How to Fix TCL L40S6500 Backlight Circuit Problem – Modified Due to Unavailable Backlight IC

A TCL LED TV L40S6500 came to my workshop for repair due to a backlight fault. The customer said that we were watching it when suddenly its display turned off. I turned on the TV and checked. I could hear sound, but there was no picture. I checked by shining a bright light on the screen, and the display was visible on the panel. This meant that its backlight was not lighting up. Now there are only two possibilities: there is a fault in the circuit or the backlight itself is faulty. I took the time from the customer and told him that I would diagnose the fault and let you know, and with your permission, I would fix it.

Visual Inspection

Now I opened this LED TV in the workshop lab to work on it. First of all, I started the physical inspection of its main board. I examined its PCB. It was in good condition. After that, I looked carefully at its power supply. I did not see anything suspicious there. Then I observed its backlight circuit very closely. I found its backlight IC a little heated. I also checked the resistance capacitors around it in the backlight circuit. Visually, I did not see anything suspicious here. Except for the IC, now let’s move on to the next step, that is, voltage testing. Now we will check the voltage of the backlight.

One Side Backlight Voltage Missing – IC Suspected

As the next step, I checked the main voltage of its backlight circuit. The voltage on the main capacitor was correct. Now I checked the output voltage of the backlight. Here I noticed a difference. The backlight of this set consists of a positive common and a negative two-line. On one of these lines, the voltage was exactly correct, while on the other line, the voltage was very low. As a precaution, I checked the backlight strings with a tester. Both were showing the same voltage and amperage. I decided that its IC was faulty. I did not have this IC with me. It was not available in the market either. So now I had the only option to install a universal driver in it.

• Key Diagnosis Points for Backlight Fault:
• ✅ Main backlight capacitor voltage was verified and found to be correct.
• ⚡ Output voltage check revealed a significant imbalance in the negative lines.
• ➖ One backlight negative line showed normal voltage; the other had very low voltage.
• 🔍 Both backlight strings tested with multimeter showed equal voltage and amperage.
• ❌ Based on the testing, the backlight driving IC was declared faulty.
• 🛠️ The specific IC was unavailable both in stock and in the market.
• 🔄 As a final solution, a universal LED driver board was installed to restore function.

TCL LED TV Backlight Repair Using External Driver Board

I took a medium-sized LED driver board for this TCL L40S6500, and I provided it with a 12-volt power supply from the main board. After connecting the BL-ON and dimmer, I now paralleled the backlight strings. Earlier, they were divided into two parts. By doing this, I had two wires, one plus and one minus. I connected them to the output of the backlight driver. After doing all this, I powered on the TV, and the backlights were on. My fault was fixed. Let me add one more thing. I had changed the backlight strings in the same set about a year ago. Now the funny thing is that this TV returned to the customer’s house with the same fault after running for two days. This time, its backlights were burnt out. I had to replace it for free.

Why Backlight Burned?

Two days later, when this TCL LED TV returned with the same fault, I had a lot to learn from it. I modified the circuit with the old backlight strings and got the set running, but what happened was that it only worked for two days? Let’s try to find out the reason. The backlight strings were more than a year old. And I know that this set is used a lot. Smart TVs are running all the time these days, so that was the reason that their backlight strings were already at the end of their life. In such a situation, when I operated them with an external driver, they burned. When I installed new strings in it, I checked the voltage on one of its LEDs, and here I got a reading of 5.3 volts, while this one LED is 6 volts. This means it is now safe.

Conclusion

I had a TCL LED for repair that had a backlight fault, but the backlight fault was in the circuit, and I couldn’t find its IC. I thought it was faulty, so I installed an extra LED driver in it, and the set worked, but after two days it came back with the same fault. Now I checked it again and found that its backlight string had burnt out. I didn’t install it and checked the voltage. One LED was 6 volts, but now it was getting 5.3 volts, which means it will last for a long time. My advice to technicians is don’t modify with the old backlight, as the old backlight has completed its life, so when you install an extra driver, it burns out after a few days, so whenever you do such a modification, change the backlight along with it.

Q1: Why does the backlight in Chinese models deteriorate so quickly?

A1: If we talk about it, I think the life of the backlight used in it is between one and two years.

A2: If your TV’s backlight is burnt out and needs to be replaced, it also depends a lot on how the technician installs it.

A3: A good technician should pay special attention to the temperature of the backlight while installing it. It is applied using thermal tape so that it transfers its heat to the body and lasts longer.

If you want to read our article about panel issues on this same model, click here.

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