Assignment 13

TYPES OF ANTENNAS

Introduction

This report discusses the common types of antennas available, their design and their attributes, which includes the radiation patterns, the gain and bandwidth characteristics.
Antenna Types
There are basically three classifications of antennas: Dipole, Reflector and Yagi. But the aggregate difference is in the design, as will be discussed.

Yagi Antennas
A Yagi antenna has several elements arranged in echelon. They are connected together by a long element, called the boom. The boom carries no current. If the boom is an insulator, the antenna works the same.
The rear-most element is called the reflector. The next element is called the driven element. All the remaining elements are called directors. The directors are about 5% shorter than the driven element. The reflector is about 5% longer than the driven element. The driven element is usually a folded dipole or a loop. It is the only element connected to the cable, yet the other elements carry almost as much current. The more directors added, the higher the gain becomes. Gains above 20 dB are possible. But the Yagi is a narrowband antenna, often intended for a single frequency. As frequency increases above the design frequency, the gain declines abruptly. Below the design frequency, the gain falls off more gradually. When a Yagi is to cover a band of frequencies, it must be designed for the highest frequency of the band. An antenna has an aperture area, from which it captures all incoming radiation. The aperture of a Yagi is round and its area is proportional to the gain. As the leading elements absorb power, diffraction bends the adjacent rays in toward the antenna.
The formula for the aperture area of any TV antenna is A=Gl2/4p where l is the wavelength and G is the gain factor over an isotropic (having physical properties that do not vary with direction) antenna (not dB). The bandwidth of a Yagi can be increased by sizing the reflector for the lowest frequency of the band while sizing the directors for the highest. But this decreases the best gain of the antenna. A better way to increase the bandwidth is to replace the reflector element with a corner-reflector assembly.
This boosts the performance on the lower numbered channels without hurting the high channels. Although the Yagi/Corner-Reflector might not be the best antenna, it is the most common UHF TV antenna, mainly because it can be mounted on the front of a VHF antenna without degrading the VHF antenna. This is a very directional antenna. If miss-aimed by 5° one can lose 1 dB of signal. If the horizon is more than 5° above horizontal, one should tilt the antenna up to point at the horizon. The overhead view shows nulls at 30° and 90° to both sides. These can be used to eliminate multi-path (ghosts) or interference. One should simply rotate the antenna until the offending signal is in one of the nulls.


Reflector Antennas
Radio waves will reflect off of a large conducting plane as if it was a mirror. A coarse screen will serve as well. Reflector antennas are very common.The double bow-tie screen reflector shown above has an average gain of 6 dB. With a bigger screen it would have more. The parabolic reflector focuses the signal onto a single dipole, but its bandwidth is a little disappointing. The corner reflector has a little less gain but much greater bandwidth. The corner reflector has roughly the gain of three dipoles. It is a good medium gain antenna, widely used for UHF. Log-Periodic Dipole Arrays (LPDA)
The LPDA has several dipoles arranged in echelon and criss-cross fed from the front. The name comes from the geometric growth, which is logarithmic.
This is a very wideband antenna with a gain of up to about 7 dB. For any frequency, only about three of the elements are carrying much current. The other elements are inactive. As frequency increases, the active elements “move” toward the front of the array. Most VHF TV antennas are LPDAs. TV LPDAs come in two types: straight and Vee. The Vee type (LPVA) has a very slightly higher gain for channels 7-13.







