28 Apr
2011
28 Apr
'11
12:10 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 28 April 2011 19:19
To: cctalk at classiccmp.org
Subject: Re: new here
across it
Right... I have an 11/44 that uses this supply, and I did repair it
without
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. getting electrocuted. In my case the problem was a
high ESR smoothing
capacitor on the 36V line which caused that supply to shut down (thus
removing the memroy supplies and the cooling fans.
[Mains smoothing capacitors]
#
I have just this minute attached a dummy load to one of the 5V outputs and
powered it on. Using a multimeter I measured 5.4V on the 5V outputs and
15.8V on the 15V outputs. I ran it with just the fans running (forgot to
disconnect them) for about 5 minutes and then switched it off. Are those
voltages within tolerance do you think? They seem close enough to me to
warrant putting some boards back, but I would like a second opinion before
risking the boards.
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.
circuitry is
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 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 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