1 Nov
2015
1 Nov
'15
6:04 a.m.
-----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
s?ndag 1 november 2015 skrev Robert Jarratt <robert.jarratt at ntlworld.com>:
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.
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.
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
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> Rob
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