11/34a problems continue

Please stop guessing and start measuring! In other words, stop swapping modules at least until you have some idea as to where the fualt is. I've suggested the 15V supply is missing. I might be right, I might be wrong. What I meant by that suggestion was that if it was my machine, I would now stick a voltmeter on the 15V supply to the backplane and see if it was correct or not. If it was, I'd check it at the -15V brick too (been caught by bad connections too many times!). If it was right there, I'd delve into the brick. If it was missing at the backplane, I'd check back to the +15V regulator PCB, etc You've swapped out a couple of parts so far I believe. Do you have any reason to believe that the replacements are good? Only last week I had 3 disk drive spindle motors behave the same way (spin up, run for a few seconds, then stop), and I thought the problem was in how I was driving them. Not so, all 3 had much the same intenral fault!. These DEC power bricks can suffer from dried-up capacitors, that could well be a problem with your 'spare' unit too. The first stages of tracing this fault properly need nothing more than a multimeter. And I can't understand how anyone can hope to maintain a minicomputer without that instrument. -tony