LF: Microvax 3100 power supply technical information.

[Dave Dunfield]

> > It's complex enough that I'd feel better going at it with a set
> > of schematics. If nothing turns up I'll draw it out myself, but
> > if someone has technical data it could save considerable time.
> 
> Sure. I am not sure if DEC ever published schematics for this supply, 
> though. And I have, alas, come to realise that for most classic devices 
> if a quite question here (or on other similar lists) doesn't turn up the 
> diagrams, it's often quicker and less stressful to trace them out 
> yourself. 

Yeah, and since nothing else has turned up, thats probably what I'm
going to end up doing.


> > > Wouldn't it be best to find out why it's doing this? My _guess_ is that 
> > > either one output is rising too high, tripping the crowbar, and thus 
> > > shutting the supply down, or, less likely, the supply to the control 
> > > circutiry has failed (the reason I mention the latter is that that fualt, 
> > > caused by a dried-up capacitor on the _primary_ side of the supply, once 
> > > led me a merry dance in a Tektronix X-terminal). I'd be inclined to start 
> > > by checking _all_ the electrolytics on that second PSU board with an ESR 
> > > meter. An high ESR capactior on the output side can put some 
> > > 'interesting' high-ish voltage spikes on the output.
> > 
> > Agreed completely - I guess I wasn't clear. I haven't done any debugging
> 
> Your first message could have been read to suggest you were thinking of 
> remvoing the second PCB altogether and somehow fooling the power-fail 
> circuit.

The idea crossed my mind as a way to bring up the system until I get the
secondary supply fixed, however on closer inspection I discovered some
things that make for an even better workaround:

- This is a VAXserver 3100/20 apparently - it has two drive levels (one on
  top of the other), and can hold something like 5 or 6 drives. To provide
  powers for that many drives, there is a main power supply that powers the
  mainboard, and one of the drive power outlets, and secondary power supply
  which powers the second drive power outlet.

- Turns out the main supply board is IDENTICAL to the one in the VAXstation
  3100, which has only the one board - this supply is marked 180W. The
  VAXstation only has mountings for two drives. so it would appear that the
  277W VAXserver 3100/20 supply is actually a 180W supply for the mainboard
  and a couple of drives, and a 97w supply for additional drives.

- Tracing the wiring of the VAXstation supply, I confirmed that all the rails
  go to exactly the same places, and also I located the power-good output
  which in my setup daisy chains to the second board.

- So I took the power-good from the primary supply and fed it into the
  corresponding pin on the connector - and as expected, I now have what
  appears to be a perfectly good 180w supply.

- As long as I don't try to use more than 1-2 drives (I don't plan to
  anyway) this should be OK for the purposes of playing with the system.
  It also lets me take out the secondary board and work on it separately
  on the bench, and reduces the desire to "get it working now", allowing
  me more time to draw it out etc. -  the downside is that I was incorrect
  in my first assessment that AC went to both boards, although the primary
  power input looks the same, with the same brown and blue wires, the
  secondary supply receives primary DC (rectified/filtered mains) from
  the primary supply - If you look on the VAXstation supply, you can see
  the connections where this power is taken off on my board. - So I'll
  have to put together an DC supply for it.


> > I too suspect a lack of regulation and crowbar trip, and yes in the absense
> > of technical data, I'll probably "shotgun" the electrolitics in the hope
> > that I'll get lucky.
> 
> I would strongly recomend getting an ESR meter if you do any (not many, 
> but any) repairs on SMPSUs. It will save a lot of time...

Thanks for the advice - I don't have one, but I do have access to one
when I need it....

Dave

--
dave06a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
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