CMB PET 2001 restoration tips?

On 7/15/06, Roberto Bazzano <r.bazzano at ulm.it> wrote: While 2114s are notorious, it doesn't sound to me like the behavior he is describing has anything to do with video RAM. "Garbage characters" on power-up is a normal state. What's also normal is that less than 1 second later, as the tube is warming up, the CPU clears video RAM. That's the step his machine is stuck at. The "problem" is that there's no one cause for having a frozen PET with garbage characters on the screen... Anything that interferes with the CPU accessing RAM or ROM will do that. As other posters have mentioned, different failures will cause different types of crashes at different points, but the visual effect is the same. If I were scoping a problem at that level, I'd pull out my Fluke 9010A and the 6502 pod and go to town; not everyone has one of those, but if you do, it's a really handy tool at this level of debugging - built-in RAM tests, ROM checksums (once you define where ROM is and what the expected checksums are), bus tests (including shorted bus-lines), etc. If you had or could build one of the modernish ROM/RAM replacements, it would go a long way to eliminating swaths of circuitry. I have one at home made by one of the cbm-hacker members... it's a daughter card that sits in the CPU socket. You pull the RAM and ROM from the machine, move the CPU to the daughter card, install a compatible ROM image to the single EPROM/EEPROM, set the DIP switches accordingly, and boot it all up. If it were not to boot, then the problem is likely some sort of stuck bus transceiver or total I/O selection failure. If it does, then you could re-install the ROM or RAM first and at least get an idea where the problem is via the time-tested divide-and-conquer method. -ethan