31 Jul
2001
31 Jul
'01
10:48 a.m.
Tony Duell wrote:
In my experience the voltage is _very_ dependant on
the load. If you
check all the regulators with no load or a small dummy load (as you
should do) you'll find they're all too high. So you tweak them down, put
the boards in and have a machine that malfunctions in strange ways. After
a lot of thought you check the 5V lines and find them sitting at +4.2V or
so (this happened on my 11/45 for just this reason).
I would recomemnd testing the supplies on dummy load and setting them
just on the high side of 5V (say 5.2V or so). Put the boards in, then
check the voltages again and adjust them properly.
Good point, I have also experienced this and have gone back to find they
need adjustment.
.
DEC used very good quality fans in these machines, probably better than
you're likely to get easily these days. If they're not actually burnt out
(check the resistance between the 2 terminal tags), then often they can
be taken apart (circlip in the centre of the blades, take off the cap,
the washers, the second circlip and the blade/rotor assembly) and the
bearings cleaned out and relubricated. My 11/45 is still on _all_ the
original fans...
While I admit that I still have some of the original fans in my 11/70,
most that I replaced were beyond any repair. I have a local electronics
surplus store that sells the cooling trays that US Robotics used to sell
with their access servers. These cooling trays had never been used and
were made up of nine of the very high quality ball bearing NMB fans that
I mentioned. These fans are used by many electronics manufacturers
today and are a direct replacement for the original fans in the KB11
CPU. I found that if you have gone to the trouble of taking the system
apart to the point where you can get the fans out, you realize that you
don't want to do it often :)
--
Doug Carman
pdp11(a)bellsouth.net