SB-200 Oscillation Problems

[Astrom, Kenth B.]

Greetings, I have a somewhat delicate problem with my SB-200 oscillating when connected to a real (antenna) load. 
Bought this baby off e-Bay a few months back, it has the Harbach soft-start module and for to start with, the amp performed pretty well. 

Late one night during the California QSO party in October, a discrete 'puff' made the amp go dark, with a plume of grey smoke slowly rising from the amplifier. 

Naturally I turned it off and in my wildest fear I naturally suspected that the HV transformer had packed in. 

It turned out that the anode capacitor C24 (1000 pF) had shorted and also completely melted the RF choke RFC3. I order a new HV capacitor online and also got hold of a choke replacement for RFC3 (1.1 mH). 

In the same sweep, I also replaced the existing 125uF filter capacitors with new 220uF ones from Mouser. 

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 swithed 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. 

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! 

I have tried two (2) different exciters (HW-101 and SB-104) and the result is the same, even if the 'Mic Gain' is turned all the way down CCW. 

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? 

I have also read someone who had a similar problem years back where the filter caps had been replaced and it turned out it was the old bleeder resistors (too low value) which caused the amp to oscillate. 

I would very much appreciate if someone could put me in the right direction here and at least give me a hint of where to start. 

Many thanks in advance. 

73 de Kenth / W1JKP
Northford, CT 


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