The choke at the output side of the amp is there primarily to prevent the
tank circuit capacitors from charging up to some DC level and arcing over.
The output tuning capacitor typically only has plate spacing enough to
handle a few hundred volts. You would never see 3000 volts at the antenna
terminal unless maybe the tune capacitor was set to minimum capacitance or a
vacuum variable was employed.
Without a choke at the output even with a good low leakage plate blocking
capacitor there would be DC across the output as the plate block capacitor
is in series with the output tune capacitor and would charge to a portion of
the supply voltage until it arced and discharged. Then the charge cycle
would start again until it arced.
Take a look at most chokes at the output. Many are rated at only a couple of
hundred mills of current. Some use no more than a 2.5 mh 125 ma type choke.
A few amps have very heavy chokes there that could handle the full current
that the power supply can deliver but not many.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of kingwood
> Sent: Saturday, August 22, 2009 6:40 PM
> To: Amp Reflector
> Subject: [Amps] one mor SB-220 fix
>
> 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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