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[Amps] one mor SB-220 fix

To: "Amp Reflector" <amps@contesting.com>
Subject: [Amps] one mor SB-220 fix
From: "kingwood" <k5jv@kingwoodcable.com>
Date: Sat, 22 Aug 2009 18:40:12 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Greetings to all,

        There is one other fix that should be really considered on the 
SB-220 that does not get much mention.  The recent thread should be taken 
seriously.

        The subject is the RF choke across the output of the amp.  This 
choke is a safety device, intended to short any DC to ground that should 
appear on the amp output.  How does DC get on the output?  With a failure of 
the plate blocking condenser.  If this cap opens, no problem, but, if it 
shorts, the entire power supply DC, approximately 3,000 volts, will be 
applied to the output of the amp.  OK, the choke will short the DC to 
ground, hopefully causing the AC line CB's to open before any real damage is 
done.  So what is the problem?  These CB's are now thirty to forty years old 
and are very unreliable.  I doubt that they were ever very reliable in 
tripping close to their rated value.

        About a year ago, I had a plate blocking cap short on an SB-220 that 
I was re-building.  The choke did it job, for a while, and shorted the DC to 
ground.  But, the CB's did not open.  So, what happened?  After some time, 
the RF choke overheated and failed, open.  The 3,000 VDC from the power 
supply was then applied to the TR relay.  This relay was never intended to 
see hi-voltage like this and the 3,000 volts arced across to the relay 
contacts to the input of the transceiver.  Of course,  the front end of my 
old faithful FT-1000D was wiped out.

        So, what is the answer to all this.  Two things.  Replace the old, 
unreliable CB's with good ceramic fuses and second, hi-pot those old, red, 
or green, doorknob capacitors that HeathKit used for plate blocking and 
hi-voltage bypassing.  Be sure that they have no leakage at, at least, twice 
their working voltage, about 6,000 volts.  If any leakage is observed, 
replace them with new capacitors.  This is a very inexpensive test that can 
really save a bunch of time and money on  receiver repairs.

73 de Lon, K5JV

1110 Golden Bear Ln.
Kingwood, TX 77339

281-358-4207
281-358-4234 FAX
281-795-1335 CELL 

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