PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Fri Jul 17 23:25:26 CDT 2015

> I had to do some cap replacement on some older Motorola tube radios,I have
> some basic soldering skills. I was under the impression that the capacitors
> in computer equipment this big from this year would have been of better
> quality and it would not be an issue.

In general I will agree with that. Of course a capacitor can fail, but it is not that
common. I certainly have never felt the need to 'recap' a classic computer PSU

> I have someone scheduled to come out tonight after i get off work and get
> it out of the rack.

You do not need to remove the complete CPU from the rack. DEC machines of
this vintage were made to be repaired. The CPU (assuming it's the 10.5" high box)
will slide out on slide rails, and there are catches to let you turn it so that the front panel
points upwards. With the CPU in various positions you can remove just about any part
of it without taking it out of the rack. In fact having it in the rack makes things easier
in some cases.

Basically, you take the top and bottom covers off, then the top cover of the power supply. Tip 
the CPU up, then unplug the cables on the distribution panel under the machine (6 pin and 15 pin
connectors). You can now test the PSU on its own. If you have to go further you will see that the 
PSU is fixed to the rest of the CPU box by 3 screws each side. If you take out the bottom 2 each side
and loosen the top one it will hinge away (the metal is cut to allow this) and you can get to the fixing
screws and connectors for the regulator modules. 

If you need to remove the unit under the transformer, you unplug the cables, undo 3 or 4 screws, and it
slides out rearwards. 

Of course you unplug the mains cable before working on anything and only plug it in when you
need to do some tests. But unlike more modern SMPSUs, there are no lethal stored voltages in this


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