My 8P appears to boot (completes countdown), but then goes berserk and everything appears to be locked up (with no sound). Most of the time, the display flashes “Insert Cartridge,” but occasionally the panel LEDs will flash and there will be a few seconds of garbled ROM data flashing rapidly on the display. For a while I thought this behavior could be the result of panel switches being held active by a faulty IC, but I have not been able to prove this. Looking at the MIDI out, I occasionally see some CCs, but otherwise the note data is nonexistent. Based on my understanding of the system architecture, I am fairly certain that the fault only exists within the controller/programmer half of the mainboard.
I borrowed the mainboard from a known working 8P that seems to work fine when swapped in this chassis, so the power supply, display, and panel boards should all be good. I tested the traces connecting ICs and connectors. I also swapped ROMs, replaced CPU and RAM just to confirm that those were good. I probed all of the pins on the suspect ICs between both boards and the only major difference I noticed was that the CPU was being sent a 32kHz interrupt signal on the bad board.
At this point, I’ve tested every IC in the vicinity of the panel switch driver circuit and CPU/data bus the with my tester. All of them tested OK, although I’m really not sure how thorough my tester is. The only IC I have no way of testing is the Roland gate array (IC2), but otherwise I have run out of ideas and things to test.
Any suggestions? This is not my first synth repair rodeo, but I’m really stumped on this one…