The front plate of the S15 head gives you access to the signal amplifier boards. You'll find 4 potentiometers adjusting the voltage and the symmetry of track A and B. A digital scope turned to X+Y provides an image of a circle, which is been adjusted to 1.2 V (if I remember correctly). However, there is no reason, why these pot's have to be re-adjusted. If your signal appears to be weak, then more likely because of a dirty encoder disc.