As mentioned above, the program must be running to enable interrupts.

Have you tested all 5 of the switch matrix ICs?
2x7400, 2x7404, 1x7432.
If one or more of those are toast, they can prevent the MPU from booting. I don't know how or why, but I've seen it many times. Just had a board with a failed 7400 (Z13 IIRC) that would make the QS80 do odd things. Marco's basic tests didn't indicate a problem.
--
Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/contact/
http://www.PinWiki.com - The new place for pinball repair info

Agreed. Very bizarre. I used the same logic that you did and thought that unless that particular IC is gating memory/device addressing, then it shouldn't matter if they were there or no. I'll post a video of what I saw with the failed 7400. I can send you the 7400 too. I thought about that.
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Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info

Here is a vid I shot just now.

After reviewing the QS80 documentation again it looks like the failed 7400, which is in the switch matrix return circuit, causes the QS80 to think that the enter button is being pressed and the sequence I'm observing are the subsequent QS80 tests.

Make sense?

--
Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/contact/
http://www.PinWiki.com - The new place for pinball repair info

An IRQ "stuck high", in one sense, is normal.
The normal state of the IRQ signal is high.
It might be more correct to say, "the IRQ signal is never pulled low".
...but I'm splitting hairs...

A peripheral device (6532) will pull the IRQ signal to ground (low) to indicate to the 6502 that some sort of processor service is required.

For System 80 MPU boards, this means that a RIOT timer has expired or that the RIOT has fielded a signal indicating that it is time to act.

The typical reason for lack of IRQ activity is that the 6502 isn't executing the software as intended. There can be many reasons for this.

...failed 6532 (usually U4)
...alkaline corrosion
...failed reset circuit
...failed clock circuit
...failed ROM
...failed U2/U3

The best way to solve most System 80 MPU boot issues is with the no longer available QuickScan80 that GPE made years ago. Failing that, Marco's test ROM using the PiniTech TC1 adapter is also a good method.

Of course, you could also send the board to a pro.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
https://www.youtube.com/c/ChrisHiblerPinball
http://www.PinWiki.com - The Place to go for Pinball Repair Info