DEC 874-D power controller

Dave McGuire wrote: Nor have I. However, I *have* seen a different DEC power control that was modified. Specifically, it was a DEC Type 811 power control, and it was part of the Type 30G Precision CRT Display attached to the PDP-1 at the Computer History Museum. After we had the PDP-1 restored to operation, we started working on restoring the Type 30. We identified a number of things that were wrong with it, including the deflection power supply (an EQR power supply made by NJE Corporation) being entirely missing. After we repaired and tested all of the Type 30's power supplies, with fire extinguisher at hand, we powered it up. We hit the power switch, the fans spun up, then there was a very loud clack and the circuit breaker tripped. What we found was that the internally wiring of the 811 was completely inexplicable. Documentation on the 811 was found in the museum's archives, and the wiring was significantly different than indicated. As wired, 120VAC was applied directly across the vane switch that closes when it sense airflow across the deflection transistor assemblies. As soon as it closed, it welded shut, and tripped the breaker. We never found out why the 811 was wired that way. We're pretty sure that it wasn't supplied by the factory that way, and that someone did a field modification, but we couldn't make any sense of how the modified version could have been useful for anything. We rewired it to factory spec, replaced the vane switch, and it has worked properly ever since. Eric