1. Check the crystal oscillator's clock signal: To determine if the crystal is functioning properly, use a digital multimeter to measure the voltage across the two ends of the oscillator. If the voltage at both ends is approximately half of the power supply voltage, or if one end is significantly lower than half the supply voltage (which is more common in high-frequency crystals), the oscillator is likely working correctly. However, if one end shows a voltage higher than half the supply voltage, the crystal may be faulty. While this test can indicate basic functionality, it doesn't confirm whether the actual frequency matches the specified value. For example, in a VCD player using an ES series decoder chip, if the 27MHz crystal is not oscillating at the correct frequency, you might notice color distortion or the boot screen appearing enlarged and out of color. Similarly, if the 16MHz crystal in the servo circuit is off, the disc may spin without being read. In such cases, replacing the crystal is often the best solution.
2. Test the reset circuit: This process requires an analog multimeter because digital meters may not capture the brief voltage changes accurately. Set the analog multimeter to the 2.5V range. Connect the red probe to the positive side of the power supply and the black probe to the reset pin. Then, turn on the power. If the needle slightly swings, the reset circuit is functioning normally. The swing’s amplitude depends on the reset time setting. This method applies to low-level reset circuits, where the reset pin should be at the power supply voltage when working properly. For high-level reset circuits, where the reset pin should be at 0V when active, reverse the probes: connect the red probe to the reset pin and the black probe to ground for testing. Always ensure that the multimeter is set to the correct range before performing these checks to avoid damage or inaccurate readings.
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