I'm guessing if you pull ALL the chips out of sockets that are on the IRQ line, then put them back one at a time, you should find 1 chip will 'hold' the IRQ high, when it was previously blinking wiht the other chips.....tedious elimination.
With all chips out, the IRQ pin might still be held high by a pullup resistor anyway.
Also to note, powering the cpu card on bench might not show the IRQ correctly, you'll need to create some activity on the playfield so the PIA's etc start generating IRQ's...
Typically, a cpu board sitting on the bench powered up wont be pulsing IRQ's at all, maybe this is part of the prob misleading your readings??
If anyone can send me a schematic/link, i'm happy to look thru it.
MM.
