Testing H7864 (MicroVAX II) PSU With No Load

Robert Jarratt robert.jarratt at ntlworld.com
Sun Nov 1 08:04:25 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Mattis Lind
> Sent: 01 November 2015 12:46
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
> söndag 1 november 2015 skrev Robert Jarratt <robert.jarratt at ntlworld.com>:
> > 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?
> >
> >
> >
> Maybe you can publish your schematic somwhere. It is much easier to come up
> with ideas to pin point problems if we all view the same schematic. I think it
> would be possible to identify the problem without comparing the two PSUs at
> least in this case where there is some kind of fundamental problem.

I do intend to publish the schematics, but right now I am pretty sure they are:

a) full of mistakes
b) not drawn logically
c) there are a couple of areas I couldn't trace without major surgery on the board.

> I would start with trying to decouple the bias voltage powering the switch logic
> so that it could be run from a bench supply while powering the main switch
> transistor and power transformer from a protection transformer and variac. If
> you run like this you could start without any dummy load at all.
> When you get to a higher input voltage from the variac it could be useful to
> have some small  dummy load.

I am not a PSU expert, and I am not sure what you mean by a "bias voltage powering the switch logic". As for decoupling it, again I suspect that is way beyond my knowledge. I do have a variac though, and I believe it is not a good idea to power a SMPS from a variac. I guess the decoupling you mention would avoid the problem of using the variac, but I don't know enough to do that.

> What kind of chip is controlling the PSU? With bias power applied is there any
> switching activity output from the chip? The RC network that usually make up
> the time constant should have some kind of sawtooth signal I would guess. If
> not it can obviously be broken or some feedback signal has caused it to shut
> down, for example due to over current feedback.

There are no chips in the PSU (apart from a couple of comparators). Quite a while ago, I did put a scope on the base of the transistor I mentioned (using an isolating transformer), and could not see a signal (or rather a very small signal), whereas on the known working one I did see a signal. It has been my aim to discover why this is, but it is too complicated for me, with no real PSU experience, to understand.

> /Mattis
> > Thanks
> >
> >
> >
> > Rob
> >
> >

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