Testing H7864 (MicroVAX II) PSU With No Load

Paul Koning paulkoning at comcast.net
Sun Nov 1 10:51:52 CST 2015

> 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 ones.

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.

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).


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