[Boatanchors] OT: power supply troubleshooting

[Bry Carling]

Yes Ian.  I have seen the same thing happen on a large amplifier with two 4 - 400A tubes.  Full wave rectifier, used 2 strings of 8 diodes with 470k balancing resistors and snubbing capacitors. The total p 53 rating was double the required voltage. Yet I still took them out twice during contests, when cranking up the 240 v at 20 A variac the primary of the pole pig!!


Ian Wilson <ianmwilson73 at gmail.com> wrote:

>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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regards,
Bry Carling

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