[Boatanchors] OT: power supply troubleshooting

[W2HX]

Wow! Thanks to everyone for the input. I have several things to follow up on now. 

 
73 Eugene W2HX
 

-----Original Message-----
From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of W2HX
Sent: Sunday, January 20, 2013 10:57 PM
To: boatanchors at mailman.qth.net
Subject: [Boatanchors] OT: power supply troubleshooting

Hi all, this is OT and solid state related (please delete me asap!) 

I have a guitar amp that is blowing fuses. The circuit seems to be AC input, fuse, transformer then to the power switch (interesting that the switch does not switch the AC Mains, but after the transformer).  When I measure the input to the switch I see 30VAC. But when I switch the switch to ON, the voltage drops to millivolts. I suspect this is because there is a short after the switch.  After the switch there are 4 discrete diodes forming a full wave bridge. Then two filter caps 470uF/35V electrolytics and then to a 7815 and 7915 regulators to create +/- 15V DC.  A fairly standard looking circuit.

First thing I did to troubleshoot this was measure the in-circuit ESR of the filter caps with an ESR tester. The ESR seems to read 0.1 ohm. I believe this is acceptable, no? but then again, the ESR would contribute to higher than normal ripple, but I don't know if it will indicate a capacitor short?  Do electrolytic caps fail short? I know that tants can.

So could the problem be the diodes? Do diodes fail short? Could 4 diodes in a full wave bridge fail in such a way as to create a short? 

If not, do I look at the regulators? 

Any general advice? 
Thanks

73 Eugene W2HX



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