PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Fri Jul 17 13:19:25 CDT 2015

> I was trying to enter a short program at the front panel and there was a
> clicking sound followed by a burning smell. I cut the power, the front
> panel is unresponsive now, so I'm going to need to look over the power

Did you see anything on the panel when it died (was the numeric display still
alight) ? 

You had better hope that the +5V line didn't go high and cook all the ICs in the
machine. DEC PSUs of that vintage do have crowbar circuits though.

> supply for starters. He did include a second empty PDP 11/34 chassis,
> perhaps the power supply in that one is in better condition.

The PSU in this machine is relatively repairer-friendly. 

There is a big mains transformer in the centre of the PSU chassis (at the back of the CPU). It
takes mains in (there are 2 primary windings, each 115V, they are connected in parallel for US
mains and series for European mains). It has several secondaries, each of about 20V-30V AC

Under the mains transformer is a little unit that contains the mains switching relay and control
circuits, and a simple linear PSU for the +15V rail. I think the LTC (line time clock), ACLO and DCLO
(power failure signals) come from that too. The main supply rails (+5V and -15V) come from 'bricks'
that fit either side of the transformer. These are swtiching regulators that take in the 20V or so from
the transformer and bring it down to the desired voltage. Although they are switchers, the maximum 
voltage inside is just the rectified input (say about 40V DC) and is thus a lot nicer to work on than 
a mains-operated switcher. The bricks are based round the 723 IC along with some transistors, an 
inductor, flyback diode, capacitors, etc.

What I would do is disconnect the logic backplane power (at the distribution connectors under the PSU)
then take the PSU covers off, take out the bottom 2 screws each side and loosen the top on so the PSU can 
hinge away from the CPU and remove the power bricks. Power up the transformer on its own (maybe with
a series light bub) and check that is OK. Then try to debug the bricks. If you have a bench PSU with current
limiting run them (one at a time) off that (they will happing run from a DC input) and see what happens. 
The +5V brick just needs the 20V-30V input, the -15V one _also_ needs a +15V supply.


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