CMB PET 2001 restoration tips?

Sounds like you are at the "real debugging" point. If you have a logic anaylzer or a storage scope, you should be able to catpture the first few accesses to ROM, and confirm that the CPU is fetching the right opcode from the right address by walking the bus lines on the CPU with your probe. Verify that what happens on the bus matches the code in the ROM contant. That fact that you have a screen full of viewable but random characters means the CPU didn't get as far as clearng the video RAM (or possibly could not write the video RAM). Assuming the former, then one of three things has most likely happened: - The program "crashed" due to reading an invalid opcode (check interface between ROM and CPU) - The program "crashed" due to faulty RAM - this usually happens at the first RTS instruction. (check interface between CPU and RAM). - The program is running, but stuck in a loop waiting for an event that never happens. Get the ROM listings from funet, and logically follow the code until the screen is cleared (iirc this is not terribly far into the code) - then note each hardware device (ROM, RAM, PIO etc.) as the code flows through this path, and using a storage scope match up the hardware accesses you see following a reset with the logic of the code. At some point they will diverge. If the CPU has crashed due to bad ROM, this can happen anywhere. If due to a bad RAM, it will likely happen at the end of a subroutine call). In either case, you will likely see bus activity which is unrelated to the logic of the program (since the program is no longer running). In some cases you may see NO bus activity (if the CPU encounted a HLT opcode). If the program is stuck looping on an event, you should see repeated accesses to the affected device and the program will not proceed past this point. Keep in mind that bad bus tranceivers etc. can make any of the above look like any ot the others, so when you are observing "funnies" on the bus, it's a good idea to verify the signals all the way through the entire address/data path. I don't have the schematics in front of me, however as this is a character oriented display, the CG ROM should sit between the video memory and the video shift register. Removing it should effectivly decouple the video RAM data from the visual display, and I would expect to see all white or all black, however with the ROM removed, the inputs to the shift register are basically undriven, and depending on the device you may just be seeing crosstalk from adjecent lines - as long as you get good recognizable characters with the CG ROM installed (even if they are randomly position around the screen), the display is probably working correctly. Scoping the inputs to the shift register should show you what is happening (and you may see lines on the display come/go as you load the pin). Dave -- dave06a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.classiccmp.org/dunfield/index.html