nctnico wrote:To fix complex power supplies like these it often helps to use a bench power supply to feed the control circuitry (make sure to fool the undervoltage lock out). The next step is to put something like 30V on the primary side instead of mains. You can also do the same for the secondary outputs. This will allow you to verify whether all the semiconductors and post regulators work without having to deal with lethal voltages.
Sometimes I think we have mental power
Guess what I did yesterday night without having read this message ?
I put a my bench power supply, set at 32Vdc, 0.25A max on the primary side and found a shorted transistor !!!
So, yes, I did on my own exactly the same you suggest!. I had previously used this same way to work on an Amateur Radio power supply (an Alinco 13.8V 32A SMPS) some weeks ago, as it was fusing main fuse. On that case, it was a matter of a faulty main diode bridge. But, as it had worked fine that time, I decided to give it a go... and, yes, it worked !
FYI, what I found is that Q9, a BU508A in my unit BIAS SUPPLY primary (note that this is different on the 520B, which uses U12 on a similar function) was shorted. I have ordered some spares from England (I am in Spain), as it is easier for me to order them online than to buy them in a shop (talk about time schedule conflicts!)
BTW, all caps checked fine on ESR tests done before the previous checking.
I will keep you posted on the final results.
Thanks a lot for your support!