Do you have a regular computer desktop LCD monitor? Preferably one close to 22" in screen size?
Plug that in to one of the VGA plugs or whatever plugs these Alphas have and see if the boot up screen pops up on the desktop monitor when you boot up the game.
At least this way you can somewhat verify whether or not the MPU, backplane, software, power supplies are any good.
Or
Plug some computer into your machine's LCD to see if the monitors pick up your computer signal....any old desktop computer should work.
There's nothing really special about these 22" machine LCDs...they're just LCD monitors really.
Maybe better fan cooling and semi-indestructable cover lens so old ladies don't scratch everything up with their diamond rings....lol
There should be some leds lights on top of the monitors next to the buttons?
Should be green if working correctly right?....any other color would be a fault of some kind....esp a red LED?
Oops, I just re-read that you tried another MPU in this problem cabinet and it worked fine.
Disregard everything I said above and keep the $50 bucks...you'll need it to help pay for a new MPU down the road.
Too bad, your MPU is screwed up....plain and simple....swap parts on the problem MPU in a known good cabinet....hard drive harnesses, hard drives, boot and juris chips, video cards check fans anything that is changeable until you've singled the root cause of why it's not working right.
The only thing we know is the backplane is okay because other known-good MPU worked in this cabinet.
Have you tried this problem MPU in a known-good cabinet?
Swapping stuff from this MPU in a known good cabinet will pinpoint which part is shot the minute you get a NO SYNC error on the good cabinet.