The mechanism of this crystal tester is very simple. If a good crystal is connected to the test lead, the oscillator will work, and an AC signal will be generated at Q1 emitter. This AC signal will flow through capacitor C3 and trigger the Q2 to light the LED indicator. The diode 1N4148 provide the back path for the AC signal. If the crystal is bad, the oscillator won’t work, and there is only DC voltage level at Q1 emitter. This DC voltage level won’t trigger the Q2 transistor since the capacitor C3 block any DC signal. You can use any high gain high frequency transistor for this crystal tester circuit, in case you can’t find exact transistor series as shown in the schematic diagram.