so ive been scratching my head on this one for a couple weeks now. my 857 is running full 100w on 20, 15, 10, no problem, even 80m is fine. but 40m it drops to maybe 30-35w max no matter what i do. menu settings look right, ALC seems to swing normally. tried different antennas thinking maybe its a tuner thing but it does it straight into a dummy load too so its definitely the radio not the feedline or anything external.

i pulled the manual and started poking around the PA section a little, the finals look ok visually, no obvious burning or anything on the board. i'm wondering if this might be one of the driver transistors being weak on that band specifically? or maybe a filter cap somewhere in the LPF board. anyone dealt with this before on the 857 or the 897 which i think shares the same PA design more or less.

not in a huge rush to fix it since i mostly run 20m for DX anyway but it's annoying me and i'd rather figure it out than just leave it.

yeah the LPF unit on those radios is a known weak spot. each band has its own switched low pass filter and the relays that switch between them can get flaky, causes exactly that kind of band-specific power drop. worth pulling the LPF board and reflowing the solder joints on the relay coils, Yaesu used pretty thin solder on those from the factory and they crack after a few years of thermal cycling. i fixed an 897D with almost the exact same symptoms doing just that, took maybe an hour with a decent iron.

if the reflow doesnt sort it out then yeah look at the driver stage, Q1017 area if i remember right for that band range, but id start with the relays first since its a much easier fix and its probably that.

had something similar on my 706 once, turned out to be a cold solder joint on one of the toroids in the filter section rather than the relay itself. might be worth just doing a full reflow on that whole board while you have it apart. also check the relay contact resistance with a meter if you can get probes on it, should be basically zero ohms when energized, anything above like half an ohm and youre losing power right there.