Circuits Of The Basic TV and What They Do - Part 1

When you finish this series of articles on the innards of the basic TV, I hope that this information will give you a basic understanding off what these circuits are called and what they do. Even knowing the basics of how a TV works and what can go wrong can be very useful, even if you do not attempt to repair the set yourself. You should be able to deal with a service technician and understand what they are telling you. More importantly, you're likely to better judge if you are being taken advantage of by a dishonest or incompetent repair center.

For example, someone once called me after getting an estimate on a TV set lacking color from a different service person. The set had a great black and white picture, but simply didn't show color. They were informed that the picture tube was bad! A bad picture tube CANNOT cause a TV to show a good black and white picture, but lack color. I would bet most consumers are unaware of this simple fact. It turned out that the color problem was a 10 cent capacitor I found to be open.

If you do not understand any terminology I have used here, please visit this TechLore thread. If the term you seek is not there, leave a note in that thread and it will be added.

If you need to ask a question about something in this article, leave a question or note in the comment box below. Any questions about a TV set fault or a repair problem will be moved to the appropriate thread. This article is intended to provide a brief explanation of different TV circuits and how they operate. Later in this series I will go over the circuits and how they work in a basic projection TV set.

Safety Guidelines

The set of guidelines below is not meant to scare or frighten you, but to make you aware that working inside of a CRT type TV or computer monitor can be deadly from the line connected power supply, as well as the High voltage power supplies. Please follow all of the safety guidelines.

  • Wear rubber soles on your shoes or wear sneakers.

  • Set up your work area away from any grounds that you may touch by accident.
  • Do not work alone! If there is an accident or you get shocked, someone else present could call for emergency assistance.
  • If you need to work on a circuit or measure a component, make sure you discharge the large power supply and High voltage capacitors with a 2 to 5 watt 100 ohm to a 500-ohm resistor, as these capacitors can give you a jolt and possibly stop your heart (this is a high current DC). Never assume anything! Always check it out with a voltmeter across the capacitor. You do not need to discharge the smaller 150+ capacitors in the circuit that are below 35 volts.
  • Do not wear any jewelry or anything metallic that could make contact with the live circuitry or any moving parts.
  • Never strike or hit the glass envelope of the picture tube, as this would cause an implosion. An implosion would send shards of glass in all directions at a very high speed.
  • Always wear eye protection when working on the back of a TV set.
  • Always connect and disconnect test leads from your meter or other test equipment with the set unplugged.
  • Attempting to repair a set when you are tired or sleepy can make you more careless.
  • Do not take shortcuts.

Have the proper tools available

You will need a basic set of decent hand tools to work on most TV sets and to make most adjustments. You do not need the best expensive tools, but cheap tools are worthless. You will need:

  • A variety of different sizes of Phillips and straight bladed screwdrivers.

  • A set of assorted hex-type socket drivers.
  • A couple different sizes of needle nose pliers and wire cutters.
  • A good pair of tweezers and dental picks.
  • A small screwdriver with a miniature 1/16 inch blade and a non-metallic tip for adjustments.
  • A low power, fine tipped soldering iron and fine rosin core solder for the small soldering and unsoldering jobs. A 25-watt iron should be fine for the smaller parts. For those bigger parts; you will need a higher power soldering iron or a small soldering gun.
  • A roll of desoldering braid or a bulb type desoldering tool.

Continue to page 2 >>

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

For those problems that occur after the set warms up, you can use what's called a circuit chiller. This allows you to spray the suspected parts to see if the problem goes back to normal. These sprays come with a small extension tube that you put onto the spray nozzle so you can localize a bad part easier.

For those problems that do not happen right away, but instead get worse as the set warms up, use a simple hairdryer and a cardboard nozzle to direct the heat to a more centralized area on the circuit board and suspected components to make the problem occur. When (if) this happens, use the circuit chiller to see if the problem goes away. If it does, you got lucky and found the intermittent part.

Intermittent problems can be the most costly and time consuming types of repairs. When you cannot find the intermittent component, there could be a small crack in the printed circuit board.

Preventive Maintenance on your TV Set

The preventive maintenance on your TV set is easy, and if followed can help the TV set last longer.

  • Keep the outside cabinet clean and do not block the vent holes or slots.

