This is likely an easy fix. If it isn't a misaligned 10 point switch it will be a diode failure on one of your diode boards. See the 3 little boards on the bottom of your last photo that have nothing but diodes on them? Turn off the game and set your multimeter to diode test and test each diode one at a time. Put the mm leads on each side of each diode and the swap the leads. They should all show the same results. If they don't, you have a bad one that needs replaced.

Are you saying your game has an old school chime box? Is that by choice or because the original sound card failed? My guess is it failed and this was the only fix available at the time. Times have changed. That sound card is being made again. I recommend you snag one. Here is the link:

https://www.flippp.fr/pifx.php

I own this game so I'm very familiar with it. Please upload a photo of your boards and a shot of this chime box. We'll get this sorted out.

Can you post a photo of your game boards in the back box? Also, if you disconnect the chime box does the scoring still run? I would also bypass all of the 10 point switches by inserting thin cardboard in the gap (like a piece of business card) and don’t forget the ones on your pop bumpers. Tape the cardboard in if you have to. There are 2 switches on each pop bumper. The top one is the “pop” while the lower one does the scoring. Report back with your findings.

The different reading tells me you didn’t replace the diode with the same type. Gottlieb’s don’t like that usually. Try to source the correct part. It might help. Try running the game with that diode removed. Does the score quit running?

I would also recommend replacing cable A1-J5 to A3-J1 with a shinny new one and re-pin connector A1-J6. These edge connectors are bad and by bad I mean really bad. They ware out super fast.

Were you able to find the scoring switch on each of the 3 pop bumpers? Can’t remember if they are 10 or 100 points but you said they weren’t scoring so they might be suspect. That is a fancy mpu. Haven’t purchased one yet but I would think it has decent testing capabilities. Does it offer up any potential problems in test mode?

Those should work. A lot of these Gottlieb parts are getting hard to source. I myself just bought a old system 1 wire harness off eBay just to get the obsolete parts off of it.

How are you testing the diodes? Are you doing like in this video?

Use this method to test all of the diodes on those 3 little diode boards. My guess is you have a second bad one or a switch is shorting out somewhere.

Sure, your meter should have a continuity test. Just clip your leads to both sides of the switch. It should have no reading when the switch is at rest and tone when the switch is closed. If it tones when it is at rest you have an issue. Hook up your leads and knock on the playfield in different places. If it doesn’t tone, move on to the next switch. Check them all including the ones on the pop bumpers. I use small alligators clips to hold the leads in place.

No tone is very rare finding. I'm thinking you didn't have good contact. Clip the MMD leads to the back side of each switch where the wires are attached and try again. At that point you should be able to apply pressure on the switch and get a tone. Besides, no tone isn't your problem. You have at least one that is registering during play randomly. I was hoping you would find one toning while pressure wasn't applied.