Am 20.05.2011 23:11 schrieb Jeff Shanholtz:
(...) Anyone know what typically fails on these cards?
There is not really much logic on these boards. Only the RAM chips and two bus drivers. Years ago I've drawn a schematic, http://john.ccac.rwth-aachen.de:8000/patrick/data/ext80col.pdf
Try PR#3, then the classic "10PRINT"HALLO";20GOTO10". If the screen fills with 80column text, the board should be 75% okay. If every second character looks weird, one of your RAM chips is faulty. In case it hangs after the PR#3, one of the driver chips (sometimes they use PALs to hide their function) is broken.
HTH Patrick