so this has been bugging me for a few weeks now. picked up an 857d off a local guy, worked fine for the first month or so, then started doing this thing where it just drops output randomly. like ill be mid-qso and the power just falls off. ALC meter looks normal, SWR is fine on my dummy load, but the watt meter is showing maybe 4-6 watts instead of the 100 im expecting.

turning it off and back on sometimes fixes it temporarily, sometimes doesnt. doesnt seem to be heat related because it can happen cold too. ive already reseated the finals connector and checked the PA board visually, nothing obvious jumping out at me. someone in another group mentioned the bias pots drifting on these but i cant find much info on that specifically for the 857. anyone dealt with this before or have a schematic section they can point me to?

yeah the bias drift thing is real on these, i had an 847 with similar symptoms a while back. the PA bias pots on the yaesu HF portables are small trimmers and they do drift over time especially if the radio got hot at some point. before you go poking around in there though, definitely check the ALC line — theres a known issue where the ALC feedback can misbehave and actually throttle the PA without the meter showing it properly. you kind of need the service manual to trace it right, the schematics are on radiomanual.net if you dont already have them.

also worth checking all the band data lines on the PA board connector. i forget which pin it is but if one of those gets flaky the radio can switch to a low power mode silently. seen it happen when the connector gets a little corroded.

mine did something similar, turned out to be a cold solder joint on the driver board. not the finals themselves. ran it for about 20 minutes on CW into a dummy load and then did a heat gun pass about 6 inches away over the driver area and it started acting up immediately, helped me nail down the location. tiny crack in a joint near Q1017 if i remember right, its been a while.