Testing H7864 (MicroVAX II) PSU With No Load

Robert Jarratt robert.jarratt at ntlworld.com
Sun Nov 1 13:52:38 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of tony
> Sent: 01 November 2015 18:47
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: RE: Testing H7864 (MicroVAX II) PSU With No Load
> >
> > If it has a transistor at the primary side, and a high frequency
> it's a switching supply.
> >  Those want a load, usually.  Especially the older ones.
> If there are large-ish electrolytics (100's of uF) with a voltage rating
> 200V and no reason to believe that there is a 200V output rail then it is
> likely to be a switcher with those being the rectified mains smoothing
> capacitors.
> Some SMPSUs (Switchers -- it stands for Switch Mode Power Supply Unit)
> a load, other's are happy without one. Occasionally DEC put the
specification of
> the supply in the printset -- they did for the 11/730, and IIRC that
supply is
> happy with no load on all outputs apart from something odd like the +12V
> I've yet to find a DEC PSU that fails destructively if not loaded, it is
more likely
> to just trip the overvoltaeg protection crowbar, shut down, then try to
> and repeat the cycle, tweeting as it goes
> > If it has a laminated iron transformer, big electrolytics for
> > filtering, and transistors at the secondary side, it's probably a
> > linear regulator supply.  Those don't need a load.  You may want to do
> > full testing under load to observe correct regulation, but for initial
testing you
> can test them without.
> Be careful!. There is a third type of supply, and it's one that DEC used a
> going back at least to 1970 (that is, it turns up in most older PDP11s,
etc). This
> has a big laminated core mains transformer, rectifiers and smoothing and
> _switching_ regualtors (not linear) to produce the right output voltages.
As with
> all switchers, that type might need a load (but my experience is that the
> ones don't).
> > Linear supplies are older; I think by the time of VAX you're likely to
> > see switchers (with the  possible exception of first generation stuff
> > like the 780, I don't remember if those big supplies are linear or
> Certainly the 11/730 is a mains switcher (it rectifies the mains to get
> 350V DC, then chops it). In fact it's 4 SMPSUs in that little box.
> The 11/780 draws a lot of power. I don't think they would have used linear
> regulators (which by definition are inefficient) in that machine.
> I would bet quite a lot that all MicroVAX PSUs are mains switchers
> Also watch out that DEC somethimes had a little linear PSU to power the
> SMPSU control circuitry, at least before the SMPSU had rattled into life.
> control circuitry (oscillator, regualtor, etc) is on the _output side_ of
> isolation barrier in such supplies, the drive signal is
transformer-coupled to the
> base of the chopper transistor. Those turn up in all sorts of things, the
> being the most common example.
> I hate to say this, but DEC seemed to like complicated and unusual power
> supply designs, and I would not want to try to repair one without a good
> schematic.

You mentioned DEC PSUs, this one is actually made by ASTEC. I will use a
dummy load and put it on an isolating transformer when using the scope.



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