I am trying to fix the motherboard to my grandfather's old //e, and have run into a bit of a problem. The board had been behaving rather strangely (not running internal diagnostics when the keyboard was detached, and giving errors for the RAM, and MMU). I've tested the chips on another board to see if I could replicated the error, but they all tested "System OK" on the new board. The most consistent error I was getting was in RAM socket #7, so I tried spraying it with some contact cleaner. At first it didn't seem to do any good, but then the board started giving me a new problem (as seen in the pictures below).
I have tested this board with two different sets of chips, and processors (the 'original' 65C02, and enhanced ROMs, as well as the 6502, and un-enhanced ROM set that came with my other board), and this is what I get on the screen. The first is with the 65C02, and the second is with the 6502. Does anyone know what went wrong?
These motherboards are not 100% alike. You will probably have to remove the chip sockets and replace them with new ones after checking all of the traces.
What I can tell you is that the IOU and the MMU chips are responsible in one way or another with the video display.
Also, the IOU chip appears to have different versions.