The Dipole
This is the simplest TV antenna. Variations on the dipole are the bowtie, the folded-dipole and the loop (a difference on the folded dipole). All four have the same gain and the same radiation field: a torroid (doughnut shape). The gain is generally 2.15 dB.The dipole has positive gain because it does not radiate equally in all directions. To get more gain, an antenna must radiate in fewer directions. If one rotates an antenna about the forward axis (a line from the transmitting antenna) the signal strength will vary as the cosine of the angle. In other words, when the antenna elements are vertical, no signal is received because TV signals have horizontal polarization
Stacked Dipoles
N number of dipoles will take in N times as much RF power as one dipole, provided they are not too close to each other. Thus a 4-dipole antenna would have a gain of 8.15 dB. Dipoles are commonly stacked horizontally (collinearly), vertically (broadside), and in echelon (end-fire).
When dipoles are stacked horizontally, the horizontal beam width becomes very narrow. This is because they do not add in-phase for directions not straight ahead. Similarly, when stacked vertically, the vertical beam width becomes narrower. A lot of dipoles stacked vertically would give the gain you needed. The vertical narrowness of the resulting beam is of little importance, but the horizontal broadness of the beam means no rotor needed.

Reference:

• Common Antenna Types, a HDTV primer © http://www.hdtvprimer.com/ANTENNASAll adapted materials are protected by the named copyright. All rights reserved.

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Assignment 12

High Voltage Section

Introduction

The TV’s high voltage section provides the very high voltage needed by the CRT. This report will discuss the common problems and possible solutions in this section of the Television receiver.

Symptoms and Troubleshooting

The high voltage section symptoms vary, but the most common ones are loss of raster and high voltage, but sound is working, blooming, corona, overloading the horizontal output amplifier and arcing.
When the raster and the high voltage is lost, it means a loss in either DC or AC supply to the HV section. One can only use a HV probe or a neon indicator to check for voltages in this section. A defective high voltage rectifier or an open filter resistor can cause this,
Blooming is the condition in which an increase in brightness causes a simultaneous raster expansion and a loss of focus. This is caused by poor high voltage regulation. This translates that the high voltage decreases as the picture tube beam current increases. This may be causes by a bad diode, a defective picture tube, damper, flyback transformer or defective hold down circuits.
A defective horizontal output transistor causes loss of raster. This defect may have generated from a shorted flyback transformer.
Corona is the result of ionization of the air, caused by very strong electrostatic fields generated by high voltage circuits. It’s known by a hissing sound and a bluish glow surrounding the point of emission. The high voltage should be measured and corrected to fix this problem. Arcing is an electrical breakdown when two points are linked by a jumping spark, causing a loud snapping sound and a flash of light. Its eliminated by putting HV putty on the two places.
To troubleshoot the Flyback Transformer, one should ground the chassis, unplug the power cable and smell the transformer. When it’s burnt, it smells like burnt paraffin. Remove the transformer from the circuit and use an ohmmeter to check each pin for the expected values. This procedure will determine whether its faulty.
With shorted components, which result with no raster, burning out is easily visible, For dim, out of focus or horizontally narrow pictures, check for leaky devices which cause a reduction in voltage. Check the output of the horizontal output circuit, if the voltage is low, there must be a leaky transistor. If a transistor is open, there will be no high voltage, the fuse will blow and the television will shut down. If there is no input at the base of the of the HV transistor, check the horizontal circuits.
The bleeder resistor is connected next to the flyback transformer (usually) and its used to stabilize the high voltage in the television, keeping the focus voltage constant. If the focus voltage is incorrect, the picture will not be correctly focused. If the focus voltage is not present at all, then there’s no raster. This means that one has to replace both the resistor and the flyback transformer. The CRT should be discharged before working on the flyback transformer.

Reference:

• Howard W. Sams, Troubleshooting and Repair Guide to TV, 1st Edition, ©1996 by Howard W. Sams and Company, Prompt Publications, Indianapolis, Indiana. Pg 150-158
• Liff, A. Alvin, Wilson, Sam J.A. Color and Black and White Television, Theory and Servicing, 3rd Edition © 1993 by Prentice Hall, Inc, A Pearson Education Company, Upper Saddle River, New Jersey, pg 445-451
  

Assignment 11

Kinds of Remote controls

Introdution
The term remote control can be also referred to as "remote" or "controller" when abbreviated. It is known by many other names as well, such as the "clicker", "channel-changer", "selector", "splat", "magic hand", etc. Commonly, remote controls are used to issue commands from a distance to televisions or other consumer electronics such as stereo systems and DVD players. Remote controls for these devices are usually small wireless handheld objects with an array of buttons for adjusting various settings such as television channel, track number, and volume. In fact, for the majority of modern devices with this kind of control, the remote contains all the function controls while the controlled device itself only has a handful of essential primary controls. Most of these remotes communicate to their respective devices via infrared (IR) signals and a few via radio signals. They are usually powered by small AAA or AA size batteries.

