I'm documenting this for anyone running into the same problem.

I had a 4101 (same as Laserjet 4100 but MFP) that had sticky solenoids in trays 1, 2, & 3 (tray 3 is optional).

After the long job of taking it all apart and fixing the solenoids, I put it all back together, satisfied that the job didn't take very long... and nothing broke in the process.

Then I tried to power on, and nothing.

I went through all the connectors, and finally determined I needed a power supply. Maybe some dust got in there and shorted something out while I was moving it around.

Got the new board in, put it all back together... nothing. No power.

I took it all back apart, plugged the bare machine in with nothing on it, not even the formatter, and I got power.

I started putting things back, one thing at a time, while powering on to check. I finally determined that the optional tray was keeping the power from coming on. I took it apart and looked at everything, but it all seemed to be the way it should. I'm too scared to plug it back in, because I don't want to blow the new power supply (replacing that thing is a pain in the arse.)

Anyway, it might also be the connector on the bottom of tray 2, but I'll never know, because I say, 'Good bye, good riddance.'

So if your 4100 isn't powering up, check the connectors leading to trays 2 & 3.