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[Amps] Strange Problerm

To: <amps@contesting.com>
Subject: [Amps] Strange Problerm
From: 2@mail.vcnet.com (Richard)
Date: Wed, 5 Jun 2002 21:37:59 -0700
'

>
>
>Joseph Staples wrote:
>
>> Twice within the past six months we have had amplifiers fail due to the 
loss 
>of all of the power supply filter capacitors in one "bank" of the supply.  
>Both amplifiers (an Alpha 76 and a SB-220) were being run on 240 V AC.  In 
>both cases the filter capacitors were "relatively" new.  In one instance 
>the failure occurred when the amp was in "standby" mode during a break in 
>a contest and in the other the unit seemed to have failed AFTER it had 
>been turned off and  prior to the next use.  At the time both cases were 
>ascribed to "random" failures, unknown cause, and possibly a "weak" sister 
>in the capacitor bank.
>>
>> However, a recent comment by an experienced HV power supply builder, has 
>brought up the subject of a possible "wild leg" in the 240 V AC line to 
>which the amps were attached.  According to this suggestion, a large 
>transient VOLTAGE swing on one leg of the 240 V mains put an over-voltage 
>on the capacitor string causing them to fail.  Both units were protected 
>from CURRENT surges and no evidence of  excessive current flow could be 
>found.   This was said to be a known fault either with residential power 
>line transformers or with improperly balanced house wiring.
>>
>> My first reaction was to take all this with a "large grain of salt" but 
then 
>there is always a chance that such a phenomena can occur.  If anyone has 
>had any similar experience or know of a rational explanation, I'd really 
>like to know about it.
>>
>> Thanks,
>>
>>
>> Joe, W5ASP
>
>When an amp is run on 240 volts there are not "2 legs" involved. The 
>neutral is not used in the HV circuit. The transformer is connected only 
>to the two hot lines so if one leg of the 240 was unbalanced with the 
>other it would make no difference. The transients are no different than 
>running on 120 volts.

Good analysis, Gary.

>Equalizing resistors ok?
>
One possibility, especially with high-ohm wire-wound resistors.  
- Also, when one side of a FWD rectifier circuit opens, half of the 
electrolytic filter caps fail due to reverse polarity current being 
furnished by the functioning side of the FWD.  This is a common problem 
with early SB-220s.  The fail-safe mod is to install a >600piv rectifier 
across each cap so that reverse current can not zap the cap.

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
end


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