Changhong color TV C2151 grating expansion and contraction fault repair example - Appliances
When dealing with the A3 movement color TV, specifically the Changhong C2151 model, a common issue is the grating expanding and contracting after startup. The symptom starts with a full-scale grating that soon begins to expand and contract. Upon measuring the B1 voltage (130V), it fluctuates between 130V and 90V repeatedly. Additionally, the base voltage of the pulse width control transistor V512 (2SC3807) varies between -3.6V and -3.0V, whereas it should be around -1.8V under normal conditions. After investigation, it was found that the feedback winding rectification filter capacitor C515 (0.015μF/63V) had degraded significantly, dropping from its original value to only 4700pF. Later, two Huangshan H2588C/R color TVs were repaired. In one of them, the power indicator was on, but the output at the 10B terminal (130V) showed instability. The base voltage of the pulse width control transistor V312 (2SC3807) was measured at 0.6V, and upon checking the feedback winding rectification filter capacitor C317 (0.015μF/63V), it was found to have no capacitance at all. Another case involved a small, dimming grating that shrank after powering on. The base voltage of V312 ranged between -2.6V and -3V, while the +B voltage fluctuated between 70V and 100V. Removing C317 and testing it revealed a capacitance of only 4460pF, confirming the degradation. From these cases, it's clear that when an A3 movement color TV exhibits similar symptoms or occasionally fails to start, checking the base voltage of the pulse width control transistor in the switching power supply can help quickly identify the problem. To prevent future issues, it’s recommended to replace the 0.015μF/63V polyester capacitor with a 0.015μF/160V metal film capacitor. Also, during repairs, it's important not to leave the TV on for extended periods if there's a large shrinkage in the grating. This is because unstable low +B voltage could cause the transistor to overheat and fail. Taking these precautions can help avoid further damage and ensure a more reliable repair.
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