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By China lcd display supplier | 10 May 2019 | 0 Comments

How to repair the power failure of LED display screen

When the power supply of the LED display screen fails, how should it be repaired?

Firstly, the fault point can be judged by detecting the voltage, resistance and waveform of the key parts, so as to quickly repair the 5V power supply for the outdoor LED screen. The power supply of LED display screen is in standby state, +5, PS ON is high level, other voltage has no output. When overhauling, the green line (PS ON) and the black line (GND) should be shortened; otherwise, even if the power supply is normal, there will be no voltage output. At the same time, a 30W/12V bulb is connected between the + 5V output terminal (red line) and the ground as a false load for no-load protection. Check +5.

The power supply of the LED display is connected to 220V, and the voltage of + 5 (purple line) terminal is checked. If there is a + 5V voltage, it can be confirmed that the input rectifier filter circuit and auxiliary power supply are working normally. The fault should be in the main power supply part: if the + 5 output voltage deviates from the normal value, it indicates that the sampling voltage feedback network is faulty, and it should focus on checking whether U2, U4 and its peripheral components are normal. When U2 is damaged, it can be replaced by P62 1/4G; when U4 is damaged, it can be replaced directly by K431, UPCI093J, YL431, WL431, S431, SD431 and AN143 1T.

Secondly, the collector of Vl or emitter of V2, i.e. "midpoint" A-to-ground resistance, is measured by a multimeter with a normal value of about 6.5 kn. If the measured resistance of midpoint A is zero, it means that at least one of the two switches is broken down. While checking Vl and V2, we should also check whether R4 and R6 are open, whether Cl and C2 are leakage, and whether the DC output circuit rectifier pairs VD12 and VD33 are broken down. If the resistance of midpoint A is about 2.5 kn, it shows that the two switches are open, the replacement can eliminate the fault.

Method 1 for checking push-pull circuits

Under normal conditions, the transformer T2 primary winding neutral point tap is driven, the B-point voltage is greater than 3.9V, the V8 and V9 neutral point tap are driven, and the C-point ground resistance is greater than 10kn. If the measured B-point voltage is obviously less than 3.9V and the C-point resistance is normal, the fault occurs in the pulse width modulation circuit: if the B-point voltage is obviously less than 3.9V and the C-point resistance is about 1.5kn, then the fault is the damage of the driving tube.

Method 2 for checking push-pull circuit

The fault points are determined by measuring the waveforms of the base and emitter of the two driving tubes with an oscilloscope. If the waveforms of V8 and V9 are basically the same, it indicates that the pulse width modulation circuit and the driving circuit are working normally: if there are waveforms, but the difference between the two waveforms is large, it may be one of the damages of V8 and V9, or the secondary winding load of the coupled transformer T2 is unbalanced. There are faults in the corresponding switch Vl, V2 and peripheral circuits: if there are no waveforms in the base of V8 and V9, the faults are in the TL494 and peripheral circuits (including). Protection circuit).

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