This is a Yaesu FL-2100B I repaired for a friend. He was using this on 29.600 FM with high carrier, around 600W out. He noticed power beginning to sag, then an arc under the hood, and the magic smoke was released.
I found the plate choke burned from a hot-spot, the plate choke bypass cap on the dc hot side blown, grid resistors blown, and a shorted tube.
I replaced the shorted tube, replaced the blown grid resistors, replaced the bypass doorknob, and coated the rf choke in the hot area. I used the grid dip meter and found a resonance around 29.700...very close to where he operates on 10m fm simplex. That's probably what burned the plate choke. I added a 2.5 mH choke between the main rf choke and the plate caps, and that pushed the resonance past 30mhz. After I tested the amp, I added an additional fan to aid in cooling when operating on 10m FM at 600w carrier.
