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