'
>
>
>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
|