so this has been bugging me for probably three months now and im at the point where i need some outside eyes on it. my 857D will just randomly drop carrier mid-transmission, like ill be in the middle of a QSO and the RF just vanishes for a second or two then comes back. SWR stays flat, power meter on the external wattmeter drops to zero then recovers. happens more on 20m than anything else but ive seen it on 40 too. 10m seems fine but i dont work 10 much so that might just be survivorship bias.

ive already reseated the PA board connectors, checked the finals with a meter and they read fine statically, swapped coax thinking it was an intermittent connector. no change. ALC looks normal on the display, mic gain is set conservatively. radio is probably 2015 vintage so not ancient but not new either. anyone dealt with this specific failure mode on this rig or have a starting point besides "send it to Yaesu"

had almost the exact same thing on an 857 a couple years back. turned out to be the PA thermal protection kicking in prematurely because of dried out thermal compound on the finals. the rig wasnt actually getting hot to the touch but apparently the sensor was seeing higher temps than the case was showing. reflowed the compound and cleaned the heatsink contact surface and it went away completely. worth checking before you go further down the rabbit hole.

the other thing i'd look at on those rigs is the power supply voltage under load - if youre running it off a supply that sags even a little under TX the 857 can do weird things. mine did something similar once when a molex connector on the supply was getting warm. measured fine at idle, dropped under load.

might also be worth scoping the ALC line if you can get in there - ive heard of the ALC feedback loop on those going a bit unstable with age on certain component values, though honestly that's getting into territory where the service manual becomes your best friend. the Yaesu SM for the 857D is actually pretty decent and the alignment section has some good troubleshooting flow. doesnt cost anything to downlod and at least youll know what youre poking at before you poke it.