Hi and sorry for the delay.. I havent been on much lately..

I think you need to focus on the MPU board for now as the 01a-leds test should run forever without problems.. both LEDs should stay alternating. Also, the blanking test should be consistent as well. The resistor and LED setup you have is sufficient and it should always have the same result upon power up. The flickering should not happen there. You can run both of those tests without the driver board and still be fine.

I think the problem is that the CPU chip appears to be crashing and restarting. This could be because of insufficient power getting to the chip, or bad connections etc. Since you have gone over the board many times for cracked joints etc you should check for clean and sufficient power going into the board.

Once you know you have good power, you will need to verify the reset circuit on the MPU board. When the CPU crashes (stops processing instructions) it doesnt automatically reboot itself. The reset circuit is supposed to hold the reset pin low for about the first 0.5 seconds after power up. This is to give the rest of the circuitry time to get clean power and be ready. After this small delay the reset circuit completes its job and allows the pull up resistor (tied to the reset line) to pull up the reset signal to a logic 1. This causes the CPU to boot and start executing code.

You can remove your resistor-LED test circuit from the blanking signal and put it on the reset signal. I believe there is a test point on the MPU board you can connect to. There may even be one on the 40 pin connector. I don't have the schematics in front of me at the moment. You should see the LED come on about 0.5 seconds after you power up and STAY on.

Unfortunately, you probably wont be able to tell if the circuit is "flickering" in this way because it could be happening faster than you can see with an LED. Once you get your logic probe you can set it to "mem" sometime after the half second and the LED on it should stay out.

As for driver board lock ups, its not always the PIA.. sometimes its the logic chips associated with it, but let's focus on getting the MPU board reliable first..

Are you putting "unregulated +5v" on your 1J2-9 pin? It is required for the reset circuit to function properly. My system 6 PSU is showing 10.4 Vdc when disconnected and 10.2 Vdc when connected. Yours should be about the same.

I also agree with Robotworkshop that a USB adapter will not have enough current to supply the entire board so I'm glad you are using the PC power supply. I have also used 400W ATX power supply on the bench just fine. You can connect the 12V of that to 1J2-9.

Also, I recommend connecting the 5Vdc to all three pins on 1J2 (4,5,6) and ground to all pins 1,2,3. This will allow the proper current to flow without stressing wires or pins.

When you also connect the driver board via the 40 pin you also need to supply the same 5V and ground power to the 2J8 pins: +5V on pins 6,8,9 and GND on pins 1,2,3. You may be able to get away with skipping this step as 5V also comes through the 40 pin interconnect, but better to be safe than uncertain.

If power looks good then verify that Q1, Q2, Q3, Q4, Q6, IC5, and all components in the reset circuit are functioning.

Do you have your logic probe yet? You need to verify the signal on pin40 of IC1

https://www.electronics-notes.com/articles/test-methods/logic-probes/how-to-use-logic-probe-hints-guidelines.php

I have never noticed any real difference between TTL and CMOS mode when using them on equipment from this era of pinball machines, but TTL is normally what I set mine to.

Use it to verify the reset signal. It should start high upon power up, and then go low about half a second later, and STAY low for as long as you run the test.

You can use the MEM function of the probe to determine if it stays low. Start with MEM off, turn the power on, and AFTER it goes low switch the probe to MEM without lifting the probe from the test point. Wait for up to about 10 seconds. If the MEM light comes on, then the probe has detected a blip in the signal and this indicates you have a bad reset circuit.

The documentation for 01b-bus explains the boot process in detail and will give you a better understanding of what should be going on with your machine.