H7874 power supply

Maciej W. Rozycki macro at linux-mips.org
Sun Mar 29 11:17:48 CDT 2020

On Sun, 29 Mar 2020, Robert Armstrong via cctech wrote:

>   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.

 There's logic included there for synchronised control of multiple H7874 
PSUs at once in a master-slave configuration via a set of auxiliary 
connectors so as to power up or down all pieces simultaneously in a 
complex machine setup with storage and/or Q-bus expansion boxes.  No 
documentation for the PSU has been chased so far as far as I know.

 Note that also with this PSU there were capacitors used that suffered 
from the issue with the quaternary ammonium salt system used in the 
electrolyte.  All mine had numerous Chemi-Con SXF parts scattered across 
all boards, some leaking.  They are prone to leaks even if unused and have 
to be replaced.  Especially those next to the 5V circuit are hard to get 
to to desolder and replace.  Also Chemi-Con SXE parts present in this PSU 
are reportedly affected although I haven't seen them leak myself.

 This PSU has been my worst nightmare as far as keeping old equipment 
alive has been concerned.  I was able to make just one working PSU out of 
three broken ones by mixing and matching individual pieces with caps 
replaced until I found one set that worked.  I consider replacing the 
large cuboid 6800µF Sprague part as well as I have seen its capacitance 
drop down to like half of its nominal.


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