[Amps] Help please: Converted Alpha 78 blowing fuses

[Louis Parascondola]

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 at flash.net>
To: amps <amps at 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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