PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Fri Jul 17 23:17:31 CDT 2015

> Thank you for the detailed information. I need to figure out how im going
> to get it out of the rack and moved to a place where i can test it over the
> next couple days where it will not be in the way.  Ill find some way to do
> a dummy load and do an extended test to be sure the supply is working
> properly. All fingers crossed, god it better not have damaged any of the
> boards, i do not know where i would get replacements.It took years for me

DEC PSUs of that vintage have a crowbar circuit which effectively short-circuits
the PSU (using an SCR) if the voltage rises too high to protect the rest of the 

On the other hand if the +5V line did get too high it could have wiped out 
just about every IC in the unit. Ouch!. I've only ever had this happen once, and
it was in a much lesser machine than a PDP11 (fortunately).

> to get the machine, who knows how long it would take to find a specific
> board that is bad. I did buy an oscillicope and a logic analyzer well in

In general you should be thinking of repairing these machines on the component
rather than board level. You may well have difficulty in finding a particular board 
(and how do you know that the replacement is good?) but finding one of the logic
ICs is very easy.

> advance in preparation for getting this machine, however short of pressing
> the power button no clue how to use them or basic troubleshooting
> procedures. Guess i just have to learn by doing....

That's how quite a few of us (including myself) learnt...

The basic procedure that I use goes like this  :

1) What should the machine be doing? In other words, from the schematics I deduce that
there should be a clock signal on 'that wire' or data flowing there with enable pulses on that

2) What is the machine doing (determined by examining said signals with a 'scope and LA)

3) Why the difference? If they are the same then that part of the machine is mostly working
properly. If not, then what could explain what I am seeing, How can I determine what the 
cause is? Aha... What is _that_ signal doing? Check it....


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