High/low plate voltage switching is passe . it was done years ago for FCC rule
compliance. You should set the amp so that the hi/ lo swich changes the ZSAC
instead. . There was NEVER an amplifier that was made which used a truly rated
relays for the purpose. A simple carbon trail virtually invisible to the naked
untrained eye can cause your problem. I just had a case last week. Bring the
amp into the 21st centry and rework it to today's standard. That was never a
good idea but was necessary for compliance. The proper relay was too expensive
and never used by any company using that switching scheme
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-----Original Message-----
From: Mike & Becca Krzystyniak <k9mk@flash.net>
To: amps <amps@contesting.com>
Sent: Sun, Jan 17, 2016 08:22 AM
Subject: [Amps] Help please: Converted Alpha 78 blowing fuses
Greetings all,
I'm looking for this groups sage advice.
A friend dropped off his Alpha-78 that was converted to a pair of
3CX800's along with adding the PWD transformer P/N "Alpha 76A-2. In CW mode
the amp powers up and works. When I hit the SSB button, I hear the HV relay
try to switch and there are sounds of arcing and then of course the fuse
pops. This is all within a few hundred milliseconds of pressing the SSB
switch.
I can't find the smoking gun here. No real evidence of what is failing
or flashing over. I took apart the filter cap stack, looing for arcing pits
or carbon scoring. I did not see any evidence of things punching down
through the rubber insulator to the chassis. Then I carefully unwound the
HV cable harness looking for a pinch that might have punched through and
found nothing. Then I also disconnected the B+ wire that goes over to the
RF deck and no change.
For grins I measured the voltage across wach cap in CW mode and 5 of the
6 had equal drops of about 290V, but one of them was lower ~245V. Not sure
what it means but is a data point.
Has anyone seen something like this before. Comments and suggestions on
what to look at next would be most appreciated.
Thanks in advance.
73's MIke K9MK/5
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