so this has been bugging me for a few weeks now. my 857 was putting out fine, like 100w on HF, then one day i noticed it just felt... weak. checked it with my wattmeter and im getting maybe 15-20w on 40m, a bit more on 20m, and almost nothing on 10m. receive seems completely normal, SWR readings look right, dummy load checks out.

i did the obvious stuff already — reseated all the connectors i could get to, checked the ALC wasn't going crazy (it wasnt), tried it on a different antenna just in case. same deal. no smoke, no smell, nothing dramatic happened that i can remember. just gradually got worse i think, or maybe it was sudden and i didnt notice for a while because im mostly listening lately.

my gut says one of the driver transistors or maybe a bias issue in the PA stage. pulled the service manual and the PA board has those RD70HHF1s in the final. anyone cracked one of these open for a similar symptom? wondering if its worth me poking around with a scope before i just swap the finals and hope for the best.

yeah that symptom pattern — power dropping more as frequency goes up — is pretty classic for a bias problem or one of the driver stages going soft. on the 857 the pre-driver and driver amps are separate from the finals so it could be upstream of the RD70s entirely. before you pull the finals i'd check the bias voltage on the PA board with the radio keyed into a dummy load. if the bias is off you'll see it there and its a much cheaper fix than new transistors.

also worth checking the low pass filter switching — there's relays that switch in different LPF sections per band and if one of those is flaky you can get exactly this kind of frequency-dependent power loss. i had an IC-706 do something similar once and it was a corroded relay contact, not the PA at all. cleaned it with deoxit and it was back to full power. might save you from going down the wrong rabbit hole.

dont rule out the driver board yet. i had an 857 with almost the exact same symptoms and it turned out to be Q1017 on the driver board that had gone leaky. finals tested fine actually. the tricky part is you kind of need a signal tracer or at least a scope to follow the RF through the chain otherwise your just guessing at parts.

theres a good thread on eham from like 2018 or 2019 that goes through the whole PA troubleshooting flow for the 857, might be worth digging up. yaesu's service manual is decent for this radio thankfully, better than some i've dealt with.