Haier After-sales Service Data; TC-2125C TV picture occasionally normal, occasionally no raster.
Fault Symptoms: The image alternates between normal and no raster with character display present. AV terminal input signal is normal. No significant reaction observed in contact devices.
Fault-related Circuit: External circuit of N101 RF AGC.
Fault Location: Poor contact of the RF AGC potentiometer RP101.
Methods, Techniques, and Tips: Black screen caused by poor RF AGC performance in A3 chassis color TVs.
Analysis and Repair: Replacing RP121 did not resolve the fault, nor did replacing the format conversion board as a whole assembly. The fault lies in the processing circuit after the video output at pin 42 of N101 (LA7680). All solder joints were re-soldered and all components checked, but the fault persisted. Upon checking the high and intermediate frequency processing circuits, it was found that when gently touching the RF AGC delay adjustment potentiometer RP101 (connected to pin D of N101) with tweezers, the fault significantly changed. It was suspected that RP101 (10kΩ) had poor contact. After replacement, the fault was eliminated.
Article Source: http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3764590.