Method of repairing LCD color TV with oscilloscope

Method of repairing LCD color TV with oscilloscope

Determine the fault scope based on the fault characteristics of the waveform

In a faulty LCD TV, the waveform changes vary greatly, but there are still some rules to follow. After analysis, there are mainly the following situations:

(1) No waveform

This situation reflects that the signal is not sent to the detection point. There may be an open circuit in the circuit, which interrupts the signal, or there may be a short circuit between the detection point and the ground.

(2) Waveform amplitude and frequency deviation are too large

If the waveform amplitude deviation is too large, it means that the circuit is not working properly. For example, if the coupling capacitor changes or the resistance of the feed signal branch increases, the waveform amplitude will generally be attenuated a lot; the change of the amplifier operating point will also cause the waveform amplitude to change.

The waveform frequency deviation is too large, which includes not only the waveform frequency deviation but also the pulse width abnormality. During maintenance, attention should be paid to observation and analysis.

(3) Waveform shape distortion

Common causes of waveform distortion include changes in capacitance and resistance, and some are caused by malfunctioning amplifiers.

(4) There are incidental noises in the waveform

There is no clear waveform on the oscilloscope, but many lines are superimposed or displayed at the same time in a disorderly manner, one of which is brighter and the others are darker, which is usually called "unclean" waveform, as shown in the figure below. The cause of this failure is the failure of the filter capacitor, leakage of some components or circuit boards, etc. Of course, if there is a high-power transformer working next to the oscilloscope or the oscilloscope is poorly grounded, this interference will also occur.

(5) Waveform inversion

Waveform inversion means that the test waveform is opposite to the normal waveform in phase, the forward pulse becomes a reverse pulse, or the reverse pulse becomes a forward pulse. It is mostly caused by malfunctioning amplifiers or gate circuits.

(6) Oscillating waves are superimposed on the waveform

The waveform is superimposed with an oscillating wave, indicating that there is a parasitic damped oscillation in the circuit, which is more common in switching power supplies or high-voltage board circuits. The frequency of parasitic oscillation is high and it is easy to radiate from the circuit and then form interference through the circuit.

There are several common methods for using an oscilloscope to repair LCD color TVs. 

1. Signal tracing method    

The signal tracing method is to measure the voltage waveform of certain key points or extended measurement points according to the fault phenomenon and along the direction of the signal during the repair of LCD color TV. The operator only needs to understand the circuit structure of the color TV, the characteristics of the signals at each key point and the normal waveform, and by testing the test points, the fault location can be quickly found. This is the most commonly used method in maintenance. 

2. Monitoring measurement method 

For faults that occur irregularly or only occur during a long test, the monitoring measurement method should be used, that is, the oscilloscope probe is fixedly hung on the suspected measurement point and the measurement is carried out for a long time. If the pin or solder joint of the measured point is too small to be convenient for hanging the probe, a thin hard wire with a length of no more than 1 cm can be soldered to this solder joint, but be careful not to short-circuit the wire with other nearby solder joints. Hang the oscilloscope probe lightly on the wire and do not lift the copper foil on the circuit board. 

3. Series probe measurement method    

Dual-trace oscilloscopes are generally equipped with two 10:1 attenuation probes. When measuring high-amplitude voltage waveforms such as reverse pulses, if there is no 100:1 probe, these two 10:1 probes can be connected in series to form a 100:1 probe. When using, the attenuation switches of the two probes are both set to the ×10 position, connected end to end, which is very convenient to use, as shown in the figure below.