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-boun...@classiccmp.org> On Behalf Of Maciej W. > Rozycki via cctech > Sent: 29 March 2020 17:18 > To: b...@jfcl.com; General Discussion: On-Topic Posts > <cct...@classiccmp.org> > Cc: r...@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
