Circuits of the Basic TV Set and What They Do - Part 2 (Page 1 of 3)
Categories: Televisions and Projectors
In Circuits Of The Basic TV and What They Do - Part 1 we discussed the standby power supply, main power supply, and the horizontal and vertical deflection circuits. In Part 2 of this series we'll dig a bit deeper inside a typical TV set, and cover degaussing circuits, tuner circuits, and common tuner problems.
Degaussing a picture tube means to demagnetize and remove any stray magnetic fields on the tube. The degaussing circuit is a relatively simple circuit. On older TV sets, the degaussing circuit consisted of a large coil of wire (usually 50 to 100 turns of wire) and a device called a thermistor.
The degaussing coil is located around the large bell around the picture tube. The bell of a picture tube is the surrounding area around the parameter, perpendicular to the face of the picture tube.
Thermistors have a cold resistance rating as well as a hot resistance rating. The cold rating is usually a very low resistance, while the hot is relatively high. When a thermistor is cold and a load such as the degaussing coil is put in series with it, and power is applied, the thermistor starts to heat up. AC power then starts to flow through the coil, thereby createing a magnetic field. The hotter the thermistor gets, the less power flows through the degaussing coil until the thermistor reaches the hot resistive rated level and stops the flow of electricity through the coil, thereby stopping the magnetic field.
There are two types of common thermistors used in TV sets; one has two terminal connections, while the other has three.
The two terminal thermistor will always have a small amount of current flowing through it as long as the AC power is applied. The three terminal type, when cold, will divert the AC power through the terminal connected to the degaussing coil until it heats up to the hot rating. Then, AC power gets sent to the other terminal to power up the rest of the TV set. This type of thermistor can fail and cause the TV set to not start up. The degaussing coil should only measure a few ohms at best, but should not measure high in resistance or shorted.
Testing a Thermistor
The best way to test a thermistor is to make sure the device under test is cold. So, after you unsolder it and take it off the printed circuit board, let it come to room temperature.
With a multimeter in the low resistance mode, the thermistor should check only a few ohms. With the meter leads still attached, take your soldering iron and heat up the thermistor. The resistance should go way up. Now, take away the heat and watch as the resistance starts to fall again until it returns to what you first measured the device at. This thermistor would be considered good.
If the resistance is over 100 ohms cold, I doubt the thermistor is good. If when you put on the heat and the part does not go up in resistance considerably, change the part.
On newer types of TV sets, there is still a coil around the parameter of the picture tube bell. However, instead of a thermistor, this circuit uses a small relay to control the voltage flow through the degaussing coil. The small click you hear on picture-tube TV sets when you first turn on the power is this degaussing relay coming on. You should (in a few seconds at most) hear it click off again, and then the picture and sound should follow. This relay is usually controlled by the microprocessor or RC time base circuit.
Testing the Relay Circuit
The best way to test this circuit would be to disconnect the degaussing coil plug from the printed circuit board and measure the resistance between the contact points of the relay when you switch on the TV set. You should also check the relay coil for resistance, which should measure anywhere from 30 to 80 ohms.
Using the Degaussing Circuit
If a strong magnetic field comes close to the front of the TV set, such as child’s toy or a large pair of speakers, and the degaussing circuit is working correctly, the discolorations should go away after a few on/off cycles of the set. You must wait in-between the on and off cycles so the thermistor will cool down. On relay controlled sets, a hard reset will be needed. A hard reset can be done by pulling the main AC plug out of the wall for several minutes.
Some other reasons discoloration can occur on a picture tube face are:
- The TV set was moved or rotated around
- A strong lightning strike occured nearby
- A piece of electronic equipment is placed on or near the TV set
If the auto-degaussing does not seem to work, or does not completely get rid of the discoloration, manually degaussing the tube may be necessary.
Manual degausses are available from most electronic part distributors, and consist of a hundred or so turns of magnet wire in a round coil. They include a line cord and a momentary switch. You simply plug in the coil and flip on the switch.
To manually degauss, bring the coil to within several inches of the screen. Slowly draw the center of the coil toward one edge of the screen and trace the parameter of the screen face. Then, slowly decrease the first parameter unitl the circles get smaller and smaller and you reach the center of the screen. Next, slowly back straight up across the room as you hold the coil. When you are about 5 to 6 feet away, you can release the line switch on the degaussing coil. This must be done in one complete step, and never release the degaussing coil switch until you are away from the TV set.
Never attempt to degauss the inside or back of the set, and near or around the neck of the picture tube. Only degauss the front, sides, and the top of the TV if needed. If you do, you could demagnetize the purity and convergence magnets, which can turn a minor problem into a major and costly repair.
In rare cases, such as when a TV set is dropped or a very large magnet comes close to the front of the TV screen, no amount of degaussing will remove discolorations. This is because the shadow mask inside the picture tube became dislodged or bent. I will discuss the picture tube elements later in this article.
More Circuits of the Basic TV Set and What They Do: Part 1 | Part 2