Monday, December 15, 2008

TV deflection coil defect

Modern TVs are designed to shutdown when problems are present either in the power supply or its loads to prevent the set from incurring further damage.

This protection feature was exhibited in a 14-inch TV which was brought in the shop for repair.

The set appears to power-up but will shutdown even before a raster is visible on the screen. A soft hissing sound can also be heard during the one or two seconds before shutdown.

The B+ supply voltage going to the horizontal output transistor's collector was measured but drops in that short moment before shutdown.

The scan-derived supplies going to the CRT panel such as the G2 and video output circuit supply voltages are both dropped.

Because of the hissing noise heard before shutdown the flyback transformer was suspected to have a high voltage leak that may be causing arcing noise. But a few tries to power-up the set reveals the source of the noise to be somewhere around the neck of the CRT. However, arcing was not visible inside the CRT neck. So the next possible source of HV arcing or leak is the deflection coil.

Next step was to unplug the deflection coil connector. When the set was powered on B+ voltage did not drop and there is a palpable rush of high voltage to the CRT. Its filament was also noticed to be lighted. This meant that a short or leak was removed when the yoke was unplugged.

When the yoke was disassembled the source of arcing was uncovered. Looking inside the yoke reveals burnt marks in the inner windings of the deflection coils.

When a deflection yoke burns it is usually at the outer edges of the windings or in the area in between windings as shown in the photo below. Notice the blackened edges of the winding on the left. In this case, the coil also reeks of a bitter smell.

Now that the cause of the trouble has been identified the only problem now is where to find a replacement part for the burnt deflection coil.

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