I have an odd problem.

The story:
I removed a Rottendog MPU from my working White Water to install an NVRAM. Upon re-installation and power up I get the D19 blanking and D21 5V LED's both solidly on. No flashing D20 - not even once. It will boot if I pull BOTH ribbon cables off the digital display board (not just one). So maybe the digital display board has a new issue? nope. I swapped that with one from my Black Rose and nothing changes. Maybe something on the MPU then? nope. I swapped the MPU from my WPC Dr Dude and nothing changes (and yes, I transferred the Dr Dude and WH20 ROM chips). I even removed the NVRAM and put it back the way it was. Still no-go.

So it's not the MPU itself, and not the display board itself. But when I disconnect the two from each other the D20 blanking LED now blinks away and D21 is solid for 5v. No light on D19. It all looks good. I DO get the single "bong" tone on startup.

Any ideas?

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SOLVED!

The last owner flipped the display board ribbon cable (J201) on both ends. It all worked, up until I installed the MPU and placed that end of the cable correctly (red on pin 1, pictured). The other end was still reversed and I didn't catch it until bobukcat's comment had me look more closely to the cables on each end.

The game wouldn't boot until BOTH ribbons J201 and J202 were installed, and installed correctly. In other words, it wouldn't boot with J201 removed.

Thanks everyone!