Progress with the IMSAI 8080 that I have with the Thinkertoy Wunderbus backplane.
Thanks to Mike and Dan for their help.
1. While probing the power supply board Dan sparked a diode, and somehow that woke it up, now functioning. There may have been something touching it.
2. Replaced a 12V Regulator on the backplane board
3. Re-assembled front panel.
4. Put in a new connector for the on-off switch wire from the front panel to the power supply board.
5. Replaced the 8080 processor chip on the CPU card.
Now the system works, but after a few minutes the front panel starts to flicker and is no longer reliably responsive.
1. Test chips on the front panel board, see if there is a short someplace, etc.