The motor is a motor-generator combo, there is no digital speed sensor that would produce a square wave. The generator is an AC generator - evidenced by the sine wave output - but the frequency is not what the speed control circuit cares about. The schematic clearly shows an amplitude (diode) detector on the generator output, so it is looking at generator voltage only. C144 and high values on adjacent resistors makes it a peak detector. Difference amplifier A107 compares the divided down generator output (peak) voltage to 5V, and drives the motor via Q112 booster. First thing is to see if the 5V is clean, stable, and within tolerance. Next check +12V into Q112 to be clean, stable, and within tolerance. Since C144 stabilizes the generator voltage, if it went bad then the generator signal to A107C would pulse and muck with the speed regulation. On an old unit electrolytic caps go bad first, I would almost bet it's a problem with the +12V or +5V supply because of bad power supply filtering.