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CHASSIS
7. VIDEO OUTPUT AMPLIFIER TDA6107JF/N3
The TDA6107JF includes three video output amplifiers and is intended to drive the three cathodes of a colour CRT
directly. The device is contained in a plastic DIL-bent-SIL 9-pin medium power (DBS9MPF) package, and uses high-
voltage DMOS technology. To obtain maximum performance, the amplifier should be used with black-current control.
8. POWER SUPPLY (SMPS)
The DC voltage required at various parts of the chassis are provided by an SMPS transformer controlled by the IC
MC44608 which is designed for driving, controlling and protecting switching transistor of SMPS. The transformer produces
126 V (Flat models) or 116 V (non Flat models) for FBT input, +/- 14 V for audio output IC, S + 3.3, S + 5 V and 8 V for
ST92195.
The ZX series of receivers incorporate a Motorola switch mode power supply using a MC 44608 regulator controller
IC. The circuit provides power to the receiver in both standby and normal operation modes.
8.1 Start Up
The switch on the mains supply is fed through the mains filter network TR801, the surge limiter resistor R828, the bridge
rectifier diodes D811/13/37/38, and reservoir capacitor producing approx. 320 volts D.C. to feed the switching MOSFET
Q801 via the primary winding of TR802 pins 6 and 7.
Start up resistor R801 feeds from a 500V coming from the mains through the adder diodes D809, D890 to pin 8 of
IC800, the IC uses 9mA current source and connects it internally to VCC at pin6 allowing a rapid charge enough for
start up. Then IC800 responds with the oscillator starting to oscillate at a 40 KHz frequency fixed by the IC manufac-
turer.
The IC then produces, pulse width modulation pulses, at this frequency on pin 5 to drive the base of the switching FET
Q801, that will then switch current on and off through the primary of TR802, which will in turn provides voltages in the
secondary windings. The secondary winding voltages being proportional to the length of time that Q801 is turned on in
each cycle. The voltage produced between pins 4 and 3 of TR802 is rectified by D804 developing aprox. 12 volts on
C810, which takes over from the start up resistor to supply pin 8 of IC800.
The Demag pin at pin1 offers 3 different functions: Zero voltage crossing detection (50mV), 24mA current detection and
120mA current detection. The 24mA level is used to detect the secondary reconfiguration status and the 120mA level
to detect an Over Voltage status called Quick OVP.
The VCC at pin6 operates between 6,6V and 13V in normal operation, when this voltage exceeds 15V then the IC
output is disabled.
8.2 Voltage Regulation
After initial start up the secondary voltages of TR802 are established. These voltages then need to be regulated to the
required levels. In a switch mode power supply such as this, it is the ON time of the switching FET Q801 that determines
the output voltages produced. To provide regulation of the supply there is a feedback loop via an adjustable zener
IC118 and an OPTO- coupler connected to pin3 of IC800. The reference voltage of IC118 is set to 2,5V to supply a B+
voltage of 115V. Any fluctuation at this pin will cause IC800 to compensate it either by increasing or decreasing the
voltage at the secondary outputs.
8.3 Voltage Protection
The MC44608 offers two OVP functions:
1- A fixed function that detects when V CC is higher than 15.4V
2- A programmable function that uses the demag pin. The current flowing into the demag pin is mirrored and compared
to the reference current Iovp (120mA). -Thus this OVP is quicker than normal number one as it directly senses the
change in current rather than waiting for a specific voltage value, and is called QOVP. In both cases, once an OVP
condition is detected, the output is latched off until a new circuit START-UP.
3- A software controlled function acts on pin52 of IC501. This pin monitors feedback from both 8V and 5V via D512,
then compares these to a reference value Vref pre-set by the hardware through resistors R545, R546, R548. In normal
mode operation 1.2V < Vref < 2.4V. Any voltage outside this window will cause the micro controller to force the TV to
stand by mode by lowering the standby port. Refer to standby mode.
8.4 Current Protection
To monitor the current drawn by the receiver the source of Q801 is returned to the bridge rectifier through a low value
resistor R807. All the current drawn by the receiver will flow through that resistor each time Q801 conducts; this will
produce a voltage across the resistors proportional to the current drawn by the receiver. This voltage is fed to pin 2 of
IC800 via R806. When the receiver is working normally the voltage across R807 is only a fraction of a volt and is not
large enough to have any effect on IC800. Under fault conditions, if the receiver draws excessive current the voltage
across R807 will rise. This voltage is monitored by the current sense input pin2.
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