tisdag 15 mars 2016 skrev Josh Dersch <dersc...@gmail.com>: > Hi all -- > > My call for a VAX-11/750 a month or so ago actually bore some fruit > (locally, even!) and as of a couple of weeks ago, I now have a very nicely > configured 11/750 system taking up most of the basement. The previous > owner got it after it was retired from a local(ish) university in the mid > 1990s and it has not been powered on since then. Apparently at the time of > its retirement the power supplies were exhibiting "random issues." (No > more detail is available than that on the history...) > > At any rate, I went through the two power supplies (and the small pilot > supply in the power controller) and found a lot of leaky capacitors (as in, > yellow/brown goo was coming out of maybe 2/3 of them) so I went ahead and > recapped the whole thing. > > At the moment I have things running on a dummy load in the 11/750 > chassis. (the harnesses are still hooked to the chassis backplane, but all > cards have been pulled, and the backplanes thoroughly checked for bent > pins, etc.) The H7104-C (2.5V) supply seems to be working fine but the main > 5V supply in the H7104-D is not doing so well (and as a result the other > voltages it's supposed to be producing are also not present). The Power > Controller lights up the "Reg. Fail" lamp (I don't know why the 5V Fail > lamp isn't also on) and the 5V supply emits a loud (somewhere around > 400Hz?) whine/squeal. I get about .3V out of it with a load. Without a > load there's no squeal and I get about 5.6V, but that's not particularly > useful. > > I've double-checked everything in the H7104-D and there's nothing > obviously wrong (no caps installed backwards, no scorched components). At > the moment the H7104-D is hooked up only to a dummy load, so it's not > anything on the backplane shorting out or causing issues. > > This is another one of those cases where I've gotten myself in over my > head with large, complicated power supplies -- anyone have any experience > with these? Any tips?
You can read the story of my 11/750 power supply repairs in this article: http://www.datormuseum.se/computers/digital-equipment-corporation/vax-11-750 Loud squealing noise was in my case due to short-ciricuit output rectifiers for the 5V. Check them. Then also chech the main chopper transistors. /Mattis > Thanks as always, > Josh > >
