I just recently purchased a Heathkit SB-200 amplifier which seems to
have a self oscillation problem. When the amplifier is keyed through
it's relay input I get RF output with zero input ! In fact there isn't
even anything hooked to the input jack. This problem occurs mostly on
10 and 15 meters. The SWR and wattmeters on my tuner pin as soon as
the amp replay is keyed. By turning the loading down to zero it will
usually (but not always) stop. At one point I while testing the amp
(when it was working right) I was able to load almost to 800 watts out
on 20 meters but when I went to 15 and 10 meters it started this
problem and cant seem to get much over 300 or 400 out now. Could this
be indicative of a bad 572B tube ? Perhaps a shorted one ? Any hyelp
would be greatly be appreciated. I am a new amp owner have just been
running 100 watts for the last 30 years !
--
Tim, K8TJ
(ex-WB8OGM)

4/7/2003 @ 6:39:04 AM

tim english wrote in message . ..
I just recently purchased a Heathkit SB-200 amplifier which seems to
have a self oscillation problem. When the amplifier is keyed through
it's relay input I get RF output with zero input ! In fact there isn't
even anything hooked to the input jack. This problem occurs mostly on
10 and 15 meters. The SWR and wattmeters on my tuner pin as soon as
the amp replay is keyed. By turning the loading down to zero it will
usually (but not always) stop. At one point I while testing the amp
(when it was working right) I was able to load almost to 800 watts out
on 20 meters but when I went to 15 and 10 meters it started this
problem and cant seem to get much over 300 or 400 out now. Could this
be indicative of a bad 572B tube ? Perhaps a shorted one ? Any hyelp
would be greatly be appreciated. I am a new amp owner have just been
running 100 watts for the last 30 years !

First of all, check see if those plate anti-parasitic osc. chokes are
OK.
The resistor, 47 OHM may have been cracked. If those are good, then
insert the same type of choke on the input circuit, between input
coils and filament where the RF input goes into. Use 2W non inductive
resistor and wind 18 GA wire for about 6 turns and solder at each end.
Hope this will help.
de K6hm