so ive been sitting on doing the mars/cap mod on my 857D for probably two years now, always felt a little sketchy about opening it up even though i know its just a couple jumpers. finally pulled the trigger last weekend and honestly it went fine, took maybe 20 minutes including finding my glasses.

the thing i actually wanted to talk about though isnt really the mod itself — its what i did after. once you have the extended TX range unlocked i went ahead and did the cooling fan mod at the same time since the board was already out. the stock fan on these runs way too conservative in my opinion, kicks in too late and i was getting thermal shutdowns running digital modes for any length of time. theres a couple different approaches people post about but i just wired the fan direct to a 12v rail through a small resistor to run it continuously at a lower speed. no more shutdowns and honestly not that much louder than before.

might do the CAT control timeout fix next, anyone done that one? i keep losing connection to fldigi after the radio sits idle for a bit and im pretty sure theres a menu setting or possibly a hardware thing i can do about it.

the fan mod is worth it, did mine probably three years ago. i actually went a slightly different route and put in a small temp-controlled circuit so it still ramps up with heat rather than running flat out all the time. used one of those little LM35 sensor boards and a basic PWM controller, total cost like four bucks off aliexpress. the 857 runs so much better on JS8Call now, i used to dread anything over an hour of solid operation.

on the CAT timeout thing — im pretty sure thats actually on the fldigi side, not the radio. theres a setting buried somewhere in the rig control config for polling interval and there might also be a keep-alive option depending on your version. i had the same issue and fixing the polling interval in fldigi sorted it without touching the radio at all. check that first before you go digging around in the hardware again.

good write up. i did the same mod on my 857 ages ago and it's been solid. one thing i'll add — double check your SWR on the extended freqs before you actually transmit anywhere outside the normal ham bands, even just for testing. found out the hard way my antenna tuner doesnt like some of those freqs and i got a little more reflected power than i expected. nothing broke but still.