Broken H724 power supply from PDP-8/e

Michael Thompson michael.99.thompson at
Sat Jan 10 08:15:06 CST 2015

On Thu, Jan 8, 2015 at 8:05 AM, Michael Thompson <
michael.99.thompson at> wrote:

> 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)
You were correct Al, it is the crowbar firing. About 18ms after power on
the  +5V output gets to 6.4V and the crowbar fires. There is nothing
obviously fried on the A2 regulator board. I have the schematics for the
power supply and the IPB and BOM for the A2 regulator board. The IPB
callouts don't include the individual reference designators so I need to
reverse engineer it. It is just a simple single sided board so that should
be easy.

There is a 'scope image of the +5V output here:

Michael Thompson

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