Saturday, February 7, 2009

Philips TV L01.2 chassis problem: criss-crossing white lines on a dark raster

TV troubleshooting is about reading the lines on the screen. And in an electronic repairer's work troubles appearing as strange lines on the screen or picture of the TV crop up from time to time. These types of trouble in a CRT TV are most often cause by a problem affecting the scanning or deflection circuits. An example of these TV troubles manifesting as weird lines on the picture was tackled in one of my previous post where a capacitor in the vertical output circuit has decreased in capacitance and caused black lines to flicker across the picture.

The other day as I was flipping through a notebook that I keep where I jot down details of troubleshooting work that I have done I came across a record of a weird lines on the picture problem. Initially the symptom is 'no picture, dark raster'. Increasing the G2 control on the flyback transformer revealed strange lines that break white and zigzag across a blank raster.
I was also able to keep a camera shot of the symptom which you can see below.

The lines tell of trouble that lies in the vertical scanning circuit. The lines are symptomatic of excessive vertical scan distortion and fold-over. The TV chassis contains a TDA8172 vertical output IC which does not feel warm when you touch it. This would mean it is not overdriven or overloaded. Voltage measurement on the vertical output IC indicate normal supply voltages. What is obviously abnormal is the voltage on the output pin 5 which measures +5V instead of 0V. Replacing the IC did not provide the cure to the problem.

I decided to scope the input signals to the TDA8172 vertical output IC and discovered that the Vdrive+ signal on pin 17 of the UOC TDA9580 was generating a rectangular wave pulse instead of a sawtooth pulse. Both the Vdrive+ and Vdrive- The UOC was diagnosed to be defective since the sawtooth pulse is supposed to be generated from inside the UOC. A normal picture was restored to the set when the UOC was replaced.

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