The first machines to be operated by remote control were used mainly for military purposes. Radio-controlled motorboats, developed by the German navy, were used to ram enemy ships in World War 1. Radio controlled bombs and other remote control weapons were used in World War 2.
Once the wars were over, United States scientists experimented to find nonmilitary uses for the remote control. In the late 1940’s automatic garage door openers were invented, and in the 1950’s the first TV remote controls were used.First TV Remote Control The first TV remote control, called "Lazy Bones," was developed in 1950 by Zenith Electronics Corporation (then known as Zenith Radio Corporation). Lazy Bones used a cable that ran from the TV set to the viewer. A motor in the TV set operated the tuner through the remote control. Although customers liked having remote control of their television, they complained that people tripped over the unsightly cable that meandered across the living room floor.
Flashmatic: The First Wireless TV Remote Zenith engineer Eugene Polley invented the "Flashmatic," which represented the industry's first wireless TV remote. Introduced in 1955, Flashmatic operated by means of four photo cells, one in each corner of the TV cabinet around the screen. While it began the concept of wireless TV remote control, the Flashmatic had some limitations. It was a simple device that had no protection circuits and, if the TV sat in an area in which the sun shone directly on it, the tuner might start rotating.
Development ChallengesZenith management loved the concepts proven by Polley's Flashmatic and directed his engineers to develop a better remote control. First thoughts pointed to radio. But, because they travel through walls, radio waves could inadvertently control a TV set in an adjacent apartment or room.
Using distinctive sound signals was discussed, but Zenith engineers believed people might not like hearing a certain sound that would become characteristic of operating the TV set through a remote control. It also would be difficult to find a sound that wouldn't accidentally be duplicated by either household noises or by the sound coming from TV programming.
The Birth of Space Command Zenith's Dr. Robert Adler suggested using "ultrasonics," that is, high-frequency sound, beyond the range of human hearing. He was assigned to lead a team of engineers to work on the first use of ultrasonics technology in the home as a new approach for a remote control. The transmitter used no batteries; it was built around aluminum rods that were light in weight and, when struck at one end, emitted distinctive high-frequency sounds. The first such remote control used four rods, each approximately 2-1/2 inches long: one for channel up, one for channel down, one for sound on and off and one for power on and off.
They were very carefully cut to lengths that would generate four slightly different frequencies. They were excited by a trigger mechanism -- similar to the trigger of a gun -- that stretched a spring and then released it so that a small hammer would strike the aluminum rod. The device was developed quickly, with the design phase beginning in 1955. Called "Zenith Space Command," the remote control went into production in 1956.
Quarter Century of Ultrasonic Remotes The original Space Command remote control was expensive because an elaborate receiver in the TV set, using six additional vacuum tubes, was needed to pick up and process the signals. Although adding the remote control system increased the price of the TV set by about 30 percent, it was a technical success and was adopted in later years by other manufacturers.
In the early 1960s, solid-state circuitry began to replace vacuum tubes. Hand-held, battery-powered control units could now be designed to generate the inaudible sound electronically. In this modified form, Dr. Adler's ultrasonic remote control invention lasted through the early 1980s, a quarter century from its beginning.
Today's Infrared Remote Controls By the early 1980s, the industry moved to infrared, or IR, remote technology. The IR remote works by using a low frequency light beam, so low that the human eye cannot see it, but which can be detected by a receiver in the TV. Zenith's development of cable-compatible tuning and teletext technologies in the 1980s greatly enhanced the capabilities and uses for infrared TV remotes.
Today, remote control is a standard feature on other consumer electronics products, including VCRs, cable and satellite boxes, digital video disc players and home audio receivers. And the most sophisticated TV sets have remotes with as many as 50 buttons.
Zenith developed the world's first wireless trackball TV remote control, called Z-Trak. The remote works like a computer mouse - click the ball and a cursor appears on the TV screen. Roll the ball and the cursor activates control menus hidden in different corners of the screen. Then, activate something from those menus - bass, treble, contrast, color temperature, channel... etc.
Manufacturers used to only make remote controls that operated one TV set. However, they are now making universal remote controls that can operate any TV set. Experts predict that someday remote controls will control almost every device in the home.

