Broken H724 power supply from PDP-8/e

Michael Thompson michael.99.thompson at
Wed Jan 7 20:24:41 CST 2015

The H724 power supply in my PDP-8/e works OK for about 5 seconds and then
the +5V output goes to about 3.0V. If I leave it on for a few more seconds
it will blow the 20A fuse for the +5V output.

I have the power supply partially disassembled and disconnected from the
chassis. I don't see anything obviously fried. There are three ring
terminals with two wires each connected to the + side of C205. Two of the
wires get hot in just a few seconds. I have not disassembled the wire
harness, but I suspect that they are the wires that go to the fuse holder.
C205 is the output filter for +5V. The capacitor resistance goes above 3k
Ohms if I leave the VOM connected for about 20 seconds, so the cap is not
the 20A short.

Should I just keep disassembling the power supply and tracing the warm

Anyone have experience repairing one of these power supplies?

Michael Thompson

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