Pete wrote:
> If anyone needs it, I can now give some
> helpful advice on how to build a small DC-DC power supply that takes 9
> volts in, and outputs 900 volts at a few microamps... :-)

I might need that info for the restoration of a DEC machine, if you can
believe it!

The Type 30G Precision CRT Display on the PDP-1 at the Computer History
Museum has the Type 33 Symbol Generator option and the Type 370 High
Speed Light Pen option. The light pen uses a fiber-optic bundle from
the pen (which contains no electronics) to a 931-A photomultiplier tube.

It's currently not working. The photomultiplier tube is inside a
Norman Jones Inc. module, part number 5000-1. It's a sealed module,
though it has a removable cover that provides access to the photomultiplier
tube for replacement. We're not yet sure whether the rest of the
module is potted, but it definitely isn't designed to be opened.

The module has a 1.5A fuse which it blows. There are two external
adjustments of unknown purpose, and an exposed 2N2139 PNP germanium
power transistor in a TO-3 package (rated 60V CB, 60V CE, 30V EB,
70W, typical beta 45 at IC of 0.5A).

The transistor is bolted down with hex socket-head screws, and they
don't seem to fit hex keys in either standard imperial or metric
sizes. But we'll try to remove it and test it.

I expect that the transistor drives a flyback transformer or the
like to produce the high voltage. Perhaps the transistor is bad,
or perhaps the oscillator, or perhaps the transformer.

Anyhow, if we can't repair the module, we might want to build a
replacement for it. The 931-A photomultiplier is rated to run
at 1000V, but I'm not sure how much current it needs.

The other option is to try to replace it with a modern phototransistor
or photodarlington. I'm dubious that they will perform as well
as the photomultiplier, though.

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