Repairs of an H786 power supply for a BA11-N

Chris Zach cz at
Sun Jun 26 19:46:19 CDT 2022

Been working on fixing an old 11/03 system in a BA11-N chassis with an 
18 bit 9273 backplane. The power supply was working at first with no 
issues, but when I left it on for a bit and came back it had blown the 
power supply fuse. Replacing fuse causes another blow, so something was 
most definitely wrong.

First place to check was the primary side of the system. Fairly simple: 
  Rectify AC to DC, then pass through a pair of BUY69A NPN transistors, 
then into a second transformer which goes to the various regulators. The 
main rectifier was not shorted and the transistors looked fine as well. 
Took the caps off to check them, no issues there either...

So I found a copy of the H786 power supply schematics (used in the 
BA11-N series of pdp11 Q bus boxes). It's on page 290 in a big PDF scan 
of a bunch of MINC-11 documents that also include the RXV21, 11/03 CPU, 
and the like. File is MP00652MNC11SchematicsJul78.2761676314.pdf if you 
need to find it.

Anyway, it also showed me another place where there could be a line AC 
short: There is of course a big rectifier that takes the 120v and turns 
it into big DC for the main switching transistors, but there is *also* a 
second little tiny rectifier buried in there that sources power for a 
"yes, 12v is there, start turning on the second stage regulator 
supplies". Sure enough it was shorted, ordered a replacement, put it in, 
and the power supply is working properly again.

The other issue was that the power fail lines were being asserted. 
That's a sidecar board, removing it allowed everything to work properly. 
Which means a problem with one of the LM339 OP amp chips. So I pulled 
and replaced both and now that works as well.

It's a fairly common unit, and now that these things are coming up on 40 
years old the silicon in them is starting to go bad. That's not good, 
but they can be fixed...

Fixing these things is never dull. Now on to fixing the MSV11-D memory 
board (2 bad 16k RAM chips) then the MiniMinc (which is basically a 
PDT11/150 with a different lid.) Not sure what that was used for, any 
ideas or thoughts?



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