> 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 machine. 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 pin. 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.... -tony