  • Allow adequate ventilation - TVs use more power than any of your other A/V components. Heat buildup takes its toll on electronic parts. Leave at least 3 inches on top and sides for air circulation if the entertainment center does not have a wide-open back panel. Do not pile other components (like VCRs) on top of the TV.
  • Clean the surface of the picture tube front with a soft cloth barely dampened with clean water, and if needed, a mild soap detergent. Never apply water, spray a glass cleaner, or a wet cloth directly onto the surface of a picture tube. Doing this can cause a serious circuit failure due to moisture seeping around the edges of the picture tube and dripping on the circuit board directly under the tube.

    If you have not cleaned your tube off in a week or two wipe it down and you will probably be amazed at the amount of filth and dirt that gets attracted to the front of a TV set. This is especially true for smokers, as the tar and nicotine will build up fast on the picture tube face. I once went out on a service call where the customer was complaining of a dull and weak picture. As soon as I looked at the set, I knew what the problem was. I licked my finger and ran it across the front of the TV set and wow, the brightness poured out of the TV set. To say the least, the customer was quite embarrassed.

  • It's important to clean the inside of the TV before you work on it.

    First, locate all of the Hex or Phillip type screws that hold on the back of the set. With the back off you'll undoutedly wonder why it is so dirty in there. The high voltage sucks in most of the dirt that gets airborne in the room when the room is cleaned.

    With the back off, use a new paintbrush and a small nozzle attachment of a vacuum cleaner. Be very careful and do not apply too much pressure while brushing off the built up dust. While true you could use compressed air, it will make a mess all over the room, and the dust is not a healthy thing to be breathing in.

    Do not try to tighten anything like screws or turn anything that you do not know what it is. They may be a critical adjustment. Also, be very careful of the Picture tube neck and anything mounted to it. The picture tube neck is very thin glass and won't take much before it snaps. The components mounted on the neck should not be disturbed as well.

    Mounted on the neck of the picture tube is the deflection yolk, and the rings are the convergence and purity magnets. Just as a precaution, I will mark those rings on the neck with some type of marker in case I hit them by accident. Some larger tube sets have a small circuit board attached to the neck. This board is the beam velocity modulator.

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

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.

<< Return to page 2 | Continue to page 4 >>

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

All Things Vertical

The vertical deflection circuit has a vertical oscillator that is locked to the video signal by the vertical sync pulse taken from the sync separator. If you get a set that is rolling, somewhere the vertical oscillator has lost the sync pulse, or there is a problem in the vertical oscillator circuit.

A vertical drive circuit is then amplified and is inserted to the vertical output device in the TV set. In almost all sets today, the vertical output circuit is built into what is known as the vertical output IC. The vertical output pulse is then inserted in to the vertical winding after it goes through a coupling circuit, and the deflection yolk will scan the electron beam inside the picture tube 60 times a second up and down the screen surface. With the horizontal and the vertical pulling on the beam, it spreads out to make the picture.

The vertical output IC will have several legs or connections to shape the vertical pulse to be amplified by the vertical outputs by means of capacitors and resistors. In addition, the B+ and the B- are connected to these leads. On a lot of TV sets where the vertical may look squashed or lines in the top or bottom or simply not enough of vertical deflection, it is mostly these small inexpensive parts.

The vertical output IC is almost all the time connected to a metal heat sink. If you ever think you need to change an IC on a heat sink, be sure to apply some heat sink compound on the back of the new device.

In many older sets, as well as some of the newer ones, you will hear a steady buzzing when the set is on. This buzzing sound seems to change with scene changes. This is the vertical windings of the TV set vibrating at 60 cycles per second when they get loose, and they start to make noise since 60 cycles per second is in the human hearing range. There's not really a lot you can do to stop this noise 100% besides changing out the deflection yolk. There are some old tricks, like sticking Popsicle sticks under the yolk windings, but I have seen tech’s crack the neck of the picture tube while doing this.


In closing of part one of this article, I want to stress that you can turn an easy repair into a expensive repair with the wrong solder or soldering equipment, or by playing with service menus, not knowing what does what, or by turning the controls inside a TV set just to see what will happen.

Many problems have simple solutions. It may just be a bad connection or blown fuse. If you ever have any doubts or do not feel comfortable with a repair or adjustment, let a professional do the repair or internal adjustments for you.

The next series will explain the degaussing circuit (and some tips on the subject), as well as the tuner circuits, IF circuits, audio output circuits, color circuits, video drivers, and the system controller circuits. Stay tuned!

<< Return to page 3 | Comment on this article

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


ican anyone help me, i am trying to fix a Sinotec tv with a vertical deflection problem, i have changed the ic but the problem still persist.


Connect With Techlore