This one board doesn't post: https://theretroweb.com/motherboards/s/u-board-tk-82c491
Actually not exactly that one in the picture but another one I own. Reset is always low and board does nothing. Chipset is UM82C491F/493F. I guess there are no datasheets for these chips, even pinout for UM82C493F.
Reset is low right from the power on, just a very fast pulse while capacitors charges but that's about it. Had to start measuring... 493F oscillator is silent but that's probably due to active reset state. Reset button has no effect to anything I could observe. RSTIN pin #171 of 82C491F is about 2.0V, so there must be something wrong.

RST signal comes from 82C493F pin #29->#171. Resistors are used to connect reset switch and power supply PWRGOOD signals then routed to 483F pin #30, so POWERGOOD keeps reset high and reset button grounds it, simple. Small capacitor leveling the signal and there's also one clamping diode to +5V. Voltage in this point was about the same 2.0V. When I soldered pin #30 up (N.C.), voltage at the capacitor rose to 5V and pin #29 down to 0V which sounds correct. I guess 493F internal logic is fried, maybe I'll order some and hope for the best result.

I draw fast schema for later use and if someone has similar case.