i never serviced the tt500, but if its anything like the 1200, the fader has contacts at zero that bypass the variable potentiometer (pitch control). when the fader is in the zero position, motor speed is regulated by a quartz crystal at 33.3 rpm. the 1200 has a small rotary potentiometer that controls the zero point for the larger pitch fader. this is adjusted until there is no jump in pitch forward or backwards when entering or exiting quartz lock. if the pitch jumps when entering or exiting quartz lock then you have two zero points. not good for mixing!

pitch range is supposed to be calibrated with a freq counter on the 1200, but i personally didn't care for factory tuning as the pots often varied between one another resulting in one table with a slightly different range and tuning. i calibrated all mine and my customer's to the +6 strobe for a perfectly matched pair of 12's.

sorry if this doesn't help.... shoulda bought the 1200s! =P