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DEGAUSSING
The ADTV chassis employs an innovative method of
degaussing, just like former Zenith TVs it is performed
through software in order to obtain a proper
demagnetization for these screens.
IC6000 generates a pulse about 1 second on pin 48 set
to high (about 4.8 VDC) through RX3900 (4.7K 5% ¼
watt).
It is applied to QX3900, which drives the 12 VCD relay
KX3900 in series with a current limiting resistor. So the
degaussing is performed when the degauss coil is plugged
to the 4T8 connector ends.
HORIZONTAL & VERTICAL DEFLECTION
The AB chassis was designed to support only one
horizontal frequency for DTV, 33.75Khz. It supplys
horizontal and vertical currents to the deflection yoke
coil and the high voltage generation for the CRT. This
section is dependent on other circuits in the main
chassis to provide drive signal, DC control voltage,s and
deflection corrections. This section can be divided into
6 functional blocks:
• Horizontal Deflection and HV Generation
• Base Drive
• HV Compensation
• Shutdown Circuit
• Vertical Deflection
PV154 - 923-03497
SERVICING
Horizontal Deflection and HV Generation Block Diagram
HORIZONTAL DEFLECTION AND HV GENERATION
The Horizontal Deflection circuitry is a common flyback
topology used in many television and monitor
applications. QX3200 is the Horizontal Output Transistor.
When QX3200 is initially turned on, current is allowed
to flow from the B++ supply through TX3100, TX3201,
and QX3200. This allows energy to be stored in TX3201.
When QX3200 is turned off some of this energy is returned
to charge the S-capacitor(s) CX3208, CX3210, and C3232.
After several cycles (repeating at scanning frequency)
sufficient voltage appears across the S-capacitors
providing an additional path producing current flow
though the horizontal deflection yoke coil (connector
4X4). The negative current path is provided by the
damper diode DX3200. The result is a positive and
negative sawtooth current through the horizontal
deflection coils (approximately 11 to 17 Amps peak to
peak). When QX3200 is turned off, about 1100 Volts
peak of flyback voltage is produced resulting from the
stored energy in the horizontal deflection coils and
TX3201. This voltage is induced to secondary windings
of TX3201 to generate the high voltage from 28 to 30
KV. The flyback (or retrace) period is about 5.0 s.
3-3
AB - SERVICING
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