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