Reference:

• Adapted from the History of Robert Adler, Remote Control© 1997 - 2007 The Great Idea Finder, All rights reserved, http://www.ideafinder.com/history/index.html.

Assignment 10

TV VIDEO IF SECTION

Introdution

The video IF section in a color tv is not very different from that of a B & W tv. The main functions are:

• amplification of the video if to 38.9MHz


• detection of the video if and the composite to form a complite color


• generate automatic fine tunning voltage for the tunner.

Symptoms and possible solutions
A complete loss of sound in a color receiver must be the result of an open in the signal flow path through the video tuner or IF amplifier. If the tuner has been found to be good, these symptoms mean that one or moiré stages in the video IF is dead. One should therefore check for bad transistors, broken connections and other faulty components. A weak picture and a weak sound with no snow and a possible buzz in the sound are those cases in which the gain of the video IF has decreased. On extreme conditions, where the IF amplifier is overdriven, the picture may become a negative and may also call for a defective AGC section.
Picture misalignment, which cause smeared pictures, loss of color, vertical and horizontal sync, and ringing, may be caused by an overdriven IF amplifier, a misaligned video IF.
Ringing means that the IF amplifier is regenerating a high peak frequency that shows at the output.The most common causes are a defective transistor, open or defective bypass capacitors, open or changed value neutralizing capacitors, open damping resistors, open or shorted AGC filter capacitors, defective tuned circuits, poor grounding and misalignment of the Video IF.
If there is no audio of video, but the raster is fine, the video detector is faulty, and there will be a soft background hum and some background noise controllable by the volume adjustment.
When testing the IF Amplifier, measure all the voltages using a voltmeter, and compare them with the indicated values on the schematic.
For an IC driven IF section, measure the power and the supply, voltages, then all the pins for comparison with theoretical values, which will indicate whether the IC is faulty or not.
Using an oscilloscope, look for an injected signal at the output of the IF amplifier and determine the expected changes. This section reacts to slight changes in component values, so a quick check of all immediate components may save a lot of time during troubleshooting.

Reference:

• Howard W. Sams, Troubleshooting and Repair Guide to TV, 1st Edition, ©1996 by Howard W. Sams and Company, Prompt Publications, Indianapolis, Indiana. Pg 85-89


• 
  Liff, A. Alvin, Wilson, Sam J.A. Color and Black and White Television, Theory and Servicing, 3rd Edition © 1993 by Prentice Hall, Inc, A Pearson Education Company, Upper Saddle River, New Jersey, pg 176-181
  
  
  

Assignment 9

TV Sound Section Problems and Solutions

Introduction

Problems that occur in a television’s sound section are usually not accompanied by any picture problems. This report will discuss the common problems that are encountered in every day use of a TV

Symptoms and Possible solutions
Problems caused by external blocks

If there’s poor sound quality accompanied by an unstable picture, the problem may lie in the AGC circuits. If the sound and the picture do not track properly, this may be a problem with the detector coil or a poor sound alignment. A hum or a running sync suggests a problem with the sync section or the AGC circuits.
Squealing or whistling, distorted or intermittent sound or no sound accompanied by a normal picture and raster will lead us to the following procedure in the sound section

