capacitor aging claim

allison wrote: We weren't too worried about transistors in the logic, as they weren't likely to cause a great deal of damage if they failed. And we weren't worried about transistors in the power supplies, because there weren't any. Most of the PDP-1 power supplies just use ferroresonant transformers with diodes for rectification and big electrolytic capacitors for filtering. We checked the capacitors out of circuit, and we checked the power supplies (with dummy loads) before we ever powered up the PDP-1 with them. We did find one diode in one power supply that had failed shorted. The deflection and HV power supplies for the Type 30G display did contain active circuitry including transistors, and as with the logic power supplies, we tested them before powering up the 30G. We did find some serious issues with these power supplies. The power supply for the photomultiplier tube used by the fiber-optic light pen had also gone bad. The fault was in the transformer, inside a potted assembly. We have temporarily replaced that power supply with a new commercial power supply module, but we might rebuild the original in the future. The most serious fault we found in the machine was that the power control for the Type 30G display was wired *completely* wrong, such that turning it on put line voltage across a closed vane switch intended to sense when the fans were up to speed. This resulted in very high current through the vane switch, well beyond its rating, causing it to weld itself closed, and trip the circuit breaker. It appeared to us that someone had tried to kludge the power control to fix some problem, and failed utterly. We believe that the demos of the PDP-1 system at the Computer Museum in Boston must have been done without the use of the power control, with the power supplies just plugged directly into an outlet strip or the like, since the power control could not possibly have worked. We rewired the power control to match the engineering drawing and replaced the vane switch, and then the power control worked correctly. We know that the deflection power supply for the Type 30 display was missing entirely in Boston; in photos of the demo there it is clearly missing and a bench power supply can be seen. The proper deflection supply for the Type 30 was a product of a third party, rather than being made by DEC. Fortunately CHM had another Type 30 with that power supply, though it was non-functional until we reverse-engineered and repaired it. Throughout the restoration process, and to this day, the team has taken great care to avoid situations where substantial damage could occur to the artifact. Eric