so this has been bugging me for a couple weeks now. got my 857 out for a portable op and noticed my signal reports on 40 were way down, checked with my wattmeter and yeah, barely 5 watts on a good day at full power setting. every other band seems totally normal, 20m is fine, 2m is fine, HF in general seems okay except 40. tried a different antenna, different feedline, same result so its not the antenna system.

the radio was fine last time i used it a few months ago, nothing dramatic happened to it that i know of, wasnt dropped or anything. i pulled it apart and did a visual inspection and nothing looks obviously cooked or burnt. im thinking maybe one of the LPF switching relays or one of the finals is degraded? the 857 uses separate LPF boards for different band groups if i remember right and 40 shares a filter with some other bands i think but im not sure those are behaving weird.

anyone been through this before with an 857 or similar yaesu? wondering if i should just send it to a shop or if this is something diagnosable at home with a scope and some patience.

yeah the 857 has a known issue with the driver stage on certain bands. 40m specifically can be tricky because of how the band switching works on that radio. first thing id check before assuming finals is the ALC -- hook it up and watch the ALC meter while youre transmitting on 40, if its pegged or doing something weird that can actually pull power down and make it look like a finals problem when its not.

also worth checking is whether the PA voltage is actually getting to the finals on that band. there's a switching transistor involved and those do fail. i had an 857 come through my bench with like 3 watts on 20m and it turned out to be a bad Q1022 or somewhere around there, one of the switching fets. pulled it, put in a replacement, back to full power. the service manual has the voltage test points labeled pretty well so if you have a DMM and the SM you can at least narrow it down before buying parts.

im not super experienced with digging into the finals and stuff but i had something similar happen with an older rig and it turned out to be a dirty relay contact. might be worth trying if you can get to it. i used some contact cleaner on the relay and cycled it a bunch of times and it helped, not a permanent fix obviously but kind of confirmed where the problem was.