Sound Section
Inject a signal into the speaker to make sure that the speaker is operational. If a hum or a tone is audible, then the speaker is not faulty. But it is still wise to check the speaker for beaks or warps in the cone. Check the speaker impedance and relate it to the indicated value.
If the speakers still do not produce sound, then there could be an open connection in or around the audio amplifier, however, trace the sound from the video section, through the sound IF, the sound detector or the amplifier to eliminate other possible causes.
If the sound for the speaker is continuously humming, then the audio amplifier is the most probable cause for this fault. A humming sound could be caused by a leaky or open transistor or capacitor, a change in resistance or voltage that could be too high or too low, Check the output of all stages and match the theoretical values to the actual ones, its best to trace back form the speaker, this may help in isolating the fault more quicker. If the sound is weak and slightly distorted, then adjust the detector coils to remove the frequency variations. If the television is using an IC, then test all pins for the expected voltages. Squealing or whistling sound may be a faulty transistor or capacitor in one of the audio stages. Check each component around the IC and compare the measurements with theoretical values.
If there’s a popping or cracking sound, check the audio amplifier stages and test the transistors. One may have to remove them from the circuit; open filter capacitors can also cause this fault. If the sound is mushy (blatting) when the volume is turned down or dull, low pitched and vibrating when turned up, there could be a faulty transistor, resistor, or poor sound alignment.
Loose wiring and open or shorted connections cans also cause intermittent sound problems. With the chassis and speaker on, lightly pull all the components to ensure that they are connected, use a cold spray to detect cold solder or heat sensitive components.

Reference:
Howard W. Sams, Troubleshooting and Repair Guide to TV, 1st Edition, ©1996 by Howard W. Sams and Company, Prompt Publications, Indianapolis, Indiana. Pg 116-120

Assignment 8

The sync seperator

Introduction
The sync seperator is very important in the tv because it syncronises the vertical section and the horizontal section.This report descuses the problems and solusions of the sync seperator.

• Loss of Vertical Synchronization or Rolling

After other obvious causes have been eliminated, this problem can be caused by a defective tube, an open integrator transistor, a shorted integrator capacitor or an open resistor between the sync separator and the vertical oscillator. If there’s a hum accompanied with the symptom, it can be caused by poor B supply filtering. The best way to approach this is to test all components and check all outputs for expected readings in order to isolate the problematic component.

• Loss of Horizontal Sync

If this symptom is not accompanied by a loss of vertical sync, it may be assumed that the sync stage is working and the problem lies in the horizontal section, otherwise it may be caused by a defective horizontal sync amplifier, and an open or leaky capacitor in the horizontal Automatic Frequency Control (AFC). A defective AFC may also cause this problem. Test all components in the horizontal sync to isolate the cause.

• Loss of Vertical and Horizontal Sync

This problem indicated that the deflection oscillators are free running and that either the video is not reaching the sync separator or the sync separator is not functioning. One should trace the signal to determine where it halts or diminishes in order to find the cause. The possible components that could be defective are a transistor in the sync or noise limiter stages, open, shorted or leaky video coupling capacitors or a change in value of the video sync isolation resistors (possibly open).

• Horizontal Bending or Pulling

This may be caused by a defect in the video strip, the AGC section, the sync separator, the horizontal AFC or the low voltage power supply. An overloaded video strip may be the cause of this symptom due to a bad hum modulation in the low voltage power supply section. One should check all the filter capacitors in this section to locate the fault. Other causes may be a defective transistor or IC or a leaky coupling capacitor. Due to the vast possibilities in this problem, one should test all components and work up the sections to the sync separator strip to best isolate the problem.

• Vertical Jitter

The picture in this problem seems to lose vertical synchronization and may be the result of poor waveform separation, such that the picture appears to bounce. This may be caused by a defective IC or transistors in the sync separator section, or a defective component in the integrator section. One should trace the signal form the video section and isolate the section which withholds the output, then isolate the defective component by testing each one.

