so this has been bugging me for a couple weeks now. my 857 is putting out maybe 20 watts max on 40 meters but hitting full 100w on every other band i've tested. checked the ALC and it looks normal, SWR is flat into a dummy load so its not an antenna thing. radio was working fine then one day just... wasnt. no idea if i bumped something or what.

already reflashed the firmware thinking maybe some setting got corrupted, didnt help obviously. im wondering if its a PA transistor issue specific to that band but i dont really know how the 857 splits up its driver stages. anyone dug into one of these before? schematics are online but theyre not exactly easy to follow when you dont know what youre looking for

yeah the 857 uses separate driver transistors for different band groups and 40m shares a stage with some of the other HF bands if i remember right, but the final amp switching is done through band pass filters and a switching network so it could also be a relay or PIN diode not fully switching over. those PIN diodes are a known weak point on the yaesu HF portables especially if the radio ever saw any RF feedback or a bad SWR event.

what i'd do first before pulling the PA apart is check the voltages going into the driver stage on 40m versus a working band. if the bias looks right then you're probably looking at the filter switching. theres a service manual floating around that has the voltage charts, way more useful than the schematic alone. also worth checking if the low power follows you if you switch to 39.something vs 41.something — if it drops off sharply at the band edge that points more toward the LPF relay than the PA itself.

had almost the exact same thing happen on my 897 which is basically the same guts. turned out to be one of those tiny relays on the LPF board had a dirty contact. i just reseated the board connectors first and it actually helped a little but didnt fix it fully. ended up hitting the relay contacts with some deoxit on a thin piece of card stock slid underneath and that sorted it. probably a temporary fix but its been two years now so ill take it lol