Correction circuit 44

**English Translation:**

**Model:** Hisense TC-2125C TV Field Sync Malfunction

**Fault Symptoms:** Poor field synchronization.

**Fault Circuit:** Field scanning circuit.

**Fault Location:** VD731 breakdown.

**Methods, Techniques, and Tips:** Inspection method for single-chip TV field synchronization issues in LA7680.

**Analysis and Repair:**

The field synchronization process of this model is mainly handled internally by the LA7680 chip. Replacing the LA7680 directly did not resolve the issue. Measuring pin 31 of the LA7680 revealed a 50/60Hz recognition voltage of 3.5V (it should be 0.4V under normal conditions). Pin 31 is the output for field scan frequency identification, indicating that the LA7680 internally misidentified the field frequency as 60Hz. Pin 32 of the LA7680 is the field drive pulse output terminal and also serves to adjust the sensitivity of field sync separation, correcting possible image distortion when playing a video recorder. Upon inspection, R452 connected to pin 32 was found to be normal. The tilt correction circuit on pin 32 of CPU N701 was then checked, revealing that VD731 (1S1555) had broken down while in-circuit. After replacing VD731, the fault was resolved.

**★ Most modern single-chip TVs lack field sync adjustment potentiometers in their circuits. For such TVs, if a field sync issue occurs, check the quality of the video signal and whether the sync separation circuit is functioning properly. If the TV has a skew (SKEW) correction circuit, also inspect the circuit between pin 32 of the LA7680 and the CPU. If no problems are found after these checks, the fault lies within the single-chip circuit itself.**

**Content Source:** http://blog.sina.com.cn/haixindianshi