• Lack of sync horizontal

The picture drifts smoothly horizontally. Depending on the difference between the video horizontal rate and the free-run frequency of the horizontal oscillator, the picture may be torn left or right is a special case where the frequencies are virtually the same. This may mean that the horizontal sync signal is missing due to a sync separator problem or that there is some a fault in the sync processing circuitry. Trace forward and isolate the cause.

• Incorrect lock horizontal

This may mean a more-or-less stable torn picture. This means that the sync signal is reaching the deflection system but that it is having problem locking to it. The horizontal oscillator free-run frequency may be too far from what it is supposed to be.

• Lack of sync vertical

The picture rolls smoothly vertically. This may mean that the vertical sync signal is missing or the deflection system is ignoring it.

Conclusion

Sync Problems usually reflect on the picture tune as irregularities in the raster but not sound (usually). One can visually determine its malfunction externally and save a lot of troubleshooting time.

Reference:

• Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Television Sets, Copyright © 1994-2007 Samuel M. Goldwasser, http://www.repairfaq.org

• Alvin A. Liff, J.A. Sam Wilson, Color and Black and White Television, Theory and Servicing, 3^rd Edition©1993 Prentice Hall Inc. upper Saddle River, NJ 07458

Assignment 7

PROBLEMS AND SOLUTIONS OF TV HORIZONTAL SECTION

Introduction
This is the section in the receiver which receives the high voltages. It is responsible for the deflection of the beam in the horizontal direction. It receives the high voltages form the flyback. The main problems may lie on the high switching diodes or the capacitors. Some of the problems would include:

• No Raster, No sound

This is usually caused by a failure in the high voltage supply, accounted by the loss of raster and sound. This points to the horizontal output transistor, which is inoperative. What could have caused the shorting of the transistor could be shorted turns in the yoke, a shorted flyback transformer, a shorted high voltage rectifier, a shorted pincushion transformer, an open tuning capacitor, a shorted scan rectifier, incorrect horizontal drive frequency and high voltage arcing. One should never eliminate one and not test other devices. In this section of the receiver, it’s imperative to be sure.

• Insufficient or Excessive Width, All Else is Functional

This can be caused by a reduced power supply voltage, a leaky capacitor other leaky or shorted components. It can also be caused by a tuning capacitor that is too small or too large.

• The Horizontal output transistor (HOT) keeps blowing

This may be caused by dried up capacitors, open resistors or chokes, bad connections, or a driver transformer with shorted windings or broken or loose core. Excessive voltage on the HOT collector (check LV regulator) can cause this problem or defective safety capacitors or damper diode around the HOT. A new transistor not mounted properly to a heat sink may cause this and probably needs a mica washer and heat sink compound. Another possible cause for random failures of the HOT are bad solder connections in the vicinity of the flyback and the HOT as well as the horizontal driver and even possibly the sync and horizontal oscillator circuits, power supply, or elsewhere

• Lose of horizontal sync

The problem lies either in the horizontal oscillator or in the sync system. If it really is a problem with sync pulses not reaching the oscillator, the picture will move around horizontally and can be brought to hold momentarily with the hold control. If the picture breaks up into strips, there is a problem in the horizontal oscillator. One should rotate the hold control: if the frequency is too far off, the picture will not settle into place at any adjustment of the hold control. One should look around the horizontal oscillator circuit: all of the oscillator parts will be right there, or check on the horizontal oscillator module.
Reference:

• Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Television Sets, Copyright © 1994-2007 Samuel M. Goldwasser, http://www.repairfaq.org

• Alvin A. Liff, J.A. Sam Wilson, Color and Black and White Television, Theory and Servicing, 3^rd Edition©1993 Prentice Hall Inc. upper Saddle River, NJ 07458

Assignment 6

Problems of The Vertical Section

Introduction
The vertical section of the tv is responsibel for the diflection of the beam in the vertical direction. The main problems are discussed belo winclude:

• Loss of Vertical Sweep

This is characterized by the raster collapsing to a single horizontal line in the center of the screen. This could be an inoperative deflection oscillator in the vertical strip. It could also mean a defective vertical amplifier or an open yoke. The best way to approach this problem is to inject a 60Hz signal from the oscillator output and trace it back to the deflection yoke. This will determine where the problem occurs.

