I've been working on a broken PC power supply and i've found a strange component wich i've never seen before.
It looks like a regular diode, but i don't think it is. It's marked 'B1' while all the diodes on the rest of the board are marked 'D'.
It sits in series with the incoming AC and the bridge rectifier - so basically, all the current going to the power supply has to pass trough it. Every time i plug the power cord in this part lights up for a second asif arcing inside. I do measure 340Volts on the main capacitor - so i don't think this part is the problem but i would like to know what it is.
It's the red circled component on the top. Click the picture for a larger one. **broken link removed**
However, if you've got 340V on the cap, then that's not the fault anyway.
A very common cause is the startup resistor going O/C, this provides a temporary supply for the chip until the PSU starts running and feeds itself. When the resistor fails, the PSU won't start.