so ive been sitting on doing the mars/cap mod on my FT-857D for probably two years now, always nervous about voiding warranty (which is already long gone lol) and just generally poking around in there. finally did it last weekend and honestly it was way less scary than i thought. just the one diode, took maybe 20 minutes including the time i spent second-guessing myself and looking up the service manual PDF for the third time.

the main reason i did it finally was i got roped into doing some emcomm stuff with the county group and they wanted us to have at least the capability to hit some of the interop freqs. makes sense i guess. anyway the mod works fine, transmits outside the ham bands now without complaining, though i still need to double check my watt output is consistent across the extended range because i feel like it drops off a bit on the low end.

while i had it open i also cleaned up the fan and reseated a couple connectors that looked a little sketchy, no idea if that did anything but felt productive. does anyone know if theres a mod or setting to make the fan kick on earlier? mine seems to wait until the thing is already pretty warm before it spins up and i run it mobile in summer and get a little paranoid about it.

yeah the fan behavior on that rig has always bugged people, theres actually a pretty well documented mod floating around where you adjust the thermistor threshold — i want to say its on one of the eham threads or maybe the 857 yahoo group archive. basically you swap out a resistor to lower the trip point so the fan comes on sooner and at lower speed instead of just kicking in full blast when its already hot. i did something similar on mine and its much better for extended mobile use. search for "857D fan mod" and you'll find a few variations, some people just wire the fan to run continuously at low voltage which is simpler but a bit noisier.

on the power output thing you're probably right that it tapers off on extended TX, thats normal for that radio outside its designed range. as long as youre not trying to run full power into a mismatched antenna on those freqs it should be fine for emcomm use.

i did the same mod on mine maybe three years ago, went fine. one thing i'll add is just make sure your SWR protection is still working right after — i had a weird issue where it seemed like the ALC was behaving a little different on 10m after i did it, might have been totally unrelated but worth checking. also if you haven't already the display dimmer mod is worth doing too, the default display brightness is way too high for night driving and theres an easy trim pot adjustment for it. small thing but quality of life improvement for sure.