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A few things to check which run into my mind:
1) Is the IRQ generator (4020) working and is there a heartbeat signal at the /IRQ pin at the 680x CPU? A common issue here: after removing the Original batteryholder the plated hole for GND is damaged and a few chips are not connected to GND anymore.
2) Is the selection for ROM's IC14 and IC20 working? A testrom only runs from IC17, there *might* be a chance the ROM's at IC14 and IC20 are not accessed properly.
Sometimes....sometimes not. I've had a repair in the past where the CPU board tested fine with a testrom at IC17. But as soon as you installed something in socket IC14 the whole thing crashed as IC14 was permanently selected.
One of the reasons I intergrated a socket test in my own testroms (as well as a IRQ gen test).
Just to be sure: the driverboard is okay and cannot be the source of the problem?
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