Friday, January 2, 2009

Vertical deflection problem: horizontal gaps on picture

Coming at the tail-end of 2008 a day before New Year's Eve, a repair technician came to me for help in the repair of a 21-inch TV.
There was a problem in the picture of the TV which looks like black horizontal stripes and gaps on the screen. The picture below would describe to you more about the symptom.

And here's another photo:

According to the technician, he has already replaced the TDA8172 vertical output IC. Unfortunately, the trouble still persisted.
The vertical output IC is obviously running hot as you can get singed just by touching its heat-sink for just a few seconds.

Based on the symptoms there appears to be non-linearity in the vertical scanning as indicated by the gaps in the picture. A slight fold-over in the top edge of the picture can also be noticed.
Since the vertical output IC has already been replaced, I set my sights on the minor components in the vertical deflection circuit.

According to the technician he has already tested the resistors and capacitors located around the vertical output IC using the ohmmeter of an analog multimeter and found no open or changed value resistors. He tested the capacitors' charging and discharging action also using the analog ohmmeter.
Digging in to the guts of the TV, I set my sights on the capacitors first as they are known to be the passive components with the highest failure rate - second only to ICs and transistors. Capacitors are better tested with a capacitance meter, not to mention the ESR meter, I skipped testing the capacitors with the ohmmeter. Testing the capacitors around the vertical output with a capacitance meter I found one mylar capacitor whose measured capacitance was way off its rated value. As indicated on its package its value should be 220nF/63V. However, this capacitor only registered 3.2nF on my capacitance meter. Apparently, this capacitor has decreased in capacitance to, approximately, less than 2 percent of its rated value.
Getting another capacitor with similar indicated value from my parts bin, I tested it on the capacitance meter to verify the rating before using it to replace the original capacitor which has decreased in capacitance.
After installing the new capacitor, the TV set was again turned on. Now we see a clean picture - the black gaps and lines have completely disappeared.
So now we have identified the cause of the problem to be the 220nF/63V mylar capacitor located a fraction of an inch to the right of the TDA8172 IC.
Tracing the connections on the printed circuit board and a review of the TDA8172 datasheet, it was found that the capacitor was connected to the output pin of the vertical output IC through a 1.5 ohms resistor. The capacitor's other pin is connected to ground pin of the vertical output IC.

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