Those KEYIN0, 1, 2 inputs to that microcontroller should be pulled up to 3.3VDC (NS3.3VD, at th VDD pin to the micro) b R5, 6, 7. The pushbuttons connect the circuit to ground. When a pushbutton is depressed, the KEYIN inputs get pulled down to a voltage the is the result of the voltage divider formed by the combination of R5, 6, or 7 and whichever series resistors are being shorted to ground by the pushbuttons depression. The micro senses and interprets the voltage at KEYIN to determine which key is depressed.
If no switches are depressed, the KEYIN voltages should all get pulled up to a solid 3.3V.
Q3 is an electrical "switch" that should only pull collector of Q3 to ground (~0.2V) like a pushbutton when the PWRTRIG signal is asserted to turn on Q3.
The STBY net s.b. pulled up to 5V b/c it's tied to the NS5V on the A/V/Psupply board. This pushbutton similarly grounds the STBY net when depressed, and should drive the U6 low when asserted.
That U6 device is I'm sure a custom programmed PAL that's interpreting all of this I/O and taking action on it.
U6 P2 0 and 1 are probably OK as you're measuring ~2.5V w/ the DMM. That's the clock for U6 so it should measure middle of the road w/ a DMM. Checking it w/ a scope would be better.
Is the PWROUT signal going high when the STBY PB is depressed?
That PWRTRIG signal at U6-17 wandering all over may be an issue. That should be set at one state or the other based on the logic of U6. It should probably be low by default based on the fact that R33 is trying to pull the net at Q3-2(base) down. That turns Q3 off so it's not pulling the KEYIN2 net down. PWRTRIG should only go high when driven high by U6. I'd definitely check Q3 to see if it's OK. KEYIN2 (and Q3-3) should rise to 3.3V when no key depressions or IR/Power control is trying to power the unit. If Q3 is OK, you might try isolating Q3 from PWRTRIG by pulling R35 just to see if Q3 shuts off properly (which should make Q3-3 rise to 3.3).
Fun stuff.
