Power Mac 6500 boot failure mystery

My Power Mac 6500/300 was working fine for months, with the exception of a dead CMOS battery that was causing the system clock to reset at each boot. But everything was in perfect working order.

I left the machine idle and unplugged for a couple of months. When I try to boot it now, the fans come on and the drives begin to spin up, but then it shuts itself back down. I cannot get it to stay on more than 1 sec or so. There is no chime.

I have tried the following:
* replaced CMOS battery with a fresh one
* let the machine sit idle, but connected to power, overnight
* pressed the reset button on the logic board
* removed all cards from PCI and COMM slots
* checked the power cables themselves, also tried different outlets
* removed RAM chips, including cache option
* disconnected power to FDD and HDD (I don't want to deal with the ZIP and CD drives, because this machine is a bear to disassemble)
* checked for any obvious blown capacitors
* read entire service manual for other potential solutions
* read all the Reddit posts in this and other subs that I could find

Ultimately, I disassembled the entire machine so that nothing is connected to the logic board aside from the power supply and the case controls. Again, this machine was working fine for months, and I made no changes to hardware before it stopped working. I'm guessing the logic board just gave up the ghost for some reason. But does anyone have any suggestions to try?