Does your game have the under-playfield pop-bumper driver boards? I am not familiar with the replacement board set by Rotten Dog. Are all your fuses good, and voltages measured OK?

Pop bumpers are self-contained with their own control boards and solely triggered by physical ball contact. There is absolutely no link between your pop bumpers and your MPU aside from the scoring switches. However, I believe the pop bumper driver boards get their 5-volts from the MPU harness.

And if your displays are now out too, did you check your power supply board for all the proper voltages? Have you looked at your fuses?

IMO, this sounds like a flaked out power supply or power supply connector. Over the years they get hot and very loose.

Was the game properly working before you replaced the boards?

This part should be easy to resolve, since the 60 VAC is only used for displays and nothing else. However, the 60 VAC goes to the power supply A2 in the head and is then rectified to 60 and 42 VDC. Test all voltages at the power supply test points.

With power off, unplug all displays and the power supply. Turn on and check voltage again.

Plug in only the power supply and see if the 60 VAC comes back up.

Assuming it goes back up to 60 volts, plug in each display one at a time and check voltage. Just remember to turn off machine before plugging/unplugging each display. See if you will find a display that is causing the load.

60 VAC being pulled down twenty volts has nothing to do with your MPU.

If it wasn't working before installing new boards, then it likely has a lot of the common Gottlieb issues; flaky power supply, bad grounds, bad capacitors, bad connections, etc.

What about the A2 power supply? The 60 VAC goes directly into this. Completely disconnect A2 and check voltage drop again.

If you still have a voltage drop, then there is a wiring fault someplace between the transformer and A2 power supply.

Sounds like you have a very serious problem within A2. It's critically important to get the A2 power supply working to spec before doing anything else.

Without A2, the Driver and MPU have no power. However, you still have power on the playfield components without any control of the driver board, which is why things are popping off when you power up.

When the machine is non-operational, you have to be methodical and start at the beginning... fix the A2 power supply first.

https://www.pinwiki.com/wiki/index.php/Gottlieb_System_80#System_80_.2F_80A_Power_Supply

I agree with Mr Sparky about the A2 power supply being the first thing to look at. The big transistor in the middle is notorious for having a bad connection.

But to answer your question about what is driving the IRQ at TP4. Note - the "IRQ" signal name.
You will find the same "IRQ" signal names at: U1-Pin4, U4-Pin25, U5-Pin25, U6-Pin25, J10-Pin16 and resistor R1. All of these pins are tied together. IRQ is supplied by the three RIOTS (U4, U5, U6). Normally you cannot tie three outputs together but this signal is different ("Open Collector outputs"). None of the three RIOTs pull the signal high but any or all of the three RIOTs can pull the signal low. Resistor R1 is responsible for pulling the IRQ signal back high. U1-Pin4 is the interrupt input to the CPU. When IRQ is pulled low - the CPU is interrupted and it then checks the three RIOTs to see which one is pulling it low (requesting service) and performs an appropriate function at that time (e.g. update displays). If the IRQ gets inexplicably stuck low, this is often caused by a defective RIOT. In your case, it does not seem to be stuck low.