So, with only the J2 plug plugged in, I get a humming sound. Because the hum is pervasive, it is difficult to determine whether it's coming from the transformer or the power conditioner (or whatever it's called; the schematic says it is "FL1" or "FLI", with a VR1 hanging on it) in the back of the cabinet.

I've read about the "hammer solution", and I tried it , but it's still humming. I don't even know what the part specifications are for that power conditioner.

So I went and swapped the transformer (& power board) between Playboy & Strikes & Spares, and now the humming is a lot less, about the same as what was for Playboy. Perhaps I should try and isolate the ground with some rubber?

The hum is not coming from the sound board. Strikes & Spares doesn't even have a sound board. And the sound board in Playboy is a brand new NVRAM board.

Can't do the rubber mounting, at least on the base, since the holes for the screws to the backbox are as they are (although I guess I could make new holes). Somehow, by swapping out the transformers for Playboy & Strikes & Spares, and performing some "hammer therapy" on them, the hum for both is now low!

Perhaps at the factory, a transformer is somehow tuned to a particular machine (or at least the power conditioner), and what I had done was swap them (since they should all interchangeable)?