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

Crystal oscillator

KDS crystal oscillator DSB221SDN

Photocoupler

During routine maintenance, it's quite common to encounter situations where a 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 about half of the supply voltage. The typical method we use for diagnosis is simply checking the voltage. When the crystal fails because of a bad coupling capacitor, we often just replace the two capacitors one by one until the circuit starts working again. But honestly, we rarely dig deeper into the root cause. That’s partly because the repair industry has become less profitable, and the time and effort required to investigate such issues aren’t always worth it. I didn't have the time to explore this further at work, but now that I do, I feel it's a good opportunity to look into it more deeply.

A few days ago, I repaired a color TV that had some serious issues with its main block. After replacing the main IC, the TV worked fine, but the next day it wouldn’t power on. I checked the voltage across the crystal oscillator’s two terminals and 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, so I assumed the crystal itself might be bad. Replacing the 8M crystal didn’t help, so I decided to replace the coupling capacitors C703 and C704 connected to the crystal. After doing that, the TV started working normally again.

After the customer picked up the TV, I took the time to examine the two ceramic capacitors closely. Looking at them externally, they seemed fine, but since the chance was there, I decided to take them to a bridge tester for a deeper analysis. I compared the faulty capacitors with a known good 22P capacitor. Below are the readings taken at different frequencies: 60Hz, 100kHz, and 200kHz. The left side shows the capacitance value, the middle window shows the equivalent series resistance (ESR), and the right window displays the test frequency.

As shown in the results, the faulty capacitor showed a higher capacitance at lower frequencies and a significantly higher ESR at higher frequencies—more than ten times that of a good capacitor. This high ESR at 8MHz is enough to disrupt the oscillation in the crystal circuit, leading to failure. It's a subtle issue that can easily be overlooked, but it clearly had a major impact on the TV's performance.

Since I still have customers waiting for repairs, I’ll wrap this up here. I hope this detailed account helps others understand how seemingly small components like capacitors can cause big problems. If you've ever faced similar issues, I encourage you to take a closer look—sometimes the solution is simpler than you think.