Some
Basic Principals of TV Servicing
by B. Van Sutphin
NRI Consultant
From "National Radio-TV News," Vol. 15 No.
7
Feb.-Mar. 1953 |
B.
Van Sutphin |
MOST
servicemen consider the simpler defects in TV receivers
to be more easily serviced than simple defects in radio
receivers. The reason is that most defects in a TV set will
point themselves out to the man who has the basic knowledge
regarding the operation of the various circuits. The complaint
itself will often lead you to the defective stage, and when
the complaint has been isolated to a particular stage or
circuit, it is usually not too difficult to locate the defective
component by voltmeter or ohmmeter tests.
Fig.
1. A block diagram that will apply to virtually any TV receiver.
"Conventional" receivers have the sound section
connected over paths 1 or 2. Most modern receivers are of
the "intercarrier" variety and have the sound
section connected as shown by 3. The sync pulses can be
obtained from the video detector (path A), or from the video
amplifier (path B).
Fig. 1 shows a block diagram that will apply to most TV
receivers even though certain circuit differences exist
between sets using electromagnetic deflection and sets using
electrostatic deflection. The signal picked up by the antenna
is transferred through the transmission line to the tuner,
or "front-end." In that stage the incoming signal
is amplified and then mixed with a signal from the local
oscillator of the receiver to produce both the video i-f
signal and the sound i-f signal. The video i-f signal obtained
at the output of the "front-end," is amplified
by the video i-f strip and fed to the video second detector.
After detection, the signal is further amplified, and finally
it is applied to the cathode ray tube.
At the output of the tuner, the audio i-f signal may be
taken off and fed into the proper amplifier as shown by
path 1. In other cases, the audio i-f signal is amplified
by one or two stages of the video i-f amplifier, and then
fed into the audio i-f system. In "inter-carrier"
sets, the two i-f signals travel together through all of
the video i-f stages, the video detector and the video amplifier
before separation.
After the audio i-f signal has been obtained by one of the
methods mentioned, it receives further amplification, and
is fed to the FM detector, which may be a discriminator
or a ratio detector. After detection, the audio signal receives
additional amplification and is fed to the speaker.
At the output of the video detector, or of the video amplifier,
the compos-ite picture-sync information is fed into a special
clipper circuit known as a sync separator. In that stage,
the sync information is separated from the picture information,
and amplifiers are then used to bring the strength of the
synchronizing information up to a usable level. After amplification,
the horizontal sync pulses are separated from the vertical
sync pulses, and each type of pulse is fed into the proper
circuit; the horizontal sync pulses are fed to the horizontal
sweep oscillator, the vertical sync pulses to the vertical
sweep oscillator. The sole purpose of these synchronizing
pulses is to maintain the oscillators at the proper frequency.
The signal obtained from the horizontal sweep oscillator
is amplified by the horizontal output stage and is applied
to the horizontal section of the deflection yoke (or to
the horizontal deflection plates in the case of an electrostatic
set). Sets using electromagnetic deflection also have a
special circuit known as a "damper" connected
between the output of the horizontal sweep section and the
deflection yoke. The purpose of the damper is to prevent
distortion of the horizontal sweep.
Also, the horizontal sweep section of a TV receiver serves
another purpose in most sets using electromagnetic deflection.
Considerable energy is stored in the magnetic field of the
horizontal winding of the deflection yoke due to the rapid
change in horizontal sweep current. The rapid collapse of
the field during retrace induces a high voltage in the primary
of the horizontal output transformer. The voltage is rectified,
filtered, and used as a second anode potential for the picture
tube. By making the high voltage dependent upon the horizontal
sweep circuit, the manufacturer provides a means of protecting
the picture tube in case of failure of the horizontal sweep.
In sets not using this system, lack of horizontal sweep
signal will cause a thin bright line on the face of the
picture tube, and the electrons bombarding this small area
of the screen will often burn the screen and ruin the picture
tube.
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