H7874 power supply

Holm Tiffe holm at freibergnet.de
Sun Mar 29 05:30:46 CDT 2020

Rob Jarratt via cctech wrote:

> > -----Original Message-----
> > From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Robert
> > Armstrong via cctech
> > Sent: 29 March 2020 02:29
> > To: cctech at classiccmp.org
> > Subject: H7874 power supply
> > 
> >   Does anybody have a maintenance print or service manual for the DEC
> > H7874?
> > This is the power supply used in the BA4xx and R400x cabinets.  As you
> might
> > guess, I have one that tries to power up but shuts down after a second.
> > Probably a bad capacitor (or several), but this thing is ridiculously
> complicated
> > and not all that easy to disassemble, either.  I'd like to be able to
> trouble shoot
> > it rather than just firing the proverbial parts cannon at it.  FWIW, none
> of the
> > electrolytic (of which there are many) have obviously failed - no leaking,
> no
> > bulges, etc.  Of course, that proves fairly little.
> > 
> This PSU is a real problem, as you say, complicated and hard to disassemble.
> I had one that didn't work, if I remember correctly in the end I had to get
> another one. I have a second machine with this PSU which has an intermittent
> problem and shuts itself down after a few minutes if it hasn't been used for
> a while, but a subsequent power up sees it work for as long as I need it.
> It looks like I reverse engineered the schematic of the 12V output board. I
> don't know if that would be of any use? How correct it is I don't know.
> Regards
> Rob
I would be interested. I have an VAX400/300 with such an working again
PSU, Changed Capacitors two years before..now it works. A friend gave me
another PSU with a similar fault, changed capacitors as before on the
other one..but no luck at all. It still powers down shortly after
switching on. Yes..thats the most ugly PSU Design I ever saw..and I'm
interested in schematics or parts of them..


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