DEC H7260 PSU fault

Jules Richardson jules.richardson99 at
Sat Jan 27 11:04:41 CST 2018

On 01/26/2018 07:15 PM, Jules Richardson via cctalk wrote:
> After that... well, I need to work out how to get the entire supply
> assembly to start up while it's open, so that I can work on it outside the
> machine - then I can at least start looking for differences between the two
> supplies (boy, are those things complex, but maybe I'll get lucky and it's
> a bad solder joint or cap somewhere)

OK, now I feel like an idiot :-)

I started looking into this, and found that neither PSU would do anything 
without being plugged into the (currently unpopulated) backplane. My 
initial assumption was that there was some interaction between the 
backplane and the "other" wires between it and the PSUs (i.e. the ones that 
don't carry +5V or +12V).

However, on a whim (and I really don't know why I did this) I plugged the 
+5V PSU connectors up to the backplane, but left the others (carrying +12V 
and the other signals) unplugged. Doing that, things behaved as before - 
the top PSU started operating normally, and the lower PSU started putting 
out around 2.5V.

It got me wondering though - maybe it was simply a load issue, and with my 
dummy loads I was right at the (lower) limit for the PSU regulation to 
work; perhaps I had just enough for the top PSU to operate normally (but 
only with the backplane hooked up too) and the lower PSU (for whatever 
reason) wanted just that little bit more than the top one.

Anyway, I tweaked my loads to draw a little more current - and bingo! I'm 
now getting stable +5V and +12V outputs on both PSUs, a working card cage 
fan, and the DC OK light comes on.

So, in summary... with no load at all, the PSUs do nothing, but with *some* 
load they either work OK, or the regulation is goofy and the outputs are low.

Time to add CPU/memory in, wire up a console cable and see what happens, I 



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