Simple Crystal tester – The idea for this crystal tester circuit sprung out of the need of testing a large number of oscillator crystals lying unused in a big hobby box. testing the crystals one by one without the appropriate device would have been very slow and a time consuming task. Commercial crystal testers are however very expensive, that is why this simple electronic crystal tester was born. The electronic circuit can be used to test crystals with frequencies from 100kHz up to 30MHz. The current consumption is low: around 10 to 14 mA.
This Simple Crystal Tester circuit uses two BC108 NPN transistors in an oscillator configuration to test whether crystals are functioning properly. The circuit is powered by a +3V supply and uses the crystal under test (connected at XTAL) as the frequency-determining element.
The first BC108 transistor forms the main oscillator stage, with the 100K and 1K resistors providing proper bias. The 50K potentiometer allows adjustment of the oscillator drive level or bias point for optimal operation with different crystals. The 0.03µF capacitor provides AC coupling, while the 1K emitter resistor stabilizes the operating point.
The second BC108 transistor acts as a detector/amplifier stage to amplify the oscillations generated when a good crystal is connected. The 56pF capacitors provide RF coupling between stages and help maintain oscillation. The 1K resistors bias this detection stage appropriately.
When a functioning crystal is inserted at the XTAL connection points, the circuit will oscillate at the crystal’s resonant frequency. This oscillation is detected and amplified, causing current to flow through the 0.05mA meter, which indicates that the crystal is good and oscillating properly. The Germanium diode rectifies the RF signal to provide a DC indication on the meter.
The 105 WW component (likely a small resistor or RF choke) and 47K resistor complete the detection circuit. If no deflection appears on the meter when a crystal is inserted, it indicates the crystal is defective or not oscillating, suggesting it’s either cracked, has poor contacts, or is otherwise faulty.
This simple tester was commonly used by radio technicians and amateur radio operators to quickly verify crystal functionality without needing expensive test equipment.
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