L&C LC-B450E: Shorted secondary switchers?

[TechGeek]

Hello all. Several months ago, I received a computer with a bad PSU. The computer didn't turn on at all. Further testing revealed that the 5VSB rail and PS_ON were present, but upon shorting PS_ON to GND to force the unit to start up, all the rails went dead.

I've done some thinking recently and I'm wondering if this is a common sign of one of the secondary switchers being shorted. Any thoughts?

FYI, not trying to repair this right now, but I'm keeping it around as a potential repair project for when I get some decent soldering equipment.

TIA

[Dan81]

Re: L&C LC-B450E: Shorted secondary switchers?

I had one like that and I fixed mine by replacing all the rectifiers on the secondary. Not sure if the same applies here, but worth a shot.

[R_J]

Re: L&C LC-B450E: Shorted secondary switchers?

What do you mean all the rails went dead? did they have voltage before? or do you mean the standby 5vsb went dead now too?
I would suspect shorted fets in the primary, when you forced the power supply on it likely blew the main fuse. so no more standby.

[TechGeek]

Re: L&C LC-B450E: Shorted secondary switchers?

What do you mean all the rails went dead? did they have voltage before? or do you mean the standby 5vsb went dead now too?
I would suspect shorted fets in the primary, when you forced the power supply on it likely blew the main fuse. so no more standby.

5VSB was present until I forced it on. Then nothing on all rails UNTIL I removed the jumper forcing it on. A second or 2 later, 5VSB came back. Fuse is intact.

[R_J]

Re: L&C LC-B450E: Shorted secondary switchers?

Here are two versions for the the LC-B250 model, might be similar

[momaka]

Re: L&C LC-B450E: Shorted secondary switchers?

The 5VSB shouldn't die, even if the main PS has a fault, because these are separate PSs. The only two cases where I think this could happen is if:
1) input NTC/thermistor has developed high resistance, thus allowing enough current for the main caps to charge and the 5VSB to run, but not enough for the main PS to run.
2) driver transistors for the main PS on the secondary side (near middle/driver transformer, typically) may be shorted and pulling the 5VSB's secondary auxiliary winding low and causing 5VSB to shut down / go out of regulation.

In any case, it should be fairly easy to fix the issue as these L&C PSUs aren't that complex really. In fact, if built well, they are pretty robust designs. Just very outdated, inefficient, and often without good output protections aside from short-circuit (and sometimes even that is not implemented properly, as I've seen some H-bridge PSUs not latch-down after short-circuit.)

Also, you don't really need a special soldering iron to work on these. A 40-Watter will typically desolder anything in this PSU. Combined with a solder sucker, you should even be able to get the heatsinked components out, if need be. Though personally, most of the time I don't bother to do that with these cheap PSUs, as they are so gutless that I can work on them often without removing much of anything.