H7874 power supply

Rob Jarratt robert.jarratt at ntlworld.com
Sun Mar 29 11:50:50 CDT 2020

I forgot to mention that I replaced a fair number of capacitors, even ones that had not leaked, because their ESR was a bit suspect. I have some notes, but it is hard to reconstruct exactly what I did.



> -----Original Message-----
> From: cctech <cctech-bounces at classiccmp.org> On Behalf Of Maciej W.
> Rozycki via cctech
> Sent: 29 March 2020 17:18
> To: bob at jfcl.com; General Discussion: On-Topic Posts
> <cctech at classiccmp.org>
> Cc: rob at jarratt.me.uk
> Subject: RE: H7874 power supply
> 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.
>   Maciej

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