To explode or not to explode, that is the question...
Many (but not all) older sweep circuits are designed to free-run at a slightly lower-than-normal frequency in the absence of a sync signal. The job of the sync signal in that case is to cause a premature termination of the sweep, thereby effecting synchronization. From that description, you can see that synchronization will fail if the sync frequency is lower than the free-run frequency.
If the sync frequency is too high, the sweep oscillator frequency might not be adjustable upward by a sufficient amount, again causing synchronization failure.
In both cases of failure, applied sync pulses will still attempt to truncate the sweep. Because the latter's frequency is no longer related to the sync pulse frequency, the result is a chaotic mess of "sweep/don't sweep" signals. Transistor currents and voltages can exceed device limits, with the typical result being a blown horizontal drive transistor (and less often, a cooked flyback).
Modern monitors tend to use PLLs to support multisync operation. Synchronization failure with these types of circuits tends not to be catastrophic (it's still possible, but the engineer had to have been somewhat incompetent to allow that to happen).
Many (but not all) older sweep circuits are designed to free-run at a slightly lower-than-normal frequency in the absence of a sync signal. The job of the sync signal in that case is to cause a premature termination of the sweep, thereby effecting synchronization. From that description, you can see that synchronization will fail if the sync frequency is lower than the free-run frequency.
If the sync frequency is too high, the sweep oscillator frequency might not be adjustable upward by a sufficient amount, again causing synchronization failure.
In both cases of failure, applied sync pulses will still attempt to truncate the sweep. Because the latter's frequency is no longer related to the sync pulse frequency, the result is a chaotic mess of "sweep/don't sweep" signals. Transistor currents and voltages can exceed device limits, with the typical result being a blown horizontal drive transistor (and less often, a cooked flyback).
Modern monitors tend to use PLLs to support multisync operation. Synchronization failure with these types of circuits tends not to be catastrophic (it's still possible, but the engineer had to have been somewhat incompetent to allow that to happen).