Repairs of an H786 power supply for a BA11-N

Douglas Taylor dj.taylor4 at
Mon Jun 27 09:52:12 CDT 2022

On 6/26/2022 8:46 PM, Chris Zach via cctalk wrote:
> 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?
> C
> CZ

I have an identical BA-11N setup and it still works (fingers crossed).  
Your post is extremely helpful should something go wrong with the power 
supply.  The date code on the box is 1970-something.


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