[Boatanchors] OT: power supply troubleshooting

[Ian Wilson]

I have had diodes fail to become something like a short. Since they
were in a series diode string, this then took out other diodes also.

You may be able to disconnect the connections to the bridge and
use a meter to check what each diode looks like in forward and
reverse directions.

73, ian K3IMW

On Sun, Jan 20, 2013 at 7:57 PM, W2HX <w2hx at w2hx.com> 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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