• Please review our updated Terms and Rules here

Newbie 2001 owner needs handholding diagnosing a problem

M

M81

New Member
Joined
Apr 8, 2026
Messages
7
Hello- I've got a 2001 in need of some care and I'm getting in over my pay grade hoping I can get some direction from this group. I have some experience rehabbing Apple II/III- so I recapped the entire machine. The end result was a much stronger and stable video image, but it behaves the same as when I acquired it- with a 1 second generation of a full screen of characters, then a " _" on the left and a "!"on the right side of screen, with an occasional "@". Unlike my work on apples- I can't swap chips from a known good machine. I've never used an oscilloscope- but have one on order to dig in a bit deeper. My next planned step is to focus on sockets. Any recognition of the issue from the screenshots, or suggested steps would be helpful. Screen Shot 2026-04-30 at 7.09.40 PM.pngScreen Shot 2026-04-30 at 9.42.28 PM.png
 
Welcome to the Vintage Computer Forum.

You have a PET 2001N, BASIC 2 machine with 8K RAM. See schematics here: https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html

A ROMulator should fix this if its bad user RAM or the ROMs. It also gives you a full 32K of RAM and choice of BASIC 2 or BASIC 4. You should procure a few 2114 Static 1Kx4 RAMs chip for the video RAM in case they are bad. Two used in the PET at UF7 and UF8.
-dave_m
 
dave_m said:
Welcome to the Vintage Computer Forum.

You have a PET 2001N, BASIC 2 machine with 8K RAM. See schematics here: https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html

A ROMulator should fix this if its bad user RAM or the ROMs. It also gives you a full 32K of RAM and choice of BASIC 2 or BASIC 4. You should procure a few 2114 Static 1Kx4 RAMs chip for the video RAM in case they are bad. Two used in the PET at UF7 and UF8.
-dave_m
Click to expand...
Thank you. Just waiting for the ROMulator to arrive... before I start asking a lot more questions :)
 
I know very little about repairing PET's but I just successfully repaired one like that by mostly reseating all the chips in the white sockets.
 
This is a second generation PET, sold overseas as series 3000. Yours would be a CBM 3008.

The original PET was a full static design, they employed custom MOS memories, later 2114s.
Then they made a major redesign of the computer to include dynamic memory. At this same point, they removed the integrated cassette and put a more proper keyboard. Also, the screen went from white to green.

I have a favor to ask you. Could you redo the motherboard picture in a more serious environment - such as a table - and give me permission to reuse? I have heard about Commodore intentionally damaging their own boards to avoid third-party expansion on them, but it is the very first time I see it and would like to show it to colleagues.
 
I forgot...

Judging for your screen I can already say something. The character generator ROM is intact, so no need to check it. From the ROM to the output is correct, and the sync signals are also correct.
What you are seeing here is just random junk from the memory initialization - so the CPU may be halted. I would start checking the CPU, reset 555 and also ROM.
 
Welcome to VCFED.

So, if the garbage screen clears after a second or so, that indicates to me that the kernal ROM (the ROM containing the reset vector) and a handful of instructions are running and the CPU is finding its way into the EDIT ROM. This is where the screen is cleared.

After the EDIT ROM has cleared the screen, and initialised the I/O devices, it RETurns back to the Kernel ROM.

If the RAM is faulty, this is where things go sadly wrong.

The couple of random characters on a blank screen may be video RAM or (more likely) the CPU going berserk after RETurning to some random memory address and going into the weeds.

Another scenario is faulty ROM. In this case, the effect could be the same.

Who's ROMulator have you bought? Has it got my PETTESTER code within it as an option?

Dave
 
My ROMulator is built! Does anyone have a handy guide to dip switch settings I should run through to narrow down where my motherboard problem is? Also have the Program/Debug module.
 

Attachments

  • Screen Shot 2026-06-01 at 4.11.27 PM.png
    Screen Shot 2026-06-01 at 4.11.27 PM.png
    4 MB · Views: 6
  • Screen Shot 2026-06-01 at 4.32.19 PM.png
    Screen Shot 2026-06-01 at 4.32.19 PM.png
    3.2 MB · Views: 6
Last edited:
Well... On my 1st go I entered this:
  • SW1 = ON
  • SW2 = OFF
  • SW3 = ON
  • SW4 = OFF
  • SW5-8 = OFF
And- IT WORKED!!

Thank you all for the help with this! When I return from vacation I'll dig in to find where the fault on the board actually is and it looks (sounds) like mine does not have any sound capability. I'm sure there is a way to do that?

Thanks again.
 

Attachments

  • Screen Shot 2026-06-02 at 2.31.26 PM.png
    Screen Shot 2026-06-02 at 2.31.26 PM.png
    2.8 MB · Views: 10
  • Screen Shot 2026-06-02 at 2.31.36 PM.png
    Screen Shot 2026-06-02 at 2.31.36 PM.png
    2.5 MB · Views: 10
  • Screen Shot 2026-06-02 at 2.31.53 PM.png
    Screen Shot 2026-06-02 at 2.31.53 PM.png
    5.4 MB · Views: 10
RetroAND said:
This is a second generation PET, sold overseas as series 3000. Yours would be a CBM 3008.

The original PET was a full static design, they employed custom MOS memories, later 2114s.
Then they made a major redesign of the computer to include dynamic memory. At this same point, they removed the integrated cassette and put a more proper keyboard. Also, the screen went from white to green.

I have a favor to ask you. Could you redo the motherboard picture in a more serious environment - such as a table - and give me permission to reuse? I have heard about Commodore intentionally damaging their own boards to avoid third-party expansion on them, but it is the very first time I see it and would like to show it to colleagues.
Click to expand...
Sorry for the delay- I must have missed this. I don’t think I’ll be removing the motherboard again. The ROMulator gives me 32k- but with those holes in the board I can’t expand the memory easily, so I’m just going to keep the ROMulator in place. Is there another picture I can take that would help you?
 
There are plenty of pictures of the Commodore PET 'hole drilled PCBs' on PET websites and VCFED. I know a few people who have even fixed the holes and expanded the memory!

See (for example) post #13: https://forum.vcfed.org/index.php?threads/commodore-pet-question.1237073/

The problem with your ROMulator is that you are not using the PETs onboard ROM or RAM.

This may (of course) not concern you - in which case, you may as well select BASIC 4 on the ROMulator.

Dave
 
I have the same pcb , but not vandalized in the DRAM area. I had considered multiple ways to fix the vandlised boards. One elegant way to do it is to have two very thin pcb's made that match the exact tracks/ plated through hole pads on the board, on both sides in that DRAM area (easy to do because you can just photograph it and make the pcb in KiCad manually) then place that onto both sides of the pcb soldering the through hole connections, then the original copper foil layout will be re-created, without having to use any messy wires.

The people at Commodore who thought that idea up, of vandalising the pcb and trying to inhibit a user/owner of their computer making a memory capacity upgrade, should have been sacked. It makes the idea of using tamper proof screws to inhibit disassembly and repairs on equipment look like Mother Teresa thought of it.
 
Last edited:
You must log in or register to reply here.
Back
Top