Crystal oscillator coupling capacitor failure cause analysis - News - Global IC Trade Starts Here Free Join

Crystal oscillator

KDS crystal oscillator DSB221SDN

Photocoupler

During maintenance tasks, it's common to encounter situations where the crystal oscillator stops working due to a faulty coupling capacitor. As most of us know, in normal operation, the voltage across the crystal is usually around half of the supply voltage. The typical method used for diagnosis is the voltage test. When a crystal oscillator fails because of a bad coupling capacitor, the usual approach is to replace the two capacitors one by one until the circuit starts working again. However, the underlying mechanism isn't often explored. In reality, this is quite frustrating, as the profitability of appliance repairs has been decreasing, and the time and effort required don’t always justify the return. Due to work constraints, I never had the chance to deeply investigate this issue. Now that I have the time, I feel compelled to look into it more closely.

A few days ago, I repaired a color TV that had a large block failure, causing serious issues. After replacing the large block, the TV worked fine initially, but the next day it wouldn’t power on. Measuring the voltage at both ends of the crystal oscillator, I found one high and one low—far from the expected half of the supply voltage. I didn’t use an oscilloscope to check the waveform. Replacing the 8M crystal (G701) didn’t help, so I replaced the coupling capacitors C703 and C704 at the same time, and the TV finally started working again.

After the customer took the TV, I set aside other tasks and examined the two ceramic capacitors more closely. Looking at them externally, they seemed fine, but since the opportunity presented itself, I decided to dig deeper. I tested them using a bridge meter, comparing the faulty ones with a normal 22P capacitor. Below are the readings for capacitance and equivalent series resistance (ESR) at different frequencies: 60Hz, 100kHz, and 200kHz. The left side shows the faulty capacitor, while the right side shows the normal one.

As seen in the images, the faulty capacitor showed higher capacitance at low frequencies, indicating poor low-frequency performance. More importantly, its ESR at high frequencies was significantly larger—more than ten times that of the good capacitor. This excessive ESR would prevent the 8MHz signal from functioning properly in the crystal oscillator circuit, leading to oscillation failure.

Since there are still customers waiting for repairs, I'll stop here for now. I hope this detailed explanation helps and inspires others who may face similar issues. If you're dealing with crystal oscillator problems, especially those involving capacitors, remember that even small components can cause big failures. A little extra testing could save you a lot of time later.