TV high voltage arcs

In a CRT-type television set there are three sections where high voltage(HV) arcing can originate:

1. the flyback transformer where high voltages used by the cathode ray tube(CRT) are generated.
   
2. the deflection coil or yoke assembly.
   
3. the CRT itself where the highest voltages are applied.
   

The HV arcs can be colored yellow, orange or red. But the arcs with the highest voltages are those that are blue in color.

The most common source of high voltage leak and arcing is the flyback transformer. Aside from producing the scan-derived supplies where some sections of the TV get power, the flyback transformer also provide the high voltages used by the CRT.

Typical voltages for color CRT's ranging in size from 14" to 34" are:

Anode : 16kV - 30kV
Screen or VG2: 100 - 400V
Focus: 300 - 600V
Cathode: 170 - 200V

Such high voltages can produce arcing. Especially if the flyback transformer has developed HV leaks. HV arcs can jump from the case of the flyback when the plastic case and internal epoxy-type insulation has developed cracks.

The flyback's ferrite core can also produce intermittent arcs that jump to surrounding heat sinks. When this happens intermittent blanking of raster will occur due to dropping of high voltage.
Most of all, the EHT cable's rubberized insulation can deteriorate and develop pin-prick cracks where the HV can arc towards the direction of the CRT, deflection coils, wires and aluminum heat sinks.

HV leaks can sometimes be remedied by dipping the flyback in insulating varnish and letting it dry for a few hours before re-installing. EHT cable can be wrapped with HV electrical tape to cover leaks. If arcing is still observed the best solution would be to replaced the flyback module.

There will also be arcing if the areas surrounding the flyback's anode cap is is covered with dust and dirt. Cleaning the anode cap and the areas of the CRT around the anode button or connector will solve the problem.

Sometimes arcing will occur inside the CRT itself when there is shorting in the internal grids or electrodes. The CRT seal can also develop cracks and air will have entered into the tube which is supposed to operate in a vacuum. When this happens the tube is considered 'gassy' and a blue arc can be seen inside the neck of the CRT. Then, there will be no other solution but to replace the CRT.

The video below shows what a HV arc will look like inside the neck of a 'gassy' CRT.

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.