H7874 power supply

Rob Jarratt robert.jarratt at ntlworld.com
Sun Mar 29 01:39:27 CDT 2020

> -----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
> 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
> 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,
> 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.



> Thanks,
> Bob Armstrong

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