What is the function of the little potentiometers that seem to be present on most XT and a lot of 286 motherboards?

There's no onboard video signal, so it wouldn't be related to brightness/intensity there, and they're present on boards that don't have CMOS memory or batteries, so it couldn't be part of the charging circuit.

Does it maybe have something to do with clock generation from passive crystals (and ceramic resonators)?

On my EAT-12 286 board, there's two of them, one right next to the realtime clock crystal and another next to the 14.3 MHz keyboard controller crystal. (the latter might be used for the ISA bus and/or FPU clock on boards)

There doesn't appear to be a third one to complement the 24 MHz crystal, though.

I assume these are factory set and not normally user (or technician) adjusted, but they're quite clearly screwdriver adjustable from the top, and could've been messed with at some point (or might do better with adjustment for other reasons, maybe if ceramic or film type capaictors in these circuits age at all).

I'd like to know if mis-adjustment might damage anything and/or just cause instability (or complete failure to POST).

I'm trying to troubleshoot why that EAT-12 board now fails to POST at all after seeming to work OK when I got it (and finally getting around trying out booting DOS on an IDE drive), but the day after that test it stopped POSTing at all. (lots of other possible factors, including oxidized or worn/non-springy DRAM sockets and the fact I swapped out RAM several times previously, and swapped the QUADTEL BIOS for an AMI one from a similar ACT chipset based board, among other things: though corrosion in the battery area is minimal and nowhere near the RAM, the sockets on the board in general seem fairly dirty and/or oxidized)

It's the same chipset as is on "Kixs's 286 to the Max" board, so a relatively fast one but no EMS support and apparently no upper memory support in RAM either. (maybe reserving that for ISA card RAM expansion, and maps 384kB of the first 1MB of RAM into the HMA, so you get more XMS RAM for what that's worth: not as nice for real-mode stuff, though)
Kixs's 286 to the Max

But I'll make a separate topic for that board (or boards), and maybe the ACT chipset in general. (I was about to do the latter when my board sopped working)

Anyway, here's pics of the sort of pots being used. (the little roundish brown things with a hole in the top and gray adjustment knob inside; one next to the 14.3 MHz crystal, to the left of the ACT chip in img 1315, and the other on the upper right, next to the front pannel I/O header pins, next to the tiny, cylindrical clock crystal: presumably 35,768 Hz)