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I have a Yaesu FL-2100B amp that apparently is developing a problem. In over a years use, I have noticed that the position of the loading knob has become increasingly lower to the point where it is now at "0". The antenna has remained the same and I,am using an external tuner which tunes the antenna to a flat match. Is it possible that some component in the amp is slowly changing value? Changing the tubes does not make any difference. Any ideas?

If I remember correctly the Plate Tune capacitor will rotate 360º but the Load capacitor will only rotate 180º. Is it possible that the knob has slipped on the shaft? That would cause the knobs pointer says 0 but the capacitor is not at 0.