A few months ago, an old customer called me and said the TV had stopped working. The Changhong LT42510HFD model had no power light at all. After checking the power supply, I found it was broken again. This was the second time I fixed it—about two and a half years ago, I replaced the TNY277 power IC. Since the TV doesn't have a power switch, the user never unplugs it. I told them that they use it for about 5 hours a day, but in reality, it's running 24/7. This time, the power block blew up completely. The L6599D secondary power management IC was shorted, and other small components were also damaged. The worst part was the switch Transformer—the primary winding was open.
Everyone knows that the most straightforward solution is to replace the entire power board, but that would be very expensive. Adding the repair cost, it could easily reach several thousand dollars. I don’t think it’s worth repairing. I advised the user to look around for alternatives. Unfortunately, the transformer is not available on the market. If replacing the board isn’t an option, we can rewind the primary coil: 1-3 has two wires with 47 turns, 2-3 has one wire with 47 turns, 4-5 is a single wire with 17 turns, and 7-8-9-10 has five strands of wire. We replaced Q922, R976, ZD915, R991, Q914, ZD912, and upgraded the TNY277 to a TNY280. We also soaked the transformer in insulating paint and let it dry properly. To prevent future damage, I added a 56 ohm, 3W resistor before the primary winding.
When returning the TV to the user, I included a power strip and asked them to plug in only the devices they needed. I reminded them that they don’t need to press the switch when not in use, as everything will be off and safer.
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Huizhou Show-Grand Electronics Co., Ltd. , https://www.sgtransformer.com