29 Mar
2020
29 Mar
'20
9:50 a.m.
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.
Regards
Rob
-----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 6800?F
Sprague part as well as I have seen its capacitance drop down to like half of its
nominal.
Maciej