28 Mar
2020
28 Mar
'20
11:39 p.m.
-----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
Thanks,
Bob Armstrong