so this has been driving me nuts for a couple weeks now. my 857 was working great, i was running it mobile and everything was fine, then one day i keyed up and my SWR meter barely moved. radio shows full power on the display but im maybe getting 5-10 watts out on a good day, usually less. receive is totally normal, sensitivity hasnt changed at all that i can tell.

i swapped antennas and coax thinking maybe a connector went bad, no change. tried it on dummy load same deal. the final amp section is where my head is going but i wanted to see if anyone else has run into this before i start poking around in there with a meter. i did notice once or twice in the last month before it died it would kind of hiccup mid transmission, like a brief dropout, not sure if thats related or i was just imagining things.

radio hasnt been dropped or anything, just normal mobile use. mounts been solid. wondering if maybe a driver transistor is on its way out or if theres something stupid im missing before i go down the rabbit hole on this

pretty classic symptom of one of the final transistors letting go, the RD70HVF1s in that rig have a reputation for going soft before they fail completely rather than just dying outright. the fact that receive is fine and it shows full power on the meter internally but nothing's actually getting out really points that direction. the internal power reading on those is measured before the finals so you can have the driver stage working perfectly and the finals half dead and the radio has no idea.

before you pull it apart though, check the ALC voltage with a scope or even just a DC voltmeter on the ALC line while you key it up, if something upstream is going haywire with ALC it can throttle the output way down. had that happen on a buddys 897 once and it looked exactly like a final failure but ended up being a leaky cap on the ALC circuit. saved him like 80 bucks in parts. that said in my experience with the 857 specifically, its usually the transistors. Yaesu used to sell them direct but check RF Parts, they usually have them in stock.

yeah i had something similar on mine a while back, turned out to be a cold solder joint on one of the driver board connectors. the vibration from mobile use can work those loose over time. worth pulling the board and just going over all the connectors with a magnifying glass before you assume its the finals, saved me from an unnecessary parts order. not saying thats definitely it but worth 20 minutes before you spend money