DEC H7260 PSU fault

allison allisonportable at
Sat Jan 27 22:29:55 CST 2018

On 01/27/2018 12:04 PM, Jules Richardson via cctalk wrote:
> 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 guess.
> cheers
> Jules
The PSUs used are a switching type and do not behave well with out about
a 5-10% load as a minimum.  They must have a load.

NOTE: the door switch does only one thing and that is to run the fans at
100% when the covers are off.  When they are
on then the switch allows the fan speed to be controlled by temperature.

FYI all this is covered in the BA123/microVAC manuals.  Seriously, if
your not familiar with Qbus and microVAX boxes
RTFM and as many as you can find.


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