**Changhong C3418PN TV: After turning on, the entire screen raster is too dim. Both remote control and button control function, but there are no characters, no image, and no audio.**
**Released by Beijing Changhong TV Repair Center on September 9, 2011.**
**Analysis and Maintenance:**
From the above symptoms, it can be inferred that the CPU is functioning normally, but there might be an issue with the horizontal or vertical retrace pulses not being transmitted to the CPU. Upon testing, the voltage at pins 37 and 38 of the CPU (DQA1) was found to be greater than 4V, which is basically normal. The voltage at pins 50 and 51 of the CPU fluctuated between 3.6V and 4V. Further inspection revealed that the voltages at pins 5 and 6 of the memory DQA07 (uPD6251) were around 4V. However, the voltages at pins 10 and 11 of NQ501 (TA8783) were only about 0.7V. Meanwhile, the collector voltages of VQA02 and VQA12 were normal, but their base voltages were abnormal. It was discovered that VDA01 (a 5.6V Zener diode) had opened up.
**Maintenance Tip:**
In this model, VDA02 and VQA12 serve as bus current amplifiers, used to enhance the load capacity of the bus. If VQA12 and VQA02 are damaged and remain in a cut-off state, the bus data cannot be transmitted to the controlled ICs, thus preventing the various control functions from being completed.
**Solution:**
Replace VDA01, and the fault will be resolved.