Found that the low-end signal observed image L, H and U three bands are relatively stable, while the high-end signal instability , and even change back and forth between two adjacent . According Symptom emphasis should check the tuning circuit , as shown in the drawings . +30 V terminal voltage measured normal . Tuning voltage of the machine is a CPU (MN1872432TW1) 46 -pin output pulse signal , the low-pass filter inverted by Q1202 and R1205, R1206, C1207, C1208 smooth composition into a DC voltage applied to the tuning tuner end BT. Measuring the tuning voltage terminal BT ( Note : When the connected image deterioration or disappearance of the multimeter , and this is due to the internal resistance of the multimeter BT total equivalent resistance value of the terminal resistor in parallel decreases the tuning voltage is decreased because of ) less stable at 13V unchanged when adjusting channels, more and more tuning voltage of 13V instability . Thus treadmill because the tuning voltage instability. CPU46 feet to Q1202b measured voltage has been very stable, while Q1202c voltage instability began to appear . Check the Q1202 and chip resistors R1205-R1207, C1207, C1208 and other components were normal , suspected adverse Q1202 performance. But after replacing the Q1202 Symptom unchanged, try to change the tuner , the fault disappears .Agnservice manual download site

From this analysis , the internal tuner varactor diode poor performance , when the tuning voltage higher than 13V, varactors and increased leakage occurs , making the low end of the band signal is relatively stable, high-frequency signal tuning voltage instability due to the emergence of running Taiwan phenomenon .Agnservice manual download site