Changhong color TV C2151 grating expansion and contraction fault repair example - Appliances

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Tantalum Capacitor

When dealing with a Changhong C2151 color TV that exhibits an issue with the screen grating expanding and contracting after startup, it's crucial to identify the root cause. In this case, the full-scale grating quickly expands and contracts. Upon measuring the voltage at terminal B1 (130V), it was found to fluctuate between 130V and 90V repeatedly. Additionally, the base voltage of the pulse width control transistor V512 (2SC3807) varied between -3.6V and -3.0V, whereas it should normally be around -1.8V. After further investigation, it was discovered that the feedback winding rectification filter capacitor C515 (0.015μF/63V) had lost its capacitance, dropping to only 4700pF.

Later, two Huangshan H2588C/R color TVs were repaired. One showed the power indicator on but no output from the 10B (130V) terminal. The base voltage of the pulse width control transistor V312 (2SC3807) was measured at 0.6V. Upon removing the feedback winding rectification filter capacitor C317 (0.015μF/63V), it was found to have no capacitance at all. Another TV had a small, dimming screen that shrank after powering on. Measuring the base voltage of V312 revealed it fluctuating between -2.6V and -3V, while the +B voltage varied from 70V to 100V. When the capacitor was tested, it had only 4460pF left.

From these cases, it's clear that when a color TV exhibits such issues or occasionally fails to start, checking the base voltage of the pulse width control transistor in the switching power supply is a quick way to locate the problem. To prevent future failures, it’s recommended to replace the original 0.015μF/63V polyester capacitor with a 0.015μF/160V metallized film capacitor. Also, during repairs, it's important not to leave the TV on for too long if the screen grating is shrinking significantly, as unstable low +B voltage can lead to overcurrent and damage the transistor.