Page 154 - Testing Electronic Components

Though crystal don’t become faulty easily like a resistor or capacitor, it is
important for an electronic repairer to know how to test a crystal.

A Crystal and its location marking in a Computer Monitor

Testing the crystal is not a breeze either. You cannot just take out your
trusty meter and test the crystal in it. In fact, there are three methods to
test a crystal: -

Using an Oscilloscope- A crystal oscillator produces a sine wave when
excited. It is appropriate then, to see a waveform representative of a sine
wave on the clock pins. If the clock is not working properly, replace the
crystal. Test the crystal with power on. Place the scope black probe to
cold ground and the main probe pin to the crystal pins. Normally
microprocessors are usually very reliable but not in this case of a Compaq
MV720 Monitor.

Monitor came in with no high voltage symptom. Using the scope to check
on the crystal revealed very unstable waveform and replacing the
microprocessor solved the no high voltage problem and the crystal
waveform shown a perfect sine wave.

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