Welcome to the BK club! I just love this game.

You might be shocked how nice you can make that PF look with a handful of Magic Erasers and some 91% isopropal alcohol. Pull out the colored paint pens for touch-ups (the colors on this PF are basic), some Novus 2, wax and you will have a great player!

Also, you either have a Black Knight LE playfield or someone modded a regular BK with the micro switches. I see them on the upper PF trough in your pictures. You don't have the LE apron, however.

Those solder joints look factory to me. I don't think the electrical engineer had a hand in that problem! They look pretty oxidized to me. That can happen when the game is stored in very humid conditions. Sometimes just age causes them to get oxidized on their own. That being said, you do have some joints that look worse than "usual". I've seen boards work fine looking like that. They will most likely need some restoration to be reliable.

Don't know if you saw this but GPE has replacement housings for your melted one if you want to restore the wiring. https://www.greatplainselectronics.com/proddetail.asp?prod=03-06-1361

Easy work to swap with one of these pin extraction tools: https://www.greatplainselectronics.com/proddetail.asp?prod=W-HT-2285

The toothpicks are definitely a "mod" for the archives!

Is the game booting to the 2500 3 mode reliably? That would be a really good sign.

You tried turning it off and back on as fast as you can with the coin door open and it will not progress past that state? Batteries or not should get the game to attract mode using this trick.

If this is the case, you either have a bad memory protect switch on the coin door or (more likely) you have a bad 5101 memory chip on the CPU board. Perfect time to replace the 5101 with NVRAM.

If you feel like messing with it some more you could verify the memory protect is working properly. With the game on and coin door open, you should see close to 0v at TP3. With the door closed, you should see 5V at TP3.

TP3 is just above and to the right of the 2 LEDs. It is just above TP7 so make sure you have the right test point.

If that is working properly, you can be pretty sure you have a 5101 problem.

Andrew makes some really nice NVRAM modules for pretty much all applications. https://nvram.weebly.com/

When the coin door opens, TP3 should be a solid 0V (or really near there). Can you try measuring from TP3 (mem protect) to TP10 (ground) which is just below and to the right of TP9. This would rule out a flaky ground connection as a possible cause.

If you still see a random signal, I would take a look into the interlock switch wiring. It enters the CPU board at Pin 1 on J3 (leftmost pin on the leftmost 4 pin connector on the top of the CPU board). As a test, you could use a jumper wire to ground that pin. In this condition, the fast on/off trick should work which would help you confirm if you have a 5101 memory problem.

Wow, sorry about that! The switch input does indeed come into the board on 1J4 pin 1. I was confused by the schematic that shows that Memory Protect input is also connected to on 1J3 pin 3 on the board, probably just for manufacturing flexibility.

1J3 needs to stay connected otherwise the displays will not work (as you found out). It provides the blanking signal to the master display board.

Grounding pin 1 on 1J3 or 1J4 should make TP3 go near 5V. If that's not working, the NVRAM may not fix anything.

All you need to do is remove 1J4 and short pins 1 (mem protect) and pin 2 (ground).

If your still not seeing 5V, IC7 (7404 hex inverter) may be bad. To confirm, with 1J4 pin 1 grounded, measure the voltage at pin 13 of IC7 (should be 0). Then measure the voltage at pin 16 of IC7 (should be 5). If not, IC7 is probably the culprit.

Of course, if you are done messing around with this (and who could blame you ), you can just try the NVRAM first.