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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