• Insufficient Height of Vertical Deflection, But Linear

This could be a cause of misadjusted height and linearity controls in the vertical sweep amplifiers, increased sawtooth time constant in the sawtooth generators or a low amplification level at the output.

If the sawtooth generator peak to peak levels are as required, then the trouble must be in the amplifier. The compliment of this is true.

Poor Vertical Linearity

This could depict misadjusted linearity controls at the vertical strip. It could also be faulty output transistors or leaky bypass capacitors for the cathode or defective output transformers in the sawtooth forming circuits.

• Excessive width or pin cushioning problems

This calls for an internal adjustment which adds a signal to the horizontal deflection to compensate for the geometry of the CRT/deflection yoke. It could also be faulty capacitors in the horizontal deflection power supply.

• Loss of vertical synchronization

This may be caused by insufficient amplitude or a change in the free running vertical sweep frequency. If the vertical hold is able to stop the picture from rolling, then its possible that the oscillator if functional. But to check through each component in the vertical strip will save time in isolating this problem.

• The Keystone Raster

The vertical deflection windings of the yoke are placed horizontally on either side of the neck of the picture tube. These windings are responsible for creating a uniform horizontal magnetic field inside the neck of the picture tube. If a shorted turn were to develop inside one of the windings, the magnetic field closest to it would be weakened. The electron beam would receive little vertical deflection when the horizontal deflection brings it near the weak magnetic field. The height of the raster near the weak magnetic field would be less than that of the opposite side, thus producing the keystone raster.

But shorted turns are not the only cause for this problem, open windings are consequently shorted through the damping resistor to the other good winding and cause this problem.

Reference:

• Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Television Sets, Copyright © 1994-2007 Samuel M. Goldwasser, http://www.repairfaq.org

• Alvin A. Liff, J.A. Sam Wilson, Color and Black and White Television, Theory and Servicing, 3^rd Edition©1993 Prentice Hall Inc. upper Saddle River, NJ 07458

Assignment 5

The Tuner Section

Introduction
the tuner section of the tv has a number of problems that can cause the viewer of the television to have a bad reseption and picture.Some of the problems will be highlighted include:

• No Picture, No Sound But the Raster Is Evident

This is the most common tuner problem, and the most basic. It could translate into lack of a signal at the antenna, so one should first check all stations to verify whether its an external problem. Trace the signal from the antenna to the tuner. If the antenna system is fine, isolate the problem in the tuner by injecting a signal and trace it through the RF amplifier, the mixer and the AFT section and verify expected outputs progressively. The cause of the problem will not give the required output signal.

A defective RF amplifier will definitely cause this symptom; one should thoroughly test it before proceeding to other sections. Any defective part will require replacement.

This could also be caused by a defective oscillator. Test the output of the oscillator to determine whether it’s producing the required frequency at the specified amplitude.

• No Picture, No Sound, But on Selected Channels

This will definitely not go beyond the local oscillator, but its advisable to trace the signal from the antenna to eliminate a defective antenna system. Then follow confirm the oscillator signal to verify its validity. This will probably prove that the problem was caused by a misadjusted oscillator. However, solder could be loose on one of the tuner components.

• Weak, Snowy Pictures

This may be a consequence of an insufficient signal to the antenna, so one should check other channels. This could also translate to an excessive AGC voltage, or insufficient oscillator voltage; a defective oscillator.

In some sets the tuner section could have a coupling transformer, and this can cause this problem. This setup would be accompanied by coupling capacitors which could be defective or open.

