H7874 power supply

Robert Armstrong bob at jfcl.com
Sun Mar 29 09:43:50 CDT 2020

> 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 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.  


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