Change Changhong plasma 3D51A5000i repeated burning PFC power correction (common disease)! ! [board number: LJ44-0023... - appliance repair

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Changhong 3D51A5000i is another one of those "invincible" transfer machines. The problem? It doesn’t power on. I don't know if you've ever seen something like this, but I've come across it many times before. Anyway, I've seen plenty of cases where the unit just wouldn’t turn on.

When I checked the power plug resistance, it was infinite. That means the power board is likely damaged. On-site, I didn’t even take it apart. I just tested a few components and concluded that the internal circuit board was faulty. The repair cost came out to be gray, black, and brown—so we agreed to fix it. Then they brought the unit back to the store for repairs. After disassembling, I found that the PFC had blown up, and some of the MOSFETs were missing from their sockets.


The power tube under the board was just soldered directly to the chassis... Like a cockroach crawling around. Big brother, are you turning off the black fire or just throwing it into the grill? Look at your soldering work—I can't even see clearly! It really hurts me to clean up after all this.

The two power tubes are different models. The bad one is 7N80, while the original one was STP14NK50ZFP, which wasn’t burned. Even if it’s not broken, you can’t just replace it with an asymmetric transistor. If you mess up during the post-repair stage, it will fail early. A few streets away, and it's done.
Suddenly, I noticed a small black spot.



No matter how embarrassing the issue is, I always catch it. My eyes and nose are different from others. My nose is especially sensitive to temperature and smells. Sometimes I can feel if a component is overheating just by sniffing. So when I use a soldering iron, I often smell it again and re-solder. Some things are very sensitive—like cracks in the circuit or burnt areas. I can sense them without even looking. I’ve never seen a board like this before, but when I scanned it, I noticed some unusual things. It’s kind of like being the king of the strongest brain, though I don’t exaggerate it too much. This sensitivity comes from an accident over 20 years ago. At that time, I was young, and I suffered serious injury to my right eye. I almost lost my vision. The size of my eyes was clearly different. My left eye was fine, but my right one was almost blind. I went to the hospital, but there was no cure. Eventually, due to financial reasons, I gave up. After school, my left eye started to get worse. In primary school, it wasn’t so bad. But by junior high, I became more and more nearsighted. Sitting in the middle of the classroom, I couldn’t see the words on the blackboard—they looked like ghosts. I tried wearing glasses, but they made me dizzy and uncomfortable. Things got worse in high school. I couldn’t read what was on the board, and my eyesight dropped below 1.0. The hospital only said I was myopic and didn’t find anything else. But things within 0.5 meters are still very clear. Since I was young, I have always hated school vision tests. I was always the last one to go for the checkup, afraid that my classmates would laugh at me. But I learned to laugh it off. Haha… Not everything is like that, but I’m lucky that my vision hasn’t affected my daily life or work. Sorry, I suddenly remembered some old memories and couldn’t help writing them down.

Following the same path, I traced the inductor feedback sampling pin. There was definitely high voltage coming in from there. How could the resistor burn like that? I also checked the power chip SPC1012T. No test, obviously, Zhou Gong (the dream interpreter) has already told me the result. Replacing these blindly would just cause more damage. Don’t miss the most critical step. This power board, whether it’s Samsung or Changhong or others, I’ve found that there are two common faults in the power inductors. Both the primary and secondary PFC inductors tend to short-circuit. The main reason is poor insulation. When the sampling winding and the primary coil are exposed to high temperatures, it’s easy for them to arc. Even Ma Yun’s so-called technical upgrades are just changing the packaging. After opening and comparing, they are the same material. The same problem exists in Samsung devices as well.

The correction inductor was removed, and the feedback winding was directly burned open.

Then I measured the primary and feedback pins, and they were shorted. You said you couldn’t fix it by blindly replacing the IC MOS resistors. 300V is flowing through without any issues, so why did the 1012T blow up?


The air cylinder was heated gently, and it wasn’t hard.

I removed the insulating tape, no need to say more.

Make sure the other one isn’t broken, and handle the insulation to avoid unnecessary repairs. Wind the feedback coil back according to the original number. Clean the main coil and ensure proper insulation. No need to replace it; just connect the wires well and secure it with silicone. Replace the bad 14NK50, check the external components of the G pin, clean the pads to prevent short circuits, replace the SP1012T and the burnt resistors. According to comparative analysis, this 20K resistor is definitely good. Learn to analyze the circuit components. Don’t touch the little things, like the drawings. Where are the many drawings? For imported machines without drawings, I usually don’t look for them. That’s why I rarely post the schematics.

At the same time, I checked the two 450V capacitors. One of them had shrunk in size and showed slight signs of bulging. I used it as is. The Panasonic board was replaced with the original one. It was a bit too big, but it fit in. Apply silicone and fix it properly.
After checking again, PS→ON was grounded, VS→ON was connected to 5V. I used a 200W bulb for 220V input, and the VS load was a 15W refrigerator bulb. The power worked normally. Then I put on the insurance, connected a 100W load, and tested the machine for 10 minutes. The temperature rise wasn’t significant.


I hung a thermometer and covered the case for one hour to make sure the temperature didn’t exceed 55 degrees.



Installed the back cover and tested the machine for a full day.




Although it’s only 100 pieces to buy one, it's way more than just learning to change 100 parts. It’s about understanding the whole system and improving your skills. Every repair teaches you something new. That’s the real value of this kind of work.