On 3/14/16 10:46 PM, Mattis Lind wrote:
tisdag 15 mars 2016 skrev Josh Dersch <derschjo at
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 to you (and others) for tips. I used the desoldering station at
work today to remove the transistors for testing (the "motherboard" PCB
in the 7104 is very thick and I was having no luck getting all the
solder out of the vias with my tools at home). I found one bad 2N6547
in the lot (everything shorted to everything else) so I'll be replacing
that tomorrow after a stop at Vetco on the way to work to pick up a
replacement. All of the diodes that I checked looked OK. Fingers
crossed that the transistor is the only issue here.
Thanks again,
Josh
> Thanks as always,
> Josh
>
>