VAX-11/750 w/o POWER OK indication
oldcpu at rogerwilco.org
oldcpu at rogerwilco.org
Fri Oct 30 23:07:10 CDT 2009
After a very long time, I have gotten very close to having a functioning power supply.
I'm still not quite there by the looks of it. With a dummy load of two automobile
headlamps (four filaments wired in parallel) for each of the 2.5V and 5V supplies, I now
have pretty decent voltages:
2.54v (2.5V supply main output)
5.21v (5V supply main output)
As far as I can tell, after going through the "H7104 Power System Tech Desc" doc,
these voltages are good. Even the slightly high 5V line is within the spec'd range of +/-
5% or 4.75v - 5.25v.
Still, I don't see a green POWER OK indication on either the power controller or the
cabinet's front panel. What I do see on the front panel is a red FAULT and a green
(slightly dimmer) RUN indication, but this is probably because no boards are yet in the
backplane. There are no lit fault indicators on the power controller itself, which
suggests that the voltages are okay.
Another list member who saw similar symptoms on his 11/750 apparently found that it
was loose contact in the Power Control ribbon cable, but that doesn't seem to be the
To test this further, I toggled the 5V supply's test switch to LO and saw the voltage drop
to 4.97V, but the POWER OK lamp still didn't light up, so I don't think the problem has to
do with the 5V output being too high.
I should put my 'scope on it to see if the output is stable. I'll do that, but I'm guessing that
it's going to be good, and I don't know if the power controller is sensitive to that
anyway. I haven't checked the POWER OK lamps themselves, and I'm guessing that
since there are two lamps, one in the power controller and one in the cabinet front
panel (and that they are not simply wired in parallel or serially) it's something else.
Maybe the bulbs are ganged together, I'd better check the FMPS again.
Before I dig into that aspect further, I wanted to check in with the group and see if
anything stands out that I've overlooked, or if you guys think I should check something
else before the bulbs.
Just want to make sure the PS is in order before loading the boards into the backplane.
 Rescued this unit two years ago this month. Turned out to have an empty card
cage, and a fault in the PS. I finally traced the PS trouble to two of the four power
output diodes in the 5V supply (shorted out, replaced all four), and a poorly contacting
connector in the 2.5V supply. Over time I have scrounged up a complete boardset so I
should have a functioning system Real Soon Now.
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