On most Fanucs that use resolvers and tachs, the tach signal is passed directly through to the servo amplifier board from the motor's tach. The ones that have pulse coders use Frequency-to-Velocity converters to generate a tach signal from the pulse coder signals, and that tach signal is passed to the servo board.
In your case, I doubt that there are any ICs on the resolver board to process the tach signal. There wouldn't be a need for any signal processing of the tach that I can see. The TSA and TSB signals should just pass through to the CN1 connector on the servo board.
The most likely scenario is that you have a blown op-amp on the servo board that is putting either +15v or -15v onto the TSA or TSB signal. Here's a test: pull the fuses on the C axis servo so it can't move, then disconnect the TSA signal (pin 7) and TSB (pin 20) from the CN1 connector on the servo board. (You can't just unplug CN1 because the servo would just alarm out and not allow you to perform this test). Enable the servo with the M18 command and measure the voltage of TSA on the servo board. (CH2, I believe). If you see 15v, then the source is on the servo board (probably the first 741 op amp that the tach signal goes to.) If you see 0v on the servo board's CH2, then the 15v is indeed coming from the main board or the resolver board, and I'm all wet.
You didn't mention what type of servo board you have. The 11M used 3-phase DC, PWM DC and AC servos. The boards are all different.