8 Jan
2015
8 Jan
'15
5:05 a.m.
From: Al Kossow <aek at bitsavers.org>
To: cctech at classiccmp.org
Subject: Re: Broken H724 power supply from PDP-8/e
On 1/7/15 6:24 PM, Michael Thompson wrote:
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
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)
8 Jan
8 Jan
7:38 a.m.
On 08/01/2015 13:05, Michael Thompson wrote:
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.
I had a similar problem when I converted a 110V version to 230V and
tried to adjust the 5V regulator according to the PDP-8/E Maintenance
Manual. Beware, for the later (ie, most common) version of the PSU the
manual is wrong where it describes the location of the pots to adjust
the voltages; it describes an early version of the regulators. As a
result I adjusted the crowbar instead of the +5V and it took me a while
to work out how to set the crowbar correctly as the manual only says
it's a factory adjustment.
--
Pete
Pete Turnbull
10 Jan
10 Jan
6:15 a.m.
On Thu, Jan 8, 2015 at 8:05 AM, Michael Thompson <
michael.99.thompson at gmail.com> wrote:
From: Al Kossow <aek at bitsavers.org>
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:
http://www.ricomputermuseum.org/Home/equipment/dec-pdp-8e/pdp-8e-restoratio…
--
Michael Thompson
To: cctech at classiccmp.org
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) 
3585
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michael.99.thompson＠gmail.com -
pete＠dunnington.plus.com