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I found this under VINYLENGINE and I hope it answers  your question:   Original model direct drive motor circuit use servo motor principle with high frequency signal generate by method such as tuned circuit in discrete transistor base/collector path. Such high frequency signal supply to control servo motor position coils. For generation of accurate high frequency signal much is depend on tolerance of circuit discrete component plus power supply remain stable. For this example component value tolerance in circuit plus variation with temperature make for signal drift over time. When later model introduce quartz crystal lock this allow for improved stable condition plus accurate control of high frequency signal to servo motor control circuit and remove much of original model component temperature plus tolerance drift problem. Later model with quartz circuit also have much improvement from original model circuit when using new design intergrated circuit to replace many discrete transistor component. This also allow for much improved stable operation of circuit.