so ive had this 857D sitting on the bench for about two months waiting for me to get around to doing the extended TX mod and i finally did it last weekend. the mod itself is straightforward, theres that one diode on the main board you clip and suddenly you've got coverage way below and above the normal amateur bands. took me maybe 20 minutes including the time i spent second-guessing myself and looking at the service manual three times to make sure i had the right component.

anyway the reason i bring this up is i also went ahead and did the MARS/CAP mod at the same time since i had the lid off anyway, and while i was in there i noticed the stock cooling fan setup is kind of a joke. the thermal management on these radios is not great especially if you're running any kind of digital modes for extended periods. i ended up sourcing a slightly higher CFM fan that fits the same mounting points and wired it to run at a lower voltage most of the time but i jumpered it so it kicks to full speed when the PA gets warm. temps are noticeably better on FT8 sessions now.

has anyone else done any internal work on the 857 or 897 series? curious if theres anything else worth doing while i have the tools out. i was thinking about recapping the audio section but not sure if thats actually gonna make a noticeable difference or if im just looking for things to do at this point

the fan mod is honestly the best thing you can do for those radios if you run digital at all. i did something similar on my 897D except i used a PWM controller tied to a thermistor i stuck to the heatsink with some thermal epoxy. gives you a nice smooth ramp instead of just off/on. the stock fan on these yaesu rigs from that era is barely adequate even for SSB if you're doing any kind of contest operating, forget about sustained digital.

on the recap question -- i wouldnt bother with the audio section unless youre actually hearing something wrong. those elna caps in there are generally fine. where recapping actually makes a difference on older 857s is the PA board if you've got one thats been sitting unused for years, some of those electrolytics get a bit leaky over time and you can see it in the power output starting to sag or get inconsistent. worth scoping the supply rails on the PA while you have it open if you havent already.

yeah i did the tx mod on mine ages ago, pretty much same experience you described. clip the diode, button it back up, done. the only thing i'd add is make sure you go through the alignment procedure afterwards if you're gonna operate much outside the normal bands, the PA is optimized at the factory for the ham allocations and you can lose a few watts if you dont touch it up. not a huge deal for most people but worth knowing.

one thing i did that was more involved was replacing the display backlighting with higher brightness LEDs, the stock ones get dim and washed out in sunlight and i do a lot of portable operating from the truck. pain in the neck to do but i can actually read the frequency now in direct sun which was not possible before