SERVO AND MOTOR FAULT SYMPTOMS
A fundamental point to remember in servicing servo and motor circuits is that capstan-speed faults will upset sound reproduction from the (mono) longitudinal track to give wrong pitch, wow or flut- ter – picture tracking will also be affected. Head-drum speed fault symptoms are confined to the picture; wow and flutter here gives rise to a lateral wobble of the picture, which will break into horizontal lines if drum speed deviates far from the norm. Loss of the reference signal (i.e. PAL or local crystal oscillator stopped) will usually greatly increase motor speed, and in many designs loss of feedback signal – faulty FG or PG generator, for instance – will do the same. In the case of a fast-running head drum, it can be slowed by friction from a finger to check for correct line frequency at normal speed; at the same time the effect on feedback signals (and their influence) can be monitored. If continuity is present, they should be acting to drive the motor faster, and the diagnosis then consists of finding out why the motor is not responding to the ‘turn-down’ signals being developed at excessive motor speeds.
A head or capstan motor running at the correct speed, but whose
phase control loop has failed, will drift in phase to give the following symptoms. Head servo out of control on record – head-switching point disturbance drifts up or down screen: check for 25 Hz head PG and field sync pulses. Head or capstan servo out of control on playback – cyclical tracking errors in which part or all of the screen is affected by noise: check PG pulses, tracking delay system, and particularly the control track pulses from the stationary head. A worn, dirty or misaligned control head will pick up insufficient pulse amplitude to operate the servo. This malfunction is often masked by the action of a muting circuit, which only unblanks video signals when the servo is locked up: the idea is to prevent unstable pictures being displayed.
In general the electrical components of a servo system are much more reliable than the mechanical components. First checks, then, should be for the presence of reference and feedback signals, which depend on crystals, plugs, sockets and transducers – in magnetic and optical form – and on transfer of control pulses to and from tape. Motors, belts and bearings are also high on the suspect list, though brushless direct-drive motors have a lower failure rate than their early permanent-magnet brush-and-commutator counterparts, still sometimes encountered and still widely used for the more mundane deck functions such as tape- and cassette-loading and reel drive.
Once the mechanical components have been exonerated, the fact that both speed and phase correction sections are closed loops is a considerable aid to fault diagnosis. A fault condition implies that the loop is broken, and at the ‘severed end’, as it were, a strong corrective signal will appear in an attempt to restore normality; it is upon tracing this that the diagnosis should be concentrated.
Setting-up procedures for servos (especially in trick playback modes) vary widely with machine type, age and manufacturer. Except in simple machines with few presets and obvious functions no attempt should be made to diagnose faults in the servo circuits without the manufacturer’s service manual and all necessary instruments. More on test equipment and diagnosis procedures will be found in Chapter 23.