29 Mar
2020
29 Mar
'20
8:43 a.m.
OK, I posted it here:
https://robs-old-computers.com/2020/03/29/h7874-power-supply/
-----Original Message-----
From: Robert Armstrong <bob at jfcl.com>
Sent: 29 March 2020 15:44
To: rob at jarratt.me.uk; 'General Discussion: On-Topic Posts'
<cctech at classiccmp.org>
Subject: RE: H7874 power supply
skinny
one
Rob Jarratt >robert.jarratt at
ntlworld.com> wrote:
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.
Sure, if you don't mind sharing. Everything helps.
Looks like there are five separate PCBs in there. There's one long, that's obviously the AC input and rectifier board,
which probably produces
something like 300VDC for the regulator boards. There are two similar but
not
identical boards with huge heat sinks that are
obviously the output
regulators. I
think this PS has +5, +3.3, +12 and -12 volt outputs,
but I've no idea
which board
does which.
And then there's a giant PCB, almost 11" square, covered with opamps,
74LS
logic and discrete parts. There are filter caps, a
transformer, and
switching
transistors on this board too so it obviously produces
yet another power
output
for something. Oh, and I think this cabinet has
variable speed fans,
right? SO
there's probably a fan speed controller on here as
well.
You'd think that will all this logic it could at least have some LEDs or
something to tell me which output has failed, but it doesn't seem like it.
Thanks
Bob