• Poor Picture Quality

Misadjusted or defective RF amplifier could be the cause of this. It could also be as basic as the wrong antenna positioning, or a poor quality coaxial cable. If these have been eliminated, inject a signal and trace it through the tuner. Check also for open coupling capacitors or an open AGC isolating resistor.

• No Color

This could be a consequence of fine tuning, assuming the obvious causes are eliminated, i.e. color control and reception. Check the AFT current and verify its required level.

• Ghosts

This means a second picture superimposed the main one. The ghost picture is usually to the right. This indicates problem which is external, i.e. two signals of the same transmission received at the same time. One should try adjusting the antenna,

• No UHF, But VHF is Functioning.

One should start with the antenna and check the UHF section of it for possible causations. If this is eliminated, one should proceed to the UHF strip in the tuner and inject a signal for verification. The problem should be encountered here.

Conclusion

Troubleshooting a tuner sometimes may mean the replacement of whole modules, if that is the case and the apparent problem extensive but isolated, it is advisable to replace whole parts because tuner circuits are precision oriented at attaining this by part replacement could be hard.

Reference:

Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Television Sets, Copyright © 1994-2007 Samuel M. Goldwasser, http://www.repairfaq.org

Alvin A. Liff, J.A. Sam Wilson, Color and Black and White Television, Theory and Servicing, 3^rd Edition©1993 Prentice Hall Inc. upper Saddle River, NJ 07458

Assignment 4

Problems and solutions of the power supply

Due to the different kinds of power supplies available, the designs are different in so many aspects, but the following components never lack, and they are usually the cause of many problems in the TV Power supply section:

1. A power switch, relay, or triac to enable main power.

2. A set of rectifiers - usually in a bridge configuration - to turn the AC into DC. Small ceramic capacitors are normally placed across the diodes to reduce RF interference.

3. One or more large filter capacitors to smooth the unregulated DC voltage.

4. A degauss control circuit usually including a thermistor or Posistor (a combination of a heater disk and Positive Temperature Coefficient (PTC) thermistor in a single package).

The TV experiences Blackouts, either intermittently or permanently

The most immediate cause may be the power wall outlet may be faulty; one should check and confirm that it’s working first. After this has been cleared, work up the cord and check for continuity up to the TV power switch. Check for a faulty power switch. Check the mains transformer and confirm that both sections, primary and secondary, are functional.

Loss of Raster, No sound

This may be the result of a break in the PCB or an open component. Check the switch, then the fuse, power resistors, rectifier diodes and the choke inductor. If these devices are confirmed to give the right output, move to the following sections of the TV for trouble shooting:

1. Degauss Posistor, check for shorts
2. Horizontal output transistor.
3. Power supply regulator (if there is one).

Loss of Raster, Weak Sound

This is usually caused by a leaky or open filter capacitor. If the readings are confirmed in their right output levels, move to the following sections of the TV for Trouble shooting:

1. Video Amp Output
2. Sound Processing Strip.

Dim, Shrunk or Poor Raster Linearity

This symptom is usually accompanied by a humming sound. It’s caused by a reduction in the supply voltage. Check the rectifier diodes or the filter capacitor for correct readings. Then check the main rectifier diode for any defects. There should be a defective component among these. If not, follow the power supply lines on the PCB and confirm continuity. There could also be a case of a loose component in the power supply section.

Overheating

This is a consequence of excessive current demand and is marked by repeated fuse blowing. This is caused by a shorted component especially in the turns of the power transformer. To find the defective component, it may be visually obvious or one has to isolate it for the circuit for testing. Check all the components of the power supply.

Conclusion

Sometimes the solution is not as obvious as mentioned here, thus other conventional approaches are not disqualified.

Reference:

• Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Television Sets, Copyright © 1994-2007

• Samuel M. Goldwasser, http://www.repairfaq.org

• Alvin A. Liff, J.A. Sam Wilson, Color and Black and White Television, Theory and Servicing, 3^rd Edition©1993

• Prentice Hall Inc. upper Saddle River, NJ 07458