VAX 11/750 power supply fault

Hi Tony No, I'm not confused. If the feedback circuit isn't running, there is no feedback. The feedback is such that it is positive at the interface between the low and high side. This way a failure in the feedback would bring the supply output to zero. To Make this work, there needs to be some way of getting things started. There is a oneshot on the high side that gives the supply a single kick on powering on. This also gets enough voltage to start the overall feedback that is negative from the output to the high side. It is just enough to blink some LEDs. A clever circuit but hard to trouble shoot. The first kick is so short, it is hard to catch on a meter and distinguish between lack of feedback and the over voltage protection( which also kills the feedback at the interface ). I spent time analyzing the overvoltage parts before realizing that it was a complete lack of feedback that was the problem and not the over voltage protection. The problem traced to an open ( poor solder ) inside the pulse transformer that connected the low side to the high side and provided the pulse width modulation for the supply. I'd call it a tricky circuit because as you seem to consider, it goes against common sense in that no feedback is no output. No feedback just means no pulse modulation and hence no output. As I also stated, the DEC supply most likely used a second powersource for the regulation feedback. Dwight _________________________________________________________________ You keep typing, we keep giving. Download Messenger and join the i?m Initiative now. http://im.live.com/messenger/im/home/?source=TAGLM