Broken H724 power supply from PDP-8/e

Michael Thompson michael.99.thompson at
Thu Jan 8 07:05:48 CST 2015

> From: Al  Kossow <aek at>
> To: cctech at
> Subject: Re: Broken H724 power supply from PDP-8/e
> On 1/7/15 6:24 PM, Michael Thompson wrote:
> > 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.
> >
> sounds like something is firing the crowbar (Q207)

Thanks Al and Marco.
I saw Q207 when I inspected the power supply and wondered what that giant
diode was for. I also saw it in the schematic and was surprised to see that
the diode installed backwards. Now I see that it is an SCR, not a diode.
R29 is easy to get to, so I will try raising the crowbar voltage a just
little. It is also possible that the regulation of the +5V is not working
correctly and the crowbar should be firing.

Michael Thompson

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