Read Sharp 28JW-74H (serv.man2) Service Manual online
50Hz Widescreen Colour Television – GA20 Chassis
Sharp Electronics (UK) Ltd
Technical Support
Issued June 02
Page 4
This cycle will be repeated until the short circuit is removed (safe restart mode). The short winding
protection will also protect in case of a secondary diode short circuit.
protection will also protect in case of a secondary diode short circuit.
Overcurrent
The cycle-by-cycle peak drain current limit circuit uses the external source resistor (R715) to measure
the current accurately. The circuit is activated after the leading edge. The Overcurrent Protection
(OCP) circuit limits the ‘sense’ voltage to an internal level.
Typical Voltage at pin 5 <0.56v
the current accurately. The circuit is activated after the leading edge. The Overcurrent Protection
(OCP) circuit limits the ‘sense’ voltage to an internal level.
Typical Voltage at pin 5 <0.56v
Soft Start
To prevent transformer rattle during hiccup mode, the transformer peak current is slowly increased by
the soft start function. This is be achieved by inserting a resistor (R713) and a capacitor (C706)
between pin 5 (I
the soft start function. This is be achieved by inserting a resistor (R713) and a capacitor (C706)
between pin 5 (I
sense
) and the sense resistor (R715). An internal current source charges the capacitor
to a voltage that is derived from Iss X R713 to a maximum value of approximately 0.5 V.
Drive (Pin 6)
This is the drive to Q702 gate. the duty cycle of this signal is dependant on the conditions at the other
pins of the TEA1507. The driver circuit to the gate of Q702 has a current sourcing capability of
typically 125 mA and a current sink capability of typical 540 mA. This permits fast turn-on and turn-off
of Q702 for efficient operation. This low driver source current reduces Electro Magnetic Interference
(EMI) and also limits the current spikes across R715.
pins of the TEA1507. The driver circuit to the gate of Q702 has a current sourcing capability of
typically 125 mA and a current sink capability of typical 540 mA. This permits fast turn-on and turn-off
of Q702 for efficient operation. This low driver source current reduces Electro Magnetic Interference
(EMI) and also limits the current spikes across R715.
HT
R
EGULATION
D758 is a variable zener diode. When the voltage increases on the control pin the zener point fails.
This control pin is connected to the 150v rail via the potential divider network of R750, R751, R752
and R753. Therefore for an increase in HT D758 turns on earlier, which in turn will turn on the opto-
coupler IC702, turning off the power supply. For a low HT then the zener point of D758 increases
keeping IC703 turned off.
This control pin is connected to the 150v rail via the potential divider network of R750, R751, R752
and R753. Therefore for an increase in HT D758 turns on earlier, which in turn will turn on the opto-
coupler IC702, turning off the power supply. For a low HT then the zener point of D758 increases
keeping IC703 turned off.
IC702
D776
R744
1K
D754
R757
47K
D712
C12
D757
Q754
C753
0.27µF
D758
R742
47R
C751
1µF
R748
330K
330K
R752
4K7
R753
47K
R751
120K
120K
R750
120K
120K
IC751
+5v
CTL
IN
150v
IC1001 pin 33
(Standby)
14
13
Part
of
T703
+7.5v
Click on the first or last page to see other 28JW-74H (serv.man2) service manuals if exist.