1 Nov
2015
1 Nov
'15
8:51 a.m.
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).
paul