so ive been chasing this issue for a couple weeks now and im pretty stumped. got an FT-897D that i picked up used maybe 8 months ago, worked fine until recently. started noticing the fan kicking on way earlier than it used to, like barely a minute into a transmission on 40m at maybe 50 watts. temps on the finals are climbing faster than they should be for that power level.

pulled it apart last weekend and checked the obvious stuff — heatsink compound was basically dried out and crumbly which i replaced, cleaned out a pretty impressive amount of dust from around the PA area, checked that the fan was actually spinning freely and not just making noise. reseated everything. still doing it.

output power looks about right on my wattmeter so its not like the finals are gone, but something is definitely dumping more heat than it should. SWR on the antenna is fine, been using the same dipole for years. wondering if maybe one of the driver transistors is partially cooked and making the finals work harder? or maybe a bias issue? i have a service manual but my bench skills are decent not great, i can do component level stuff but i dont want to just start shotgunning parts at it

bias drift is actually pretty common on those older 897s especially if the previous owner ran it hard. the idle current on the PA stage can creep up over time and yeah it'll run hot even at modest power levels while still showing normal output. worth checking the PA bias adjustment — there's a trimmer for it and the service manual has the procedure, you basically need a current meter inline and adjust it down to spec. its finicky but not that hard if you take your time.

also worth scoping the drive signal if you can, sometimes a driver transistor that's going soft will cause the PA to spend more time in a weird operating region. but honestly start with bias, thats the most likely culprit on a rig that age. seen it a bunch of times on the 857 too which shares a lot of the same PA design.

did you check if maybe its an issue with how its mounted in whatever enclosure youre using? i ask because my buddys 897 was doing something similar and turned out the way he had it in his rack was blocking the bottom vents. sounds dumb but worth ruling out before digging deeper into the bias stuff