I've been working on this problem 8753B CPU card for a few months now with very limited results. The original problem was no video, which I tracked down to a bad PAL 16R1 chip, replacing that got me a screen with random lines and various flashing characters. I thought that problem was bad DRAM so I replaced them, still no change. Watching the status lights while changing the position of the A9CC jumper and pressing the "PRESET" button I was able to get it to occasionally start to work, but not all the time.
The short story is I can now get it to run every time by following this procedure; turn the unit on with the A9CC jumper in the "NORMAL" position, then move the jumper to the center position and press "PRESET", then move the jumper to the next position to the right and press "PRESET". This will display "DRAM Passed" on the screen, then move the jumper back to the "NORMAL" position and press "PRESET", at which time the unit will start up and operate normally as long as power is applied.
Any change in that sequence and it will not operate. I've swapped the PROM's into a working board and they work fine.
Anyone have any idea what is going on with this?
Chuck
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