Welcome to the adventure that is System 80.
Scott (above) is 100% correct. All three RIOTs need to be working almost 100% for the board to boot.

IRQ staying high means that the processor isn't running enough to have the RIOTs doing their jobs, including interrupting the 6502 when necessary.

I have Marco's test ROM that I use with a PiniTech board that plugs into TC1.
I also have Ed's (GPE) QuickScan80, which is NLA.

I mostly use Ed's QS80. It tests the RAM and all three RIOTs.
Most frequent reasons for failure to boot...
...failed reset section
...failed clock section
...blown U4 (switch matrix). You can diode test Z11-Z15. If one of those is blown, there is a 50/50 chance that U4 is blown.
...failed 6502
...failed U2 or U3
...failed RAM

If you'd like to send the board to me, I can take care of it.
Otherwise...Happy Probing!
--
Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/Contact ... for board repairs
http://www.PinWiki.com - The Place to go for Pinball Repair Info

Nothing. That's just random.

Here is the other option. Currently not available either....
https://www.pinitech.com/products/gottlieb_universaltester.php

I remember the bad old days when I didn't have these test aids. It's a tough road to hoe.
--
Chris Hibler - CARGPB #31
http://ChrisHiblerPinball.com/Contact ... for board repairs
http://www.PinWiki.com - The Place to go for Pinball Repair Info