Presenting Problem: A DEC Rainbow 100B presented all seven diagnostic lights at boot.

Diagnostics:

1. 8088 and Z80 - verified good
2. ROMs - dumped and verified good
3. Capacitors - recapped

It finally occurred to me to check the clock signal on the 8088. Nothing. The clock signal on the Z80? Nothing. The clock signal on the video timing chip? Nothing.

The DEC Rainbow 100B uses a 24.07342 MHz oscillator for the entire system. Its output pin was dead as a doornail.

Solution:

1. I used an Arduino and the Adafruit SI5351A clock generator chip to generate the right frequency (25 * (28 + (11101/12500))/30).
2. To simplify this, I soldered the SMA connector to the board and hooked up my oscilloscope probe with a BNC coupler.
3. I hooked onto the clock's pin on the bottom side of the main board.
4. Booted up the Rainbow and... viola! No Massive Hardware Failure!

The Rainbow is now fully restored and operational, and I sourced replacement oscillators.