Dr. Joshua Reichard Builds a "Defibrillator" to Revive a Dead DEC Rainbow 100B Dual-CPU Desktop

Vintage computing enthusiast and computer scientist Joshua Reichard, PhD, has brought a vintage DEC Rainbow 100B computer back to life — by building a "defibrillator" out of an Arduino compatible microcontroller board and a clock generator to diagnose and temporarily fix the problem.

"Presenting [a] problem: a DEC Rainbow 100B presented all seven diagnostic lights at boot," Reichard explains of the issue he sought to solve with the vintage machine, a desktop computer launched by the Digital Equipment Corporation (DEC) in 1983 and boasting an unusual heterogeneous dual-CPU design with one Zilog Z80 and one Intel 8088. "Diagnostics: 8088 and Z80 — verified good. ROMs — dumped and verified good. Capacitors — recapped."

With the usual suspects discounted, Reichard turned to the system's clock, which should have been ticking away courtesy of a 24.07342MHz crystal oscillator. Tested at the Intel 8088, no clock signal was found. Tested again at the Z80, still no signal. Even the video chip wasn't getting a clock signal, suggesting that the oscillator had failed — but was that the only problem?

"I used an Arduino and the Adafruit SI5351A clock generator chip to generate the right frequency (25 * (28 + (11101/12500))/30)," Reichard explains of his testing to see if the lack of oscillation was indeed the problem. "To simplify this, I soldered the SMA connector to the board and hooked up my oscilloscope probe with a BNC coupler. I hooked onto the clock's pin on the bottom side of the main board. Booted up the Rainbow and… viola! No Massive Hardware Failure!"

Having confirmed that a correctly-specified oscillator solves the problem, Reichard acquired replacement parts and brought the machine back to life without the need for the external "defibrillator" circuit — saving a classic machine from the scrap heap.

Reichard's full write-up is available on his Hackaday.io page.