The fault involves the circuit; Intermediate Frequency (IF) processing circuit. Fault location: IF oscillator crystal 2171 is damaged.
Methods, Techniques, and Tips: Beijing Hisense TV after-sales service telephone number; Key points and characteristics of IF circuit inspection for LA7687.
Analysis and Inspection: Upon checking, the high-frequency signal processing circuit is normal, meaning that the MB voltage, TU voltage, band-switching voltage, and ACC voltage of the high-frequency tuner are all normal. Replacing the high-frequency head U101 did not resolve the issue. Next, the IF processing circuit should be checked, focusing on the pre-IF amplifier circuit, 38MHz processing components, and video channel. Adding interference signals to the video signal output terminal (pin ⑧) of N201 (LA7687), interference patterns appear on the screen, indicating that the channel processing is basically normal. The components and voltages of the pre-IF amplifier V101 circuit were also detected to be normal. Checking the IF input (pins 48 and 47) and IF ACC voltage of N201 also showed no abnormalities. After replacing the IF oscillator crystal Z171 connected to pin 5 of N201, the fault was eliminated.
★ In the single-chip color TV IF circuit of LA7687, there is no image detector IF transformer or AFT IF transformer commonly used in conventional TVs. Instead, an IF oscillator crystal is used to generate the IF carrier for image demodulation. Therefore, when encountering an imageless fault in this model, in addition to the regular inspection points of the IF circuit, the IF oscillator crystal at pin ⑤ of LA7687 should also be checked.
Hisense TC-2139 TV has no image ★ Article source: Beijing Hisense TV repair hotline http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3764590.
Fault symptoms: No image appears on the screen, only a raster is present.