At the end of the production process, all STK-S4 pickups are fine tuned for hum-canceling. Each pickup is essentially 'injected' with hum, and measured for the amount hum it cancels. To achieve the maximum amount of hum-canceling, a small resistor is often used to fine tune each pickup. Since each coil that we wind is very slightly different, the value of that resistor is not always the same, and in some cases, the resistor is not needed because the pickup is already rejecting the maximum amount of hum.
WRITTEN ON JUNE 15, 2015, BY