H7874 power supply

Rob Jarratt robert.jarratt at ntlworld.com
Sun Mar 29 10:43:22 CDT 2020

OK, I posted it here:

> -----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
> > 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,
skinny one
> that's obviously the AC input and rectifier board, which probably produces
> something like 300VDC for the regulator boards.  There are two similar but
> 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,
> logic and discrete parts.  There are filter caps, a transformer, and
> transistors on this board too so it obviously produces yet another power
> 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

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