[Boatanchors] OT: power supply troubleshooting

[Al Parker]

hi Eugene,
	The ESR meter won't indicate a direct short, it can't discriminate 
between a direct DC short and a very low ESR (which is measured at a 
high freq., maybe 1 kc or higher, depending upon the meter).  You can 
use a "regular" ohmeter to check for a short, if it stays at a low 
reading, rather than starting high and coming down, it's a low 
resistance short.  Your 0.1 ESR reading might be a short, but a good 
470uF/35V will have ESR not too far from that (I don't have a chart 
handy now).
	I think diodes usually fail open, but ??  Can you open the ckt just 
after the diode bridge and b4 the caps?
	Strange arrgt for the switch, after the xfmr.  Not a good thing, but at 
least it's fused.
73,

Al, W8UT
www.boatanchors.org
www.hammarlund.info

"There is nothing -- absolutely nothing -- half so much
worth doing as simply messing about in boats"
Ratty, to Mole

On 1/20/2013 10:57 PM, W2HX wrote:
> 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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