Good luck bump

I've brought 3 of these back from the dead, all 3 of them had issues with their blanking circuitry, one had bad traces near the CPU as well and also needed the CPU replaced. If you suspect that there was ever alkaline damage, the 555 timer and associated circuitry on the blanking section needs to be replaced (a few small value caps, resistors, diodes, and 2 transistors I believe). One of the transistors is a specialty transistor, so don't clip it at the leads in case you cannot replace it. I believe the other is just a 2n3904 or something similar. It is an easy fix.

Even if it was working previously, if battery leak damage was not neutralized, it would continue to corrode those areas until they eventually stop working.

If it is clean, it could be many different things. Have the capacitors ever been replaced on the PSU or MPU? Are any ICs socketed or was any previous board work done? Any rust visible on socketed ICs like the CPU and EPROMs?

Also, read this through: http://techniek.flipperwinkel.nl/desega/index1.htm

The blanking circuit is on the far left, upper mid region. Wherever the 555 8-pin IC is located. Look for greened over solder pads and/or components.

Before replacing parts though, grab a DMM and check the voltage on pin 40 of the CPU with the game on. If it is high (5v), then the blanking circuit is working, check the CPU and or traces near/around it.

If you end up having to remove the MPU to do work, it is difficult to remove alkaline-laden solder. Best thing to do is neutralize the area first with 50/50 water and white vinegar. Then re-flow new solder on top of the old crap with a very hot iron, like 750+ degrees hot. The trick is to add just enough new solder over the greenish alkaline solder to get it to flow a bit, then you can suck all of it up together so you're left with a nice clean pad. You won't be able to remove it with just a desoldering station alone, it needs the additional new solder flowed over it to get it to melt enough to be removed at once. Otherwise, it just breaks up into clumps and makes a mess.

Also be careful how long you hold a hot iron like that on a pad. It'll seem like nothing is happening for a few seconds, then you'll physically see the alkaline "juice" and smell it as well as it sweats off of the original solder. That's when you add the new stuff to it. It doesn't always take the new solder well, and sometimes you end up having to just clip the components and try and pull them out as best as you can and then sand down the pad to expose the copper. That's in really bad cases though.

Good luck.