Frequent Hamfests and electronic flea markets, or any other type of surplus outlet, you know the pros and cons of buying from those sources. On the one hand, they’re an excellent source of hard-to-get parts as well as a haven for bargain hunters. On the other, however, just about everything is sold “as is.” with no guarantee of any kind—it’s strictly “let the buyer beware.” If you’ve ever come home with a pile of components only to find out that half of them were useless, you know that not all bargains are what they seem.
The ideal solution to that problem of course, is to find sonic way to weed out the obviously bad parts before you buy them. The circuit I’ll be describing here has proved useful for just that purpose when digging through stacks of crystals. as well as in troubleshooting my equipment. It is small. easy-to-build, and will, at a glance, let you know if a particular crystal will oscillate. Let’s look at the circuit shown in Fig.1
Transistor QI, a 2N3563, 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 1N4148 diodes and filtered by C1, a .01 uF capacitor. The positive voltage developed across the capacitor is applied to the base of Q2 another 2N3563, causing it to conduct. When that happens, current flows through LED 1 causing it to glow.
Since only a good crystal will oscillate, a glowing LED indicates that the crystal is in deed OK. The circuit is powered by a standard nine-volt transistor-radio battery and the SPST pushbutton power-switch is included to prolong battery life.
The circuit is easy to build, with size for easy portability—the only real consideration, While just about any construction technique will work well, it’s easiest to use a small piece of perforated construction-board.
To use the crystal tester, simply connect a crystal to the test leads and close the SPST pushbutton power-switch. If the crystal is OK the LED will glow brightly. If the LED does not glow, or just glows dimly the crystal is bad and should not be used.
One note on the intended use for the tester is in order here, however. This tester will check any crystal for oscillation. However, it will not necessarily make the crystal oscillate at the frequency that it is supposed to: SO you can’t use this tester with a frequency counter to test for that. What the circuit null do is give you a way to quickly weed out crystals that are obviously bad, and, after all, that is half the battle.
Copyright by Bill Bytheway, K7TTY February 2012