27 Jan
2018
27 Jan
'18
8:29 p.m.
On 01/27/2018 12:04 PM, Jules Richardson via cctalk wrote:
On 01/26/2018 07:15 PM, Jules Richardson via cctalk
wrote:
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.
Allison
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