[Amps] Strange Problerm

[Gary Schafer]

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.

Equalizing resistors ok?

73
Gary  K4FMX