Picked up a non-working M100. I've opened it and attached bench power supply, verified all ICs get proper voltage (5v.), I see display darken and all pixels seem to be on (or maybe off state).
I picked up an recap kit and installed that, but no change. Thinking the ram might be bad, I unsoldered the RAM module, and installed a socket, and tested Ram optional module #1 and #2 in it, but no change. I then got my scope, and verified crystal is working on pin 2 of the 80c85. On first power up, I do it see it try to talk to the bus but then it stops after X clock cycles. Probing address lines at M1, I noticed that AD7 goes low and stays low (unbuffered side), A7 stays low the entire time. Verified MR2A, MR1A resistors are not shorted or open (correct values). Maybe M1 is bad? Though M1 still might be good, as M2 could be pulling it down, or the 80c85 itself could have a bad pin. At this point I not sure how to dig deeper unless I buy replacement ICs. Thinking maybe the ROM has gone bad, I tried to read it. The ROM it has the 3256C07 in it (verified VCC is at pin 28), I then pulled it out and tried to read it using my TL866 programmer. But it fails on Pin detect on same pins, I've selected every variant of 27c256, and still these pins fail. [cid:0b86da58-d862-428a-a3bb-2ef0653cb789] I've unchecked Pin detect, and forced a read but data looks incorrect, and when I try to read it again, it will not match. At this point, I'm going to pause and think about what I could do next. Any help be greatly welcomed! Thanks, Jerry
