AUDIO/CONTROL HEADS
Because of the low linear tape speed, the narrowness of the longitudinal audio track, and the lack of a pressure-pad as used in audio-only tape recorders, problems are more prevalent with the audio head area of a videorecorder. Symptoms of a worn or dirty audio head are low, muffled sound lacking in treble response; and of a worn control-track head erratic servo lock on playback – especially of its own recordings, since the head will have been used twice. In machines where a signal-mute circuit is used on playback there will be no output signal with the servo unlocked, though trick-mode pictures will be displayed.
For checking and setting up A/C heads an alignment tape is required. This has a standard audio track with which adjustment of height, level and azimuth can be made. Height setting is only required when the head assembly has been replaced, and is made by adjustment of a single nut or three screws for maximum volume on replay of the test tape. A lack of treble response from ‘foreign’ (i.e. recorded elsewhere) tapes indicates an azimuth error; it is corrected by a spring- loaded screw or nut at one side of the head-mounting platform.
Adjust for maximum level of high-frequency (typically 6 kHz) sound from the appropriate section of the test tape; if a large adjustment is necessary, recheck height before finally setting and sealing the azimuth-adjusting screw.
Whereas the azimuth adjustment rocks the A/C head assembly sideways, a further tilt adjustment screw is provided to rock the head fore-and-aft. This is called tilt or zenith adjustment, and ensures that the top edge of the tape (audio track) bears correctly against the head for consistent sound. Where sound level fluctuates (especially with four-hour and longer tapes) the back-tension should be checked before the azimuth adjustment screw is adjusted to slightly tilt out the upper face of the head: too much tilt will result in intermittent transfer of the bottom (control) track.
The final aspect of A/C head adjustment is its positioning along the tape path, which governs the relative placement on tape of control- and video-tracks. Its main importance is in establishing compatibility, whereby a correctly recorded ‘foreign’ tape will not require any offset of the tracking control during playback. It is set by lateral adjustment of the A/C head for perfect tracking of an alignment tape, with the tracking control in its normal (click-stop) or default (auto-tracking off) position. Where the auto-tracking can- not be turned off, the service manual gives an alternative method of setting the lateral (X) position of the ACE head, typically involving the use of a double-beam oscilloscope to compare timings of CTL and SW25 pulse waveforms.