Power supply problem?

[fasteddy]

I have a Pyramid PhaseIII 20 amp power supply that every time you turn on the switch it blows the fuse. Any pointers on where to start? I thought maybe the rectifier circuit might be bad, but I have no way to tell.

Thanks

fasteddy


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[Captain Kilowatt]

The rectifiers may be bad but I would look at the filter capacitors.Turn off the power and short out the capacitor(s) and then check it (them) with an ohm meter.Disconnect one lead of the cap first to isolate it from the rest of the circuit.The meter should kick over to the right and then settle back showing a high resistance.Next I would check to see if the crowbar circuit is tripping due to high voltage.When the regulator pass transistors go bad it can cause the output voltage to soar to the pre-regulator voltage and this will trip the crowbar circuit which is an SCR device that automatically shorts the output and blows the fuse.I would lean towards bad pass transistors.If you have a variac (variable AC transformer) then hook it up to the input and watch the output voltage of the supply as you SLOWLY turn up the AC input to it.If the output is at +/- 13.8 v before the AC is up all the way then the pass transistors are shorted.Good luck.



P.S. You can check the rectifiers by disconnecting the lead from them to the filter cap and seeing if the fuse blows.No blow = good rectifier.

Garth 9VE01 PE993 CDX993 Learn from others mistakes.You can not live long enough to make them all yourself.</p>Edited by: <A HREF=http://pub86.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=qrn>QRN</A> 

at: 1/28/04 4:31 pm