I was working on a Hisense color TV model TF25R08N, which had an issue where the screen displayed an "M" character with snowflake-like points when turned on, and the remote control wasn't responding. The on-screen "M" indicated that there might be a bus status or data loss problem.
First, I tried activating the copy data boot button, and the menu characters appeared normally. However, the access signal data was out of sync. Despite this, there were normal sounds and visible color stripes, suggesting that the audio and video signals were not completely broken.
I suspected a data issue and re-copied the data, but the fault remained unchanged. Next, I checked the crystal oscillator at [8.000M], but it didn’t resolve the problem. I measured the voltage at various pins of the chip, including 5V, 8V, and 3.3V, and all were within normal ranges.
The AFC pin (pin 14) was measured at around 4.8V, which should have been normal. However, when using a multimeter set to a low resistance range, the line frequency didn’t seem to change. Normally, measuring the AFC pin with a low internal resistance should cause a noticeable change in the line frequency. I noticed a slight change in the horizontal bars on the screen, which led me to suspect that the AFC circuit was not functioning properly.
After checking the peripheral circuit of the 14th pin, I discovered that capacitor C207 (8200PF) had a leakage resistance of several tens of kilohms. Replacing the capacitor resolved the issue, and the TV booted up normally.
At first, I only checked the voltage at the 14th pin without examining the external components. The leakage in C207 didn’t significantly pull down the voltage, but it was enough to affect the AFC circuit’s performance. This case highlights the importance of checking both the voltage and the condition of surrounding components when troubleshooting. A low-resistance measurement can often help identify such issues. This repair example is shared for reference.
Acrylic Letter,Acrylic Led Sign,Acrylic Channel Letter,Modern Acrylic Wall Art
Wuxi Motian Signage Co., Ltd , https://www.makesignage.com