28 Apr
2011
28 Apr
'11
11:18 a.m.
Interesting you should mention the PDP11. The PSU I
need to test is
from a PDP11/24, it is a H7140. I think it is Switched Mode, but it is
pretty heavy
How do I put this... You've been thrown in the deep end :-)
Am I correct that your 11/24 is in the 10.5" (6U) box? Boards going in
vertically from the top? If so, it is the PSU I am thinking of. I came in the 11/44, and it's one of the most
complciatred PSUs you are likely to
come across.
Yes, this is an exact description of what I have. I saw those capacitors and realized I need to steer
well clear of them after
power has been applied.
Worse than that they can store significant amounts of energy after the
mains has been unplugged. Yes, there are bleeder reissotrs, but I am not
about to trust my health to soemthing like that ;-).
The offiical procedure is to uinplug the mains, wait 5 minutes, rmeove th
ecovers and then measure the voltage between the red and black wires on
the 2-wire barrier strips. If it's more than a few volts, find some way
of safely discharging those capacitors beofre handling anything.
I will admit I didn't always 'wait 5 minutes'. The thing should discharge
a lot quicker than that. And I would run it with the covers off too. But
after disconnectig the mains, I alwaysed checked that voltage before
handling anything, pulling boards, etc.
Thanks for the warning. I find PSUs worrying at the
best of times, even
before your warning this one already scares me rather more, which is why I
am trying to do things more correctly than usual.
As I said, this one is nicer than some in that a lot of the control
circuitry is not directly conencted ot the mains. So you can work on that
wihtout too much danger. But there is a lot of live circuitry in there.
If you need to repair it, I am happy to help.
I found three types of connector from the PSU to the
rest of the machine.
There are six 15-pin connectors (5 rows of 3 pins), with only three
Those are the stnadard DEC backplane power conenctors. You cna check
voltages and attach dummy loads t those.
connectors actually connected. Then there are two
connectors I don't know
how to describe but are each a single row of about 10 or so pins, the
I thought there ws at least one ribbon cable connecotr that goes ot the
frontpanel keyswitch. At least on the 11/44 said switch can turn on the
memroy supply only, for example,
connector is very like the ones you find on a MicroVAX
II power supply if
you have seen one of those.. Finally there were two wires with spade
connectors on them which connected to a couple of tabs on the PSU.
I am pretty sure those are the power to the cooling fans (all 3 fans are
wired in parallel, BTW. Do nto short either of those to ground or you
will blow some transsitors in the H-bridge circuit on the PSU control
board. Don't ask how I know that :-)
The fans, BTW, are spceical. But fortunately the on;y special part ios
the motor windigns, really. You can use bits of other fans of the same
make to fixe them. I know I had to rpelace the bearings in one of mine,
which wasn;t too difficult.
-tony