Transcript Video IF Amplifier

Video IF Amplifier
Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
UEAB2006
Three parameters considered as
waveform for Video IF amplifiers
Amplitude
 Frequency – when the amplitude and the
frequency are displayed it is called
frequency domain displayed
 Time – when the amplitude and time are
displayed it is called time domain display.
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Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
UEAB2006
Intermediate frequency
41.25 MHz Video free interference
frequency
 41.25 Sound free from possible
interference frequency
 Tuned to 25.75 MHz and 21.5 MHz
oscillator frequency range from 80 MHz to
212 MH z
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Falls on channel 5-13
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IF Stage
Sound takeoff is obtained from the last IF
amplifier and is feed to a separate sound
IF detector
 Today’s sound takeoff points occurs in the
video IF Stage
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Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
UEAB2006
Four Major Function of Video IF
Strip
To compensate for vestigial sideband
transmission
 To provide the most of the selectivity
required by the receiver for interferencefree reception
 To provide most of the picture and sound
signal amplification of the receiver
 To frequency change the IF into the video
signal at the video detector
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Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
UEAB2006
IF Response
45.75 MHz the desired IF signal produced
at the picture IF carrier
 41.25 MHz for the sound IF carrier
 Upper adjacent picture at 39.75 MHz
 Lower adjacent sound carrier at 47.25
MHz
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UEAB2006
Inter-carrier Buzz
Misalignment can cause excessive
amplitude modulation at 4.5 MHz
 Possible loss of sound and picture carriers
may result to a 60 Hz rate of the vertical
sync pulse
 Inhibition from AM signal is not completed
that can cause or may result to Buzz
signal
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Compensation for Vestigial
sideband Transmission
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Three signal components
Lower sidebands extend to 1.25 MHz bellow
the carrier
 Upper sidebands extend 4.2 MHz above the
carrier
 Partial sideband called Vestigial
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Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
UEAB2006
Color Receiver IF response
The Video Sound IF
 The picture IF
 Upper and lower Sidebands associated
with the black and white or luminance
signal: these signal range from 0.75 MHz
bellow the carrier to 4.2 MHz above the
picture carrier
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Television 1 Jess Role @
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Video IF traps
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Reduce Video IF response by means of
highly selective resonant filters
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Common types of Traps
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Series resonant shunt traps – mild short
circuiting
Parallel resonant series traps – signal path
blocking
Transformer traps- filtering through magnetic
induction using the secondary as the output
Absorption traps – eliminating undesired signal
Bridge T traps – using resistors by determining
the amount of attenuation
Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
UEAB2006
Television Video IF requirements
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Over-couples tuned transformer- primary and secondary
with a degree of mutual coupling adjusted to exceed
critical coupling
Bifilar transformer- unity coupling
Loaded tuned circuits- using loading resistor to balance
the circuit
Synchronous tuning- tuned circuit with the same
frequency
Stagger tuning – obtaining a wide band-pass amplifier
synchronizing other amplifiers
Walking IF- adjustable bandpass circuit
Surface acoustic wave (SAW) filters – using piezoelectric
materials to maintain a specific frequency
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Television 1 Jess Role @
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Transistor Video IF using Impedance
coupling between stages
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Television 1 Jess Role @
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Television 1 Jess Role @
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Television 1 Jess Role @
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IC Video Amplifier
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Equipment needed for Sweep
Alignment
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Consider Manufacturers alignment service
manuals
Make sure that SG has an accurate marker
combination
An oscilloscope can cope with low frequency at
low level
Matching pads- terminate SG 75 ohms
transmission line
Bias supplies- AGC bias clamps etc
Demodulator probes- using manufactures
alignment manuals observing frequency
response
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Video Alignment hookup
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B/W Video detector is consist of the
following
The composite video signal (0 to 4.2 MHz
 4.5 MHz sound IF
 A dc Component corresponding to the
average illumination of the picture
 Residual IF and higher-frequency
harmonic comonents
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Television 1 Jess Role @
UEAB2006
Video detector Used in Black and
White
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Television 1 Jess Role @
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Video detector
used in Color
Television
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Troubleshooting Video IF problems
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No picture no sound raster ok
No picture sound ok raster ok
Weak color and black and white picture, weak
sound, raster ok
Overloaded or negative picture buzz sound
raster ok
Inter-carrier buzz in sound
Poor vertical and horizontal synchronization
tunable ringing or oscillation in the picture
Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
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Television 1 Jess Role @
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Television 1 Jess Role @
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Continuation
Smeared picture
 Sound bars in picture
 Poor color fit
 No color
 Best color and best black and white have
different fine tuning position
 Hum in the picture
 Intermittent deffects
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Possible causes
Defects caused by an increase or
decrease in total amplification of the Video
IF
 Defects caused by changes in alignment
 Other defects such as intermittent
operation or hum modulation
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Solving some issues in the Video IF
Using Bias Clamp – variable power supply
that is placed across the output of the
AGC system
 Use plastic precision tools in adjustment
(incase adjustment is tampered)
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Common causes of Intercarrier
Buzz
Defective transistor or IC
 Overload signal
 Misadjusted traps
 Improper alignment of Video IF stage
 Overload IF amplifiers
 Hum modulation of the signal
 Misalignment causing poor low frequency
signal
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Causes of regeneration and
oscillation
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Defective transistor
Open decoupling and bypass capacitors
Open or change value neutralizing capacitors
Open damping resistors
Open or shorted AGC filter capacitors
Defective tuned circuits
Shields that have been removed or disconnected
Poor grounding connection
Improper lead dress or component placement
Misalignment of video IF
Television 1 Jess Role @
UEAB2006
Television 1 Jess Role @
UEAB2006