Crystal Tester Schematic (3)

Parts List:
   R1 = 33K          C1,C2 = 1nF (1000pF)
   R2 = 1K              C3 = 0.1uF (100nF)
                        C4 = 0.005uF (5nF)
Q1,Q2 = 2N3607         La1 = Lamp, GE #2180
D1,D2 = 1N914           S1 = Momentary Push-button, SPST

Misc: crocodile clips, Lamp socket, 9V battery clip, wire, solder, etc.

Description:
Your 40- or 80 meter crystal isn't oscillation--what's wrong? Is it a wiring error? A solder bridge? A faulty component? Maybe it's the crystal! A simple circuit that tests whether or not a crystal is good, this is a real chestnut. First noticed in October 1989 issue of "Popular Electronics Magazine", but the original circuit is credited to Mike Kaufman in Wayne Green's out-of-print book Practical Test Instruments You Can Build according in an article in 73 Amateur Radio Today (1/92, p.22) that updates of the circuit by featuring an output for an oscilloscope or frequency counter and uses an LED instead of the lamp shown. I am offering the orignal tester here because it still works very well.

Transistor Q1, a 2N3607, and its associated components form an oscillator circuit that will oscillate if, and only if, a good crystal is connected to the test clips. The output from the oscillator is then rectified by the two 1N914 signal diodes and filtered by C1, a 1nF (1000pF) ceramic capacitor.
The positive voltage developed across the capacitor is applied to the base of Q2, another 2N3607, causing it to conduct. When that happens, current flows through La1, causing it to glow. Since only a good crystal will oscillate, a glowing lamp indicates that the crystal is indeed OK. No oscillation, no light. Simple as that.

Parts layout is not critical. If possible, first prototype the circuit on a breadboard to check if it works *for you*.
To check the tester, use a crystal of a frequency you can monitor on your receiver, put the tester close to the input of the receiver and close the switch. If you hear the signal and the lamp lights, the crystal, and the tester, and receiver all work. After that test, you can assume that if the lamp lights a crystal under test is good.

The tester should operate well for any crystal with a fundamental frequency from about 2 to 8 MHz. Harmonics are also generated by the tester, although due to the lack of tuned circuits overtone crystals will oscillate at their fundamental frequency.

The NTE85 is a direct replacement for the 2N3607, but any of the cheapo plastic, common, NPN transistor like a 2N2222 will do fine.

Ceramic capacitors work fine.

Any miniature 5 or 6-volt lamp will do instead of the GE #2180.

The circuit is powered by a standard 9-volt alkaline battery and the SPST push button power-switch is included to prolong battery life.


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Page Copyright © 1995, Tony van Roon
Article Copyright © 1989, Mike Kaufman.