|NotTaR of Television Sets : Subsystems of a television set
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TV Receivers 101
A TV set includes the following functional blocks:
- Low voltage power supply (some may also be part of (2).) Most of the lower
voltages used in the TV may be derived from the horizontal deflection
circuits. Sometimes, there is a separate switching power supply but
this would be the exception. Rectifier/filter capacitor/regulator from AC
line provides the B+ to the switching power supply or horizontal
deflection system. Degauss operates off of the line whenever power is
turned on (after having been off for a few minutes) to demagnetize the CRT.
- Horizontal deflection. These circuits provide the waveforms needed to
sweep the electron beam in the CRT across and back some 15,734 times
per second (for NTSC). The horizontal sync pulse from the sync separator
locks the horizontal deflection to the video signal.
- Vertical deflection. These circuits provide the waveforms needed to
sweep the electron beam in the CRT from top to bottom and back 60 times
per second (for NTSC). The vertical sync pulse from the sync separator
locks the vertical deflection to the video signal.
- CRT high voltage (also part of (2).) A modern color CRT requires
up to 30 kV for a crisp bright picture. Rather than having a totally
separate power supply, nearly every TV on the planet derives the HV
(as well as many other voltages) from the horizontal deflection using
a special transformer called a 'flyback' or 'Line OutPut Transformer (LOPT)
for those of you on the other side of the lake.
- Tuner, IF, AGC, video and audio demodulators. Input is the antenna or
cable signal and output are baseband video and audio signals. There is
usually someplace inside the TV where line level video and audio are
present but it may not be accessible from the outside of the cabinet
unless you paid for the more expensive model with the A/V option.
Very often, the tuner is a shielded metal box positioned on the bottom
right (as viewed from the front) separate from the main circuit board.
Sometimes it is on the main circuit board. The IF section may be in
On older or cheap TVs with a knob tuner, this is usually mounted to the
front panel by itself. There are usually separate boxes for the VHF and
- Chroma demodulator. Input is the baseband video signal. Outputs are
the individual signals for the red, green, and blue video to the CRT.
- Video drivers (RGB). These are almost always located on a little
circuit board plugged directly onto the neck of the CRT. They boost
the output of the chroma demodulator to the hundred volts or so needed
to drive the cathodes of the CRT.
- Sync separator. Input is baseband video. Output is horizontal and
vertical sync pulses to control the deflection circuits.
- Audio amplifier/output. The line level audio is amplified to drive
a set of speakers. If this is a stereo TV, then these circuits must
also perform the stereo demultiplexing.
- System control. Most modern TVs actually use a microcontroller - a fixed
program microcomputer to perform all user interface and control functions
from the front panel and remote control. These are becoming increasingly
sophisticated. However, they do not fail often. Older TVs use a bunch
of knobs and switches and these are prone to wear and dirt.
Most problems occur in the horizontal deflection and power supply sections.
These run at relatively high power levels and some components run hot.
The high voltage section is prone to breakdown and arcing as a result
of hairline cracks, humidity, dirt, etc.
The tuner components are usually quite reliable unless the antenna experiences
a lightning strike. However, it seems that even after 20+ years of
solid state TVs, manufacturers still cannot reliably solder the tuner
connectors and shields so that bad solder connections in these areas are
common even in new sets.
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