so this has been bugging me for a couple weeks now. my 857 was putting out full power (100w on HF) no problem and then one day i keyed up and the meter barely moved. like maybe 20-25 watts on a good day and sometimes it dips even lower depending on the band. receive seems totally fine, SWR is normal on my dummy load so its not the antenna side.

i did the obvious stuff already, checked the power supply voltage under load (stays solid at 13.8), reseated the mic connector, checked the ALC setting isnt doing something weird. nothing obvious jumped out at me. im wondering if its a driver stage issue or maybe one of the finals is going soft but i honestly dont know how to tell without just shotgunning parts at it.

anyone dealt with this on an 857 before? i dont really want to ship it off to a service center if i can avoid it, i have a decent scope and a few other bits of test gear so im not totally helpless. just not sure what to probe first.

yeah the 857 finals are RD70HVF1s if i remember right, and they do fail gradually sometimes rather than all at once which makes it annoying to diagnose. first thing id actually do is pull the top cover and look at the PA board while its running — check if one side of the push-pull pair is running noticeably hotter than the other. if one device is partly dead the other ends up doing most of the work and you can usually feel the difference even just by careful touch after a short TX burst (be careful obviously).

also worth checking the bias trimmer on the PA board, sometimes those drift. there's a procedure in the service manual for setting idle current and if its way off that could explain partial power loss. the service manual is floating around online, just search for the 857D service manual PDF, its not hard to find. i went through something similar with mine about two years ago and it ended up being a cold solder joint on the driver board which i only found by flexing the board gently while watching the power meter. not saying thats your problem but worth a look before you start swapping transistors.

dont overlook the LPF relays, ive seen those cause weird power issues on band changes on a few different Yaesu rigs. they get oxidized contacts and the insertion loss goes up enough to matter. probably not your main culprit if its happening across all bands but worth knowing. also what band is worst? if its noticeably worse on 10m vs 40m that tells you something different than if its flat across everything.