The Circuits and Inner Workings of the TV set

Most problems in today's TV sets happen in the horizontal deflection, vertical deflection, high voltage power supplies, and main power supplies because they run at high power levels, and many of these components run hot. Heat is a TV's worst enemy. In this article, I will cover these sections, as there is a lot to learn and absorb.

Standby and Main Power Supplies

All modern solid-state TV sets have a standby power supply and main power supply circuit. The standby supply will provide the voltage needed to run the microprocessor IC in the TV set. This consists of a low voltage power transformer feeding one or more sets of rectifiers, filter capacitors, and possibly regulators.

When you push the power switch on the front of the set (or push the power button on the remote control), the microprocessor will have a turn on signal that will start up the main supply, which in turn will engage the horizontal circuit. If all is well, a small signal comes from the High voltage supply, or what is called a sweep derived power supply, and keeps the main power supply active.

Some sets have a main power switch on the rear of the set. I can't count how many times I have seen a dead set, and it was because someone turned off the main power switch.

All main power supplies have:

1. A power switch, relay, or triac to turn the main AC power on.
   
   
2. A set of rectifiers, most of the time a bridge rectifier, to convert the AC into DC.
   
   
3. One or more large filter capacitors to smooth the unregulated DC. In the U.S., this voltage is around 145-165 V DC. In countries with 220 VAC power, it will be around 295-325 V DC.
   
   
4. An IC or hybrid type of a regulator to provide a steady DC voltage to the horizontal deflection system. Sometimes a small pulse or voltage from a secondary output of the flyback.
   
   
5. A degauss control circuit. When power is turned on, a high AC current is applied to the degauss coil wrapped around the outer envelope of the CRT. The thermister heats up, increases
   in resistance, and slowly lowers the current to nearly zero over a couple of seconds.

Getting Horizontal

The horizontal deflection circuit starts out with a sync separator circuit, which once the sweep is started up, takes a sample of the video signal and separates the vertical and horizontal sync pulses, then sends them to the appropriate sweep circuits. These sync pulses lock the horizontal and vertical sweep to the video signal for stabilization of the picture.

Then there is a horizontal driver circuit. The Sync separator, horizontal and vertical oscillators, and driver circuits are (most of the time) in a main IC called the "driver chip" or part of a Jungle IC.

Then we get to the horizontal driver circuit, and that simply amplifies the horizontal waveform to drive the horizontal output transistor. Never try to measure or touch the collector of the horizontal output transistor, as doing so will burn out your meter or give you one heck of a shock because of the very high frequency pulse there. This is also known as the "HOT". That pulse drives the flyback or the "Line Output Transformer" (as it is known in Canada), which makes the high voltage for the picture tube and the sweep-derived voltages needed for the other parts of the TV set.

The horizontal deflection then goes through what's called a waveform shaping circuit (known as the pincushion circuit) and finally through the horizontal deflection winding on the deflection yolk around the picture tube neck. This in turn scans the electron beam from the picture tube to scan back and forth horizontally on the inside of the picture tube... some 15,734 times per second in an NTSC type TV.

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More Circuits of the Basic TV Set and What They Do: Part 1 | Part 2

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