so this has been driving me nuts for about two weeks now. my 857 is running maybe 20-25 watts on 40 meters when it should be doing 100. every other band is totally normal, 80m fine, 20m fine, 10m fine, even 6m is putting out properly. just 40m is being weird.

i already checked the obvious stuff — ALC is behaving, no overtemp, SWR is fine on a dummy load so its not the antenna. swapped coax just to rule that out. power output is consistently low no matter how long i transmit so its not a thermal thing.

ive been poking around and somebody mentioned the driver transistors can fail partially on a single band but i honestly dont know enough about the RF stages in this rig to know if thats plausible or if im barking up the wrong tree. anyone dealt with something like this before? feels like it should be diagnosable without just throwing parts at it but im not sure where to start probing.

yeah that band-specific power drop usually points to either the LPF switching or something in the driver chain. the 857 uses switched low pass filters for different bands and if one of those relays is corroded or not fully seating you can get weird insertion loss on just that band. worth pulling the covers and checking if the relay for the 40m LPF is actually clicking when you transmit — you can usually hear them with everything quiet.

if the relay checks out then id be looking at the pre-driver stage, there's a small transistor in the exciter board that can degrade and lose gain in a specific frequency range, its not super common but it does happen especially in rigs that have seen a lot of use or been transported a lot. do you have a service manual? the Yaesu one for the 857D is actually pretty good for tracing the signal path. check the voltages on the PA board while youre in there too, a drooping supply rail on just that section would also do this.

had something similar on an older 706 and it ended up being a cold solder joint on the filter board. not saying thats it but worth reflowing the connections around the LPF section before you go deep into the transistors. saved me a lot of headache once i finally found it, looked totally fine visually but under magnification there was a tiny crack around one of the relay pins.