HEC-250LR-T with (or rather, without) exploded resistor

[Agent24]

Seems like PSUs like blowing up on me recently...

Plugged in this old P3 system I have, didn't get far after hitting the PSU switch and there was a small popping noise

Took me a while to find it (I guess the lack of burnt silicon smell should have been a good sign) but eventually I found two leads either side the silkscreen of an R where there should have been a little resistor.

It seems to be part of the circuitry for the controller\monitor ICs

Fortunately I have an identical PSU so I can find the value easily but why would this resistor just outright explode (either that or vaporised, I can see no traces of it) I can see no damage to surrounding components either...

[everell]

Re: HEC-250LR-T with (or rather, without) exploded resistor

In recent weeks I have worked on a HEC-300AR-TF which is probably similar. It has a AZ3843 for pwm chip and P10NK60Z FET (10 amps 600 volts) for the switching transistor. There are only two transformers between the two heat sinks, the smaller one for the 5vsb circuit and the larger one for the main switching circuit. Since this psu does not have a third transformer to isolate the pwm chip from the main switcher - when the main switcher shorts, as in the power supply I worked on, the 320 volts from the main caps goes to the pwm chip, taking out any small resistors in the way.

So.........measure the resistance of the main switching transistor. If it is zero, or near zero, you can figure that the pwm chip was also fried, as well as some other components in the vicinity. Perhaps challenging, it can be fixed.