Any corrosion on the board or battery holder where the batteries were?

What do you get at TP7?

4.3 is good.
Any chance one of your door or coin switches is closed?

Maybe a bad PIA. Do you have a good known PIA you can swap out?

It's IC11 on the solenoid driver board.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiGqOrUuM37AhU1KX0KHQomAlAQFnoECAkQAQ&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FPeripheral_Interface_Adapter&usg=AOvVaw1ToeQ6RvwsOKy7wNuA8XVQ

You have 3 PIA's on the board. If they are socketed, try swapping one of the other ones with IC11 and see it that makes any changes. May identify if IC11 is bad.

You’re not replacing the soldered in PIA on the MOU board. Replace IC11 on the driver board. It’s already socketed. If you want, just swap it with one of the others on that board and see if you can get into diagnostic test to verify that’s the problem

You can replace 6820 with 6821. There are other substitutes as well.

Maybe a bad socket or two?
Are the chips AMI brand by any chance? If so, they have a history of going bad and would be well to replace.

Can you post high resolution pictures of both sides of the board?

From what you describe, something during the repair went south. Take a close look at what might have bent and shorted, for instance, what I circled yellow on the attached pic. Also, I would double check all the pin/socket connections that were unsoldered/soldered for open or bridged traces. I can't see clearly in the pics.

driver bd (resized).png

Something got worse after you changed out 2 chips. Can't see clearly in the photo. Would you be able to take (and upload) high res pics of the front and back of the board, preferably in indirect sunlight?

Have you changed out the 40 pin connector? Hard to see in the pic provided, but maybe a couple pins are bad.