PDP-11/83 Power Supply Wierdness

From: Kevin Handy <kth_at_srv.net>
Date: Fri Sep 5 12:31:00 2003

Christopher McNabb wrote:

>My PDP-11/83 has recently (past few weeks) been displaying some odd
>symptoms. The system is a PDP-11/83 in a "Corporate Cabinet". Most of
>the time, when the power switch is turned on, the "run", "halt", "ready"
>lights blink on then go off. Also, the Red light on the TK-50 will come
>on then go off. You also hear a relay closing and opening in the rack
>power supply.
>
>About every 10th try, the DC OK light will come on and the system will
>start to power up. Then all the lights go off after a couple of seconds
>and the rack power supply relay clunks.
>
>When the switch is turned off, "run", "ready", and the TK-50 light blink
>on and you hear what sounds like the relay in the rack power supply
>closing and opening.
>
>Kevin, do you remember it doing anything like this when it was yours?
>
>
>
Never had that problem myself. It sounds like the power distribution module
in the bottom of the cabinet is turning off. The distribution module
turns on
power to the other boxes when the CPU box is turned on, and turns it off
when
the power to the CPU box is turned off.

Check the control cable that runs from the CPU box into that module and make
sure it hasn't come loose (it's a fairly small cable). The CPU box is
plugged
in a always-on extension in the distribution box, and when power is
turned on,
it sends a signal down that wire that turns on everything else. If power
in the
CPU box fails, then the signal disappears, and the rest of the system
will also
shut down (and the relay in the distribution module will clunk).

It sounds like the CPU box power supply is shutting down. Toggling the
power switch usually resets it. I's guess it's complaining about the power
it is receiving (rightly or wrongly).

There is a small toggle switch on the power distribution panel
(accessible from
the back of the machine) that controls how it behaves. Try putting it
in manual mode, which will cause everything to always be powered.
If everything else retains power when the CPU box fails, then it is
something with power in the CPU box. If all power still fails, then it
is probably
the power distribution box.

If it doesn't fail, you may have too many things plugged into the wall
power outlet
to handle the initial startup current, and the CPU box may be shutting
down due
to bad(low) power, pulling all the other boxes off. Try leaving the
tape drive off
during power on to reduce the initial power requirements (power switch
is on the
front of the tape drive, if you haven't noticed it). RSTS/E, at least,
doesn't need it
to be on when the system is powered up.

Also check to see if the power supply on the CPU box if full of crud, and
overheating or otherwise causing a fault. (Blow it out with compressed air
while power is turned off). Power supplies seem to like packing themselves
with all the dust they can possibly suck in.

Another possibility is too many things in the CPU box. You could
try moving one or two boards to the expansion box.

If the power supply is failing, I believe it is the same one used in the
BA-23
boxes, and it should be easy to find a replacement. Actually replacing
it may
not be as easy as finding one ;)
Received on Fri Sep 05 2003 - 12:31:00 BST