VAX 11/750 5V PS troubleshooting
oldcpu2 at rogerwilco.org
Tue Dec 4 16:48:55 CST 2007
J Blaser wrote:
> Quoting Tony Duell <ard at p850ug1.demon.co.uk>:
>> But I can be eaily persuaded to look at the PSU schematics sometime and
>> see if I spot anything obvious. Or at least tlak you through the design
>> (which might be somewhat unconventional if other DEC supplies are
>> anything to go by )
>>  This might not be a problem for you. If oyu don't have any experience
>> of SMPSUs, any one is as good/bad as another to get started with. The
>> problems generally come if you have fixed ideas ofg how supplies should
>> work and then you get something strange thrown at you.
> Thanks much for the offer to work me through this! There are a couple of things
> that I want to do tomorrow night when I get a chance.
I finally got a little time to take another look at this 5V supply.
I removed all connections to the backplane, etc., then connected my
4-filament rig of headlamps as the only load and powered up. I see the
same results as before, about 0.8V (a bit variable) on the 5V output.
So I guess we can safely say that the problem lies within the PS itself,
and not in anything that it's connected to. I'll add that I hear a
not-loud-but-noticable 'DZZZ' sound from the PS (not just now, it's been
there from the beginning), probably the crowbar doing something. Still,
I think I have a suitable load with the headlamps.
Tony, if your offer still stands, I'd appreciate some advice on how best
to proceed. If you could give me a couple of ideas about what would be
good test points to get started with, I'll give it a go. As I said
earlier, I have a modest set of tools, e.g., VOM and oscilloscope.
I have the 2.5V (H7104-C) and 5V (H7104-D) PSU printsets, found at:
I admit, I'm not making much sense of them at the moment.
FWIW, I have no experience working inside of switching PSUs, so I have
no knowledge (good or bad) to draw upon.
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