In the maintenance work, it is often encountered that the crystal oscillator stops vibrating and the whole machine stops working due to the failure of the crystal coupling capacitor. As we all know, the crystal voltage of the crystal oscillator in normal operation is half of the power supply voltage in most cases. The judgment method we usually use is basically the voltage method. When the crystal oscillator is stopped due to the failure of the crystal coupling capacitor, we only replace the two coupling capacitors one by one until the crystal circuit can work normally. Investigating its mechanism, we generally do not pursue it. In fact, it is really helpless, because the benefits of home appliance repair are getting worse and worse, and investment is too big to recover. I didn't have time to study this problem for work reasons. Now I can freely control it in time. Therefore, I have every reason to ponder over this issue. I have a chance, and this opportunity will come soon. Dongguan Guancheng Precision Plastic Manufacturing Co., Ltd. , https://www.dpowergo.com
A few days ago, I repaired a cottage color TV that entered the color TV and was pissed around the large block to cause large damage. After replacing the large block, the color TV could work normally, but it could not be turned on the next day. The voltage of the two poles of the crystal oscillator is measured, one high and one low, but they are far from the half of the power supply voltage, and the waveform is not observed by the oscilloscope. Replacing the G701 8M crystal oscillator is invalid, so the coupling capacitors C703 and C704 of the G701 are replaced at the same time, and the machine can finally work normally.
After the customer took the TV set, put down the work at hand and carefully examined the two ceramic capacitors. Looking at the innocent anomalies from the outside, I thought that since the opportunity came, why not take these two capacitors into a deeper look? Then open the bridge and compare the fault capacitance with a normal 22P capacitor on the bridge. The following is the reading of the capacitance and equivalent series resistance of the bridge frequency set at 60HZ, 100KHZ, 200KHZ, brown. The capacitor is the fault capacitor and the yellow one is the normal capacitor.
The left side of the bridge is the capacitance value of the capacitor, the middle window is the equivalent series resistance of the capacitor, and the right window is the frequency value of the bridge. As can be seen from the figure, the failed capacitor becomes larger at low frequencies. Many times, the low-frequency characteristics are worse; the high-frequency equivalent series resistance is very large, which is more than ten times that of the normal capacitor. This excessive high-frequency equivalent series resistance is too large for the 8M signal of the crystal oscillator circuit, causing the crystal oscillator circuit to fail to continuously oscillate normally.
Since there are customers waiting to repair the electrical appliances, this post will be written here for the time being, I hope to help and enlighten the friends who saw this post.