SB-200 Oscillation Problems

[Kenneth G. Gordon]

> First time I tuned it up everything seemed to be working and I was
> tuning a good 600W into a 50 ohm dummy load. But as soon as I switched
> to a 'real' load, like my G5RV the amplifier oscillated. I just had to
> touch the key or the PTT for a split second and it put out a self
> oscillating carrier for the duration of the VOX delay. 

That sounds to ME as though you are getting some RF feedback into it 
from your G5RV being at very high SWR. Do you have an antenna 
tuner in the line somewhere? Has something, perhaps, happened to 
THAT?

> 
> Here is the funny part - it never oscillates if I am using the dummy
> load, but as soon as I use a real antenna (with tuner!) it oscillates,
> and get this - that with absolutely NO driving power at all! 

Again, it REALLY sounds to me as though you are getting some RF 
back into the amp from the antenna. Is the case in proper contact with 
the chassis? Are all bypass capacitors good? Is it possible that the 
coax between the exciter and amp has a bad shield? What about the 
coax connectors on EITHER end of the exciter cable? Are they properly 
grounded?

I repeat: it sounds to me as though RF output is getting back into the 
input somehow. It may even be through the line cord if the line cord 
passes near the grid input circuitry, or plate output circuitry.

Perhaps the RF choke hasn't enough inductance, or has too much 
distributed capacitance, and is not properly isolating the plate circuit 
from the power supply. Perhaps the RF choke has a resonance at the 
operating frequency, but then again, if it did, it would normally go visit 
its ancestors.

> To me the words neutralization and positive feedback rings a bell, but
> I also understand that these G-G amplifiers cannot be neutralized, Is
> that correct? 

Not entirely, but it isn't trivial. However, your amp worked properly 
before it blew the plate cap and let the magic smoke out of the RF 
choke, so I would suspect something else.

Ken Gordon W7EKB

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