Old 21-inch Changhong "color cast" repair case - Appliances

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It was an old user who brought in a Changhong 21-inch TV for repair. Previously, it had been fixed with no faults found, using the LA7688 chip and some spare parts, along with an old schematic. The user reported a color cast issue. Here's the image: Psb.jpg

The first thing I thought was that this might be a green gun problem or an issue with the LA7688. I turned off the color saturation and checked the picture. It was black and white, which meant the green gun circuit and the entire tailgate matrix were working fine. Then I measured the voltage on pin 34 of the LA7688, which is the green gun output. It was higher than the red and blue. Even after turning off the color balance, the three gun voltages remained balanced, so the issue wasn't with the resistors in the line. I started to suspect the baseband delay IC, like the LC89950, but the only component directly related to the color issue was the LA7688. I had a spare part on hand, so I replaced it confidently. But after installation, the screen turned green again—green, green, green! I was frustrated. I took out the schematic and carefully checked from pin 1 to 52. Only pins 33–39 were related to the color issue, and they all looked normal. At this point, I was really stuck. The user was watching closely and said, "Master, take your time, this one is tough." That made me feel better. I took a deep breath and started thinking again. Maybe I missed something obvious. There are three video standards: PAL, NTSC, and SECAM. I tried switching between them using the remote, but the screen stayed green. The user added, "It’s always green," which made me think again. Wait a minute—NTSC should display in black and white if there's a color issue. Why was it green? The only explanation was that the TV was stuck in the SECAM mode. SECAM is known for its greenish tint. I remembered that the standard conversion IC, the 4052, handles both image and sound systems. I checked the voltage on pin 1 (AV/SECAM/SIFIN) of the LA7688. In normal TV mode, it should be around 1.7–2.6V for PAL/NTSC, but it was only 1.3V—clearly in SECAM mode. That explained everything. The problem wasn’t the LA7688 itself, but a faulty capacitor connected to pin 1. It was a 103PF ceramic capacitor (C173) with a 1K resistor. After checking, I found it was leaking. I replaced it, and the TV worked perfectly. Here’s the before-and-after: Psb (7).jpg

You can see the capacitor on the back, marked C173, and it's definitely a 103PF one. Psb (6).jpg

Here’s the final repaired image: Psb (5).jpg

This was my first time encountering this specific issue, and I hope it helps others in the future.

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