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Commodore 2001-8 Not Booting

smarks15

New Member
Joined
Apr 4, 2022
Messages
1
Hi everyone,

A friend suggested that you guys may be able to help with my Commodore PET. It currently just boots to a junk screen. I used a Romulator to bypass the ROM/RAM and no luck. Then, I swapped out the video RAM with some of the other RAM chips from the board and, while the random characters did change a bit, it still didn't work. I then tested the CPU by swapping in a known-to-be-working 6502, but that did not change anything either. I then tested the reset circuit with a logic probe and it's working fine. I'm unsure of what to try next. I could clean all the chips with deoxit, but I'm wondering what you guys suggest.

Thanks!
Sam

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jucole

New Member
Joined
Apr 8, 2022
Messages
9
Location
UK - Lancashire
Hi Sam, I bought a PET 2001 the other year that presented the same issue. I bought a ROMulator and switched out the ROMs (no luck) then tried switching out the ROMs & RAMs, and it booted up.

This is only a guess but If the ROMulator is working correctly it looks like the CPU can't clear the video RAM contents? Just out of interest what happens if you keep the tape rewind button down and power up the PET, does the motor start spinning straight away and stay spinning? or does it spin then stop, then spin continuously?
 

daver2

Veteran Member
Joined
Jun 19, 2012
Messages
7,376
Location
UK - Worcester
Welcome (both of you) to VCFED.

Don’t forget to update your profile with your location.

You will also be under moderation for your first ten posts, so holding a ‘sensible’ conversation will be difficult until then.

However, if you look at my other posts, you will see that we are currently fixing a slightly different version of the PET, and the steps are similar.

Have you found the schematics for your machine? Look on http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/index.html. Be aware that there are a few different variants of the PET, so the first thing is to identify which one you actually have. Look at the part numbers on the RAM and ROM. If you can’t work it out, post a high resolution photograph and I can work it out for you.

Well, you’ve already done a lot of things that I would do to start with! I suspect some vital signal is missing on the CPU, but that is purely a guess of course at this stage.

I see you have a logic probe. Can you check what is happening on pin 7 (SYNC) of the CPU. This pin should pulse HIGH every time the CPU executes an instruction. No pulsing, no clearing the screen!

What test equipment do you have and which ROMulator have you bought?

I developed a PETTESTER (that replaces one of the ROMs) - and this has been included into some of the ROMulators. The code and documentation is freely available from my Google drive if you can burn a suitable EPROM.
Dave
 
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daver2

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Jun 19, 2012
Messages
7,376
Location
UK - Worcester
I travel to Lancaster/Heysham on a regular basis (or at least I did before Covid of course)...

Dave
 

jucole

New Member
Joined
Apr 8, 2022
Messages
9
Location
UK - Lancashire
I travel to Lancaster/Heysham on a regular basis (or at least I did before Covid of course)...

Dave

Ah ok, in that case you'd fly past the turning for the M55 which would lead you to the home of a PET 2001-8, 320008. There's a nice coffee shop in Lancaster - me and the missus were there at the weekend getting some supplies. A strong coffee helps me not put my IC in the wrong way round!
 
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jucole

New Member
Joined
Apr 8, 2022
Messages
9
Location
UK - Lancashire
I've just been reading over some of the past PET repair threads - wow some of them have been a real rollercoaster ride to follow! packed full of really useful info! And your diagnostics ROM project code is really useful!
 

daver2

Veteran Member
Joined
Jun 19, 2012
Messages
7,376
Location
UK - Worcester
Oh yes, some faults just keep on giving!

>>> And your diagnostics ROM project code is really useful!

That’s what I was hoping...

I have a few more additions in the pipeline as well. A slightly better initial memory test. Some faults in pages 0 and 1 of memory can ‘sneak through’ my simple memory test and cause the later code to hang or crash. I have also added the ‘chime’ for machines that have the sounder. Also thinking about adding a user and IEEE port test at the moment. I am trying to think of the 95% solution that can test the main devices without the need for a loopback connector.

Hopefully, I will have then filled the ROM up! Just taking a break at the moment.

We really must set-up a ‘Commodore Convention’ in the UK at some point...

Dave
 

jucole

New Member
Joined
Apr 8, 2022
Messages
9
Location
UK - Lancashire
I must confess after reading those posts I do feel a bit lazy now just stuffing in a RAM/ROM replacement to get mine working. I think i'll create a new thread and see if I can diagnose the issues with my board; and I have another PET board which I managed to pick up as a potential donor board, so I can have a go at fixing that up too! Time to.. pull the dust sheets off my test gear - get some coffee in - and fire up the lab!! lol

The new tweaks to the PET tester sound pretty interesting!

"We really must set-up a ‘Commodore Convention’ in the UK at some point..."
Yes, that would be nice!
 
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