Mitchell Spector wrote:
Is it possible the 65C816 is dead, or the oscillator socket not making a proper contact? I'll have to pull another IIgs out of storage to test the card in there, but since the original 65C816 DIP works I doubt the motherboard is at fault. I hope it's a socketed part on the ZipGSand not one of the surface mount ASIC at fault. Judging by the symptoms, I'm guessing the former.
I'd try the ZipGS in a different machine first.You know the processor socket on the motherboard is only good for a few insertions/extractions before it gets sprung and begins to lose contact. The pins on the ZipGS are so much smaller than the pins on the original processor, so I would expect it to make better contact even if the ZipGS can't.
How many TWGS and ZipGS boards have been discarded as dead, just because they made poor contact? I've come across a few.
If the ZipGS doesn't work in the next machine, try replacing the cable - especially if it's the original factory cable.