IIsi No Chime, White then Black Screen

I just finished cleaning and recapping both the logic board and power supply on my IIsi. The fans turn on, and the hard drive spins up. The speaker on the machine clicks as it powers on, there is a white screen for just a second, and then it turns black. Nothing happens after that. It does shut back off with the power button on the machine.

I have a PRAM battery installed. A mishap did occur when I was wrapping up the recap - I installed a small cap on the PSU backwards, and it blew instantly. I shut the machine back down and replaced the cap.

Quick update: I checked the external floppy connector, and I am missing -12v. I took the PSU back apart, and the cap that I reversed on accident is one of the last components on the -12v rail before it exits the power supply. That trace connects to two separate big coil thingies and a diode labeled 10DL2CZ (here's a link to the datasheet).

According to the Bormac schematics, pin 5 of the external floppy connector is connected to -12V, but I don't see any trace connected to it when looking through the board using bright light and I also can't measure -12V there. Even Apple lists this pin as not connected

So there simply are no -12V to measure on this port

Thanks @RFO, I'll try to find another place to take this measurement.

Only time I had this issue crop up on a IIsi was when the fan contacts shorted out the SCSI port on my original machine, which had issues with both the speaker cutting in and out and also the fan sometimes just stopping. In my 20 year old stupidity, I smacked the fan downward with the machine powered up and the case lid removed. Fried the motherboard's SCSI chip. A few days later, picked up another IIsi for $175 (this was back in '96), dropped my drive and NuBus adapter card into it, and kept going. Swapped the floppy drive from the old machine into an old A9M0106 800K floppy drive case eventually, so I could have two SuperDrives.

Re: your power supply issue, it's possible that having the capacitor installed in reverse damaged a chip on the board, if you do find out that there is -12 volts DC being provided by the PS.

I have the power supply out of the machine, and would like to test it's output directly. Is it possible and safe to bench test a power supply like this? I've shorted a couple of pins on an ATX power supply before to get it to power on - I'm not sure if these Macintosh power supplies work the same way.

Testing the PSU without it plugged into a logic board/hard drive etc might not give you reliable readings as there won't be any load on the PSU (I'm not sure the IIsi PSU will do anything on its own actually). Best to test via the logic board IMO.

I was able to verify that -12.35 V is coming from the PSU on the -12 V rail. This suggests that there's something happening on the logic board. I tried an SE/30 ROM SIMM, which caused it to hang on a white screen, instead of briefly flashing white and going black.