PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sun Aug 9 10:40:47 CDT 2015

When we first powered up the PDP-12 the main fuse for the VR12 display
blew. A replacement fuse did the same. We thought that the brown goo in the
bottom of the chassis had leaked from the high-voltage power supply, and
the high-voltage power supply is directly connected to the input, so that
was the first suspect.

We bench tested the high-voltage power supply using a Variac on the input.
With a 10VAC input there was no output at all. Increasing the input voltage
did not change the missing output voltage.

I hate to mention this but...

The two capacitors in the voltage-doubler circuit are connected in series
between the output lead and ground. We connected a current limited lab
power supply to the output lead and ground and slowly increased the voltage
while watching the current draw. With the voltage stable the current draw
was a few microamps. We increased the output voltage of the power supply to
the 64VDC max, disconnected the power supply, and measured the voltage
across the caps. It very slowly decreased, so maybe the caps were OK.

We reconnected the Variac to the input and with 10VAC the high-voltage
power supply had a 1000VDC output. We put 10x 500kOhm resistors in series
across the output and increased the Variac voltage. By measuring the
voltage across one resistor we could see that the output was more than
10,000VDC. The resistors started smoking so we knew that we had a lot of
high-voltage available.

So, once again the magic of reforming capacitors saves another piece of

Michael Thompson

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