so ive been chasing this one for about three weeks now and im starting to think im missing something obvious. the radio puts out full power on VHF and UHF, no problem, SWR looks clean, everything normal. but on HF - doesnt matter what band - im getting maybe 8-12 watts when i should be seeing 100. ALC looks weird too, kinda slams hard right away instead of settling normally.

i already swapped the mic, tried different antennas, checked the coax, reseated all the connectors i could get to. ran the radio into a dummy load just to rule out antenna stuff and same thing. i did have it sitting in a pretty hot car a few months back which i know is not great but it worked fine after that for a while so i dunno if thats related.

before i send it off to a tech or try to dig into it myself - anyone seen this before on the 857? feels like maybe a PA bias issue or something in the driver stage but i honestly dont know enough about that particular radio to be sure. the service manual is... dense.

yeah the heat thing is probably not a coincidence. those finals on the 857 are reasonably robust but if it got really cooked in the car you could have a partially degraded PA transistor - they dont always fail hard, sometimes they just get weak and you end up with exactly what you're describing. the ALC slamming is a pretty good clue too, that suggests the radio is trying to push power and something isnt responding right so the ALC loop is going nuts trying to compensate.

if you're comfortable with a meter and the service manual i'd start by checking the bias on the PA stage, there's a trimmer for that and its not unheard of for those to drift or for a solder joint nearby to go a little cold after thermal stress. also worth just doing a visual on the board around the PA area with a magnifying glass, cracked solder joints around heat-cycled components are super common and easy to miss.

if you dont want to dig in yourself, W4 area there are usually a few guys who do Yaesu work reasonably. sending it to one of the authorized places isnt that expensive compared to what a misdiagnosed repair costs you in time.

had almost the exact same symptom on my 897D a while back, turned out to be the low pass filter relay. one of the contacts was flaky so it wasnt switching properly for the HF bands - VHF/UHF bypass that whole section so they were fine. might be worth looking at that before assuming its the PA itself. lot cheaper fix if thats what it is.