The conundrum of the Blanking signal.....

No 5V at blanking test point and no 5v at pin 37 - jumping 5v to pin 37 does nothing and game still does not boot. This is because Blanking is fine and is actually doing its job by pulling voltage low.

Your fault lies elsewhere, so we need to look at the PIA's - also check that the clock is running on MPU - i have had a crystal fail on me.

I take it you get no diagnostic code on the led on the CPU board???

If ALL voltages are present at test points then we can look at signals on the CPU, make sure clock is running - check IRQ etc on MPU chip - are your sockets good??
Do you have a logic probe at all?