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nice writeup!

I have a System 32 here that has no audio and the audio RAM always tests as GOOD, even when removed.....wish I could absolutely rule out the custom as good with an oscilloscope before dumping more time into it

Check the Clock and Reset signals on the Z80. It also could be a bad Z80 Audio CPU like I encountered. I think they fail more often than the customs do.

I already swapped in a Z80, it isn't that. this made me check the unit again and I've just realized that probing pins 6-8 on ICs 11 and 12 causes a reset or a freeze. I'm wondering if maybe IC13 (attached to both) could be at fault?

Lucky it wasn’t that damn custom