so this has been driving me nuts for about two weeks now. my 857 started putting out maybe 5-6 watts on HF when it should be doing 100. SSB, CW, doesnt matter. all bands affected more or less the same, maybe slightly worse on 10m but hard to tell. receive seems totally fine, SWR meter in the rig reads normal when i key up into a dummy load.

already swapped the mic, checked the ALC meter (barely moves which is suspicious), reflashed the firmware just in case, and done a factory reset. nothing changed. pulled the covers and nothing looks obviously burned. all the finals look fine visually, no scorching on the board near Q1034 or the driver area that i can see. voltage on the finals rail reads about 13.4v which seems right.

the weird thing is it happened kind of gradually over like a week, wasnt a sudden death situation. one day i noticed contacts werent coming back to me as strong and checked the wattmeter and yeah, way down. i half wonder if its the driver stage rather than the finals themselves since all four finals failing at once seems unlikely. anyone dug into one of these before?

the ALC barely moving is actually a pretty solid clue there - on the 857 when the driver stage is weak the ALC loop never really gets a chance to do its thing because there isnt enough signal making it to the finals to fold back. id put money on Q1010 or something in that driver chain before blaming all four finals.

if you have a scope you can probe the gate drive signal going into the finals and see if its actually there and at the right level. without a scope you can at least do a rough check by measuring DC resistance on the finals with them out of circuit - they shouldnt read as shorts but thats not a guarantee they're good either. the gradual onset really does sound more like a bias drift or a driver part going soft than catastrophic final failure. Yaesu service manual has the alignment procedure for the TX chain, the bias pots are worth checking too, VR3035 i think from memory but double check that.

had almost the exact same thing on mine a couple years back, low power on all bands, looked fine on receive. turned out to be a bad solder joint on one of the connectors on the PA board, the one that carries the drive signal over. intermittent connection so the drive level going to the finals was all over the place. reflowing that fixed it completely, like an hour of work total once i figured out what it was.

worth just going over the board with a magnifying glass and reflowing anything that looks even slightly dull or cracked before you start swapping parts. these rigs are old enough now that cold joints are pretty common especially if it ever got warm in a car or whatever.