I recently repaired a Hisense color TV model TF25R08N, which had an issue where the [M] character appeared on the screen with snowflake-like points and the remote control was unresponsive. The on-screen M symbol indicated that there was a problem with the bus status or data loss. I first tried activating the copy data boot button, and the operation menu characters appeared normal. However, after checking, I found that the access signal data was out of sync. Despite this, there were normal sounds and visible color stripes, suggesting that the audio and video functions should be working correctly.
I suspected that the issue might be related to the data, so I attempted to re-copy it, but the fault remained unchanged. Next, I checked and replaced the crystal oscillator [8.000M], but that didn't resolve the issue. I then measured the voltages at various pins of the chip, including 5V, 8V, and 3.3V, all of which were within normal ranges. I also checked the voltage at pin 14 of the AFC chip, which was around 4.8V—this should have been normal. However, when using a multimeter set to a low resistance range, I noticed that the line frequency voltage did not seem to change. Normally, measuring the AFC pin with a low internal resistance would cause a noticeable change in the line frequency.
When I observed the screen, the diagonal bars showed a slight variation, which led me to conclude that the AFC circuit was either not functioning or operating abnormally. I then thoroughly checked the peripheral circuit around pin 14 and eventually discovered that capacitor C207 [8200PF] had a leakage resistance of several tens of kilohms. After replacing the capacitor, the boot image returned to normal, and the TV worked as expected.
At the beginning of the diagnosis, I only measured the voltage at pin 14 and didn’t check the external components. The leakage in C207 wasn’t enough to significantly pull down the voltage, but it was sufficient to affect the AFC circuit's performance. This case highlights the importance of checking both internal and external components during troubleshooting. A low resistance measurement can often help identify issues that aren’t immediately obvious from voltage readings alone.
This repair example is provided for reference and may help others facing similar problems with their TVs.
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