It's a pretty clever little circuit. It's a four pin connection. Pin
4 is ground, 3 is 3.3V. Pin 1 goes to the menu button with a basic
pull up resistor, switched ground configuration. Pin 2 goes to all
the other buttons in a voltage divider configuration. Power button
pulls pin 2 to ground, the other buttons switch in different legs of
the divider. The voltages measured are
CH+ .50V
CH- .88V
V+ 1.30V
V- 1.68V
Input 2.51V
Power 0V
This corresponds to the circuit tracing. By that, I mean the CH+
button swtiches in 1.5k, CH- switches in another 1.5k for 1.5k + 1.5k,
V+ switches in 1.5 + 1.5 + 2.2k, V- 1.5 + 1.5 + 2.2 + 2.7k, and input
1.5k + 1.5k + 2.2k + 2.7k + 10.5k. All are divided into themselves
plus the pullup of 10k (for V- it would be (1.5 + 1.5 + 2.2 +
2.7)/(1.5 + 1.5 + 2.2 + 2.7 + 10). I'm awful w/ ascii schematics, I
can give it a shot if anyone wants. It looks like the circuit's fine,
so it's somewhere else downstream that either isn't measuring
properly, or is doing the wrong function for the wrong voltage. Not
suprising for a lightning strike.

Steve