Thursday, December 25, 2008

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.

No comments:

Post a Comment

For comments and inquiries about this topic: