DIP ICs for almost 50 years, and even now I sometimes get a pin out of line when inserting an IC into a socket.
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Old 20th Sep 2022, 12:18 pm   #19
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by paulsherwin View Post
While not wanting to discourage you, making component level repairs to computers is very challenging, particularly to pre-PC era hardware.
It's a lot easier than component-level repair on modern computers!

More seriously, it does depend -- a lot -- on your experience. I can find my way around a minicomputer (several hundred or more TTL logic chips) a lot more quickly than I'll find my way round an old TV. But then I work on such machines a lot and know what to look for.

Vintage computer repair can include just about all other areas covered by this forum with the exception of RF tuned circuit type stuff. You will meet :

Digital electronics (obviously)
Analogue electronics
RF theory at least for transmission lines (a computer bus should be designed as such even if it rarely is)
Switch mode power supplies
Video monitors, so the 'second half' of a TV
Magnetic recording (disk and tape drives) with the attendant mechanical issues sometimes.

As for test equipment, a logic analyser -- a good one -- can be very useful. Yes, it's a specialised instrument, but no more specialised for computer repair than, say, a spectrum analyser is for somebody working on radio transmitters or a distortion meter for somebody who takes audio seriously.

A logic probe -- which indicates '1', '0' or 'changing' can be very useful as first test for spotting a data bus line which is shorted to ground or a clock that's not running for some reason. I normally use one first to spot the obvious faults.

On a PET I'd start by replacing all the IC sockets with turned-pin ones even if they look good. They do not have a good 'track record' in my experience. At least then you know that any fault are 'real' ones.
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Old 20th Sep 2022, 1:42 pm   #20
BrackenFix
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Default Re: 1978 Commodore PET project

Do you have any recommendations for a logic analyser?
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