al smith wrote: > Situation is a PC type switching supply that is failing, mulitple units. > Power source is 120VAC, being supplied via a UPS system. > > One theory is that the fact there is a 10-20% total harmonic distortion > present, that this may be an issue. It could be, but do you know what is failing? Is the unit dead and the switching transistors blown apart (the fuse will probably be blown in this case) or do they look intact? If they are OK, it could just be the oscillator that has failed - or the PSU to it. The chip oscillators usually have a large 1W 330K resistor coming down from the +ve rectified mains which drops enough voltage to supply the oscillator chip. The across the chip will be a small electrolytic. This voltage is just enough to start the chip and then there is probably some sort of transformer feeding voltage back to the chip after that. It is common to see the cap across the chip worn out or this feed resistor OC. Brian. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body