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. Allison
