Testing H7864 (MicroVAX II) PSU With No Load

Mattis Lind mattislind at gmail.com
Sun Nov 1 06:46:05 CST 2015

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 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.

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.


> Thanks
> Rob

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