> [Original Message]
> From: Joseph Staples <w5asp@earthlink.net>
> To: <amps@contesting.com>
To: <amps@contesting.com>
> Date: 6/5/02 7:24:25 PM
> Subject: [Amps] Strange Problerm
>
> 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
>
> Too much data is missing to speculate on the cause of the demiase.
Questions can be raised, though. Since the capacitors were somewhat new,
that means they were replaced. What else whas replaced or at least
checked. What about the equalizing resistors. Were they new? If so, what
kind? Were they checked prior to the failure? Were they checked after the
failure?
Problems at the primary source would probably first affect the fuses, the
transformer and the diodes. Generally heat and/or voltage excess causes
capacitor failure (unless they simply have dried up). It does not take a
lot of variation in the equalizing resistors to allow an excess voltage to
appear on on of the capacitors.
The original equalizing resistors in both the Alpha and the Heath pushed
the voltage limit, so give them a good look.
Colin K7FM
> --- Joseph Staples
>
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--- COLIN LAMB
--- k7fm@teleport.com
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