Testing H7864 (MicroVAX II) PSU With No Load

Robert Jarratt robert.jarratt at ntlworld.com
Sun Nov 1 11:37:00 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Paul
> Sent: 01 November 2015 16:52
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
> > On Nov 1, 2015, at 7:12 AM, Robert Jarratt <robert.jarratt at ntlworld.com>
> wrote:
> >
> > Some of you may recall I have a faulty H7864 PSU, which failed a while
> > ago with a loud pop, but no obvious physical damage. I replaced the
> > blown transistor (on the primary side of the large transformer), but
> > when I power it on,  the transistor does not switch and there is no
> > output, so clearly there is still a problem somewhere.
> >
> >
> >
> > I have been spending some time drawing schematics for almost the whole
> > thing. I am now at the point where I intend to compare it with a
> > working one, probing each one side by side, to see up to where it
> > appears to be working. It would be awkward to have two dummy loads,
> > just for lack of suitable equipment. Does anyone know if it is safe to
> > run these PSUs with no load? Would my testing be valid without a load?
> If it has a transistor at the primary side, and a high frequency
transformer, it's
> a switching supply.  Those want a load, usually.  Especially the older
> If it has a laminated iron transformer, big electrolytics for filtering,
> 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.
> 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 switchers).
> 	paul

Hmmm.... Until your email I was pretty confident this was a switcher. But
your description of a linear PSU seems to fit. I have posted three photos of
the main board here: http://1drv.ms/1KQkTBp can you tell from that what kind
of PSU it is?

The big TO-3 on the left is the one that I found had failed. The replacement
does not appear to be switching, but I don't know why.



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