Mistracking due to a shallow groove in the vinyl
Mistracking can occur if the groove of the record becomes too shallow. To understand this we need to consider the nature of the groove:
In an old-fashioned mono record, the groove simply moves from side-to-side to encode the signal.
Stereo records use a V-shaped groove in which one wall carries the left signal, the other wall carries the right. They are modulated independently.
If the input signal happens to be mono, or not to have a great deal of stereo content, then the result will be a groove that does indeed only move from side to side.
The vertical component will be zero. In the worst case however if the signal is out of phase (i.e. the original signal was identical in both channels - mono - then one channel was inverted), then the groove will not move from side to side at all, only up and down.
In the extreme, the depth of the groove could be reduced to zero. When there is no groove, the stylus has nothing to track and will slide into the adjacent groove. This will happen repeatedly.
Often such a locked groove is the result of a scratch, but it has happened in the past that records have been released where the groove did indeed become dangerously shallow and not all record players were able to track properly.
Although pure out of phase signals should be rare, the thing to look out for is high bass levels panned left or right, since these are the next worst offenders.
The mastering engineer's brute force cure is to mix low frequencies together, so that the bass content of the audio is mono, while higher frequencies are still stereo.
There is a further advantage in 'monoing' the bass. In popular music, it is often thought desirable that the bass is at a high level.
If the bass is panned center, and plays through two speakers, then obviously it will be 6 dB higher than if it were panned hard left or right. This applies to CD too.
If the worst comes to the worst and records are pressed where the groove in places does become very shallow, they will be difficult to play even on a good system. The cure may be drastic - extreme stylus force, or ultimately if the record just has to be played, a coin placed on the headshell may keep the stylus in the groove. It is recommended that this is done only once so the record can be copied onto another medium. Bear in mind that this procedure risks damaging the cantilever of the stylus.