VAX 11/750 5V PS troubleshooting

[J Blaser]

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 [1])
>>
>> [1] 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:

http://vt100.net/mirror/hcps/h7104-c.pdf
http://vt100.net/mirror/hcps/h7104-d.pdf

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.

Many thanks.

- Jared