Ken Shirriff's Careful Restoration of a 1969 Analog Computer Leads to a Surprise PSU Discovery

Engineer and vintage computing enthusiast Ken Shirriff has written of a power supply repair with a difference: it required reverse-engineering and understanding the operation of the 1969 analog computer to which it was attached.

"We recently started restoring a vintage analog computer. Unlike a digital computer that represents numbers with discrete binary values, an analog computer performs computations using physical, continuously changeable values such as voltages," Shirriff explains of the project. "Since the accuracy of the results depends on the accuracy of these voltages, a precision power supply is critical in an analog computer.

"The computer uses two reference voltages: +10 V and -10 V, which the power supply must generate with high accuracy. (Older, tube-based analog computers typically used +/- 100 V references)," Shirriff continues. "The power supply also provides regulated +/- 15 V to power the op amps, power for the various relays in the computer, and power for the lamps."

With such an old device - its components are date-stamped 1968 and 1969 — there's a risk on power-up that something has gone wrong in the power supply that could permanently damage the machine. As a result, the process of restoration starts with removing the power supply and investigating it for potential issues — which, given a complete lack of documentation or schematics, is akin to reverse engineering it.

Removing the voltage regulator cards from the system, everything seemed to be going smoothly - apart from one thing. "We tweaked the potentiometer on the +15 V regulator to get exactly 15 V output. The -15 V regulator seemed temperamental and the voltage jumped around when we adjusted it," Shirriff notes. "I suspected a dirty potentiometer, but it settled down to a stable output (narrator: this is foreshadowing).

"We put all the power supply boards back in the cabinet, put the power supply back in the computer, and powered up the chassis (but not the analog computer modules)," Shirriff writes. "Some of the indicator lights on the control panel lit up and the +15 V supply showed up on the meter. However, the -15 V supply wasn't giving any voltage, and the op amp overload lights were illuminated on the front panel, and the reference voltages from the op amps weren't there.

"I studied the printed circuit board and noticed that there wasn't a PCB trace between the transistor and the connector! Instead, part of the current path was through the heat sink. The heat sink was screwed down to the PCB, making a connection between the two red arrows above. After I tightened all the screws, the board worked fine."

Shirriff's full write-up, and a link to an earlier investigation into the operation of the computer itself, can be found on his blog; updates will also be available via his Twitter account.