so ive been sitting on doing the mars/cap mod on my FT-857D for probably two years now, kept putting it off because honestly i was nervous about voiding warranty on a radio i paid decent money for. anyway the warranty is long gone at this point so i finally cracked it open last weekend.

the mod itself is dead simple, just moving a couple diodes on the main board, theres a million guides online and they all basically say the same thing. took me maybe 20 minutes including the time i spent second guessing myself and triple checking which diode was which. radio came back together fine, transmits on extended ranges now no issues.

but heres the thing i actually wanted to ask about — while i had the thing open i noticed the cooling situation in there is kind of grim. the fan doesnt kick on until the radio is pretty warm and on extended digital mode sessions like i do with FT8 it runs hot. has anyone swapped in a higher cfm fan or done anything to improve airflow in these? i know theres a mod floating around where you tie the fan to always on at low speed but i cant remember where i saw it and searching hasnt turned up much. the stock fan is just so loud when it does run that id almost rather have a quieter one running continuously than the on/off cycling.

yeah the thermal management on the 857 is pretty mediocre for a radio that size, yaesu kind of crammed a lot in there. i did the fan mod on mine maybe three years ago, basically just put a small resistor in series with the fan power line so it runs slower all the time instead of the on/off thing. its quieter overall and keeps temps more stable. i think i used something like a 47 ohm resistor but honestly dont remember exactly, you'd want to measure your fan's current draw and dial it in properly. there's a thread on eham somewhere that goes into the specifics, might take some digging.

the diode mod is solid btw, been running mine that way for ages, never had any issues from it.

I went a slightly different direction on mine — ended up just modifying the thermistor threshold so the fan kicks in earlier rather than messing with the fan itself. It's a bit fiddly to get right because you're basically adjusting when the control circuit triggers, but the end result is the fan runs at normal speed just... sooner. Temps during WSPR beaconing sessions dropped noticeably. The stock fan is actually decent if it just ran more aggressively earlier in the thermal cycle. That said if the fan noise bothers you the resistor trick the other guy mentioned is probably the easier path. I've also seen people swap in a Noctua fan cut to fit but that seems like more work than its worth to me personally.