Commodore PET 2001 Repair

[Videogamecollector123]

So I just managed to pull a Commodore PET 2001 out of my University's storage closet. It was used in the electrical engineering department and appeared to have some non-standard equipment attached to it, but after rearraging the cables to the stock positions, I managed to get the machine to boot up. However this is what came up on the screen

 

My next step is to reseat all of the chips but I noticed that there might be a chip missing. I saw in pictures of this machine that there are four chips in the second row while mine only has three. Is this correct and if not, what chip is supposed to go in the empty spot?

[Videogamecollector123]

I also found two of these boards that look like they would fit in one of the open slots

 

[UKRetrogamer]

PETVET is a comparatively cheap way of diagnosing problematic PETS but you'd need to check its compatibility with your system.

http://www.bitfixer.com/bf/petvet

[Videogamecollector123]

Is there a way to do the testing without that device? I do have access to oscilliscopes and logic analyzers in the electrical engineering labs here.

[awhite2600]

It looks like either bad or missing ROM. The PETs had some empty sockets for expansion ROMs. I'm wondering if the daughterboard that you removed replaced one of the stock ROM chips.

 

The fact that you are crashing into the ML monitor in the Bxxx address range tells me that code is jumping into that address range. The range from B000 to BFFF was reserved for extra ROM. A standard PET wouldn't attempt to access that area on bootup. Can you put the board back and see if the PET will boot?

[Videogamecollector123]

It looks like either bad or missing ROM. The PETs had some empty sockets for expansion ROMs. I'm wondering if the daughterboard that you removed replaced one of the stock ROM chips.

 

The fact that you are crashing into the ML monitor in the Bxxx address range tells me that code is jumping into that address range. The range from B000 to BFFF was reserved for extra ROM. A standard PET wouldn't attempt to access that area on bootup. Can you put the board back and see if the PET will boot?

Well the board wasn't actually in the machine when I found it, it was in a box next to it. These macines were modified to be hooked up to oscilliscopes and other electrical equipment so it's possible there was a custom rom of some kind involved. I did try plugging both of the boards in that I found and trying each position on the switch to no avail. Though sometimes I would get a screen of garbage before it went to the screen I got without it in. There is apparently a professor on campus who still has one that might be working. I'm going to see if I can see how his is set up.

[Videogamecollector123]

The daughterboards I found are only have EPROMS on them, which I'm guessing was part of what the computer was used for. I believe we have a rom writer here, so I could potentially make the missing rom chip, does anyone know if the roms are availible online and which one normally goes into the empty fourth slot on in the second row?