When it comes to frequency generating circuits or rather precise
oscillator circuits, crystals become a crucial part, especially because
they play an important role for generating and maintaining accurate
frequency rates of the particular circuit. However these devices are
prone to many defects and are normally difficult to check through
conventional DMM units.
The shown circuit can be used for checking all types of crystals
instantly. The circuit itself is a small transistor oscillator circuit
which starts oscillating when a good crystal is introduced across the
indicated points in the circuit. If the crystal is a good one, the bulb
lights up showing the relevant results and if there’s any defect in the
attached crystal, the bulb remains switched OFF. link
