76FW-53H (serv.man13). DA100 and DW100 training course notes - Sharp TV Service Manual (repair manual). Page 6

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Colour Television – DA100/DW100 Chassis
Sharp Electronics (UK) Ltd
Technical Support
September 99
Page 3
S
LAVE 
P
ROCESSOR 
P
OWER 
S
UPPLY
This part of the circuit has changed compared to previous chassis, where this supply was derived by
means of a bridge rectifier with two additional 68k resistors and a 5v1 zener diode to limit the voltage.
This has been replaced by a series regulator circuit, which is supplied from one side of the mains, with
an earth return via the bridge rectifier.
D708 and D707 forms part of a bridge rectifier, two diodes of the main bridge rectifier are used for the
other part. Therefore, at the junction of D707 and D708 there is a 100Hz signal comprising of positive
halve cycles at mains potential.  Each time this signal exceeds 47v D729 will conduct, turning on
Q717.
When Q717 turns on the gate of Q715 is taken to ground. Since this is a N Channel MOSFET Q715 is
turned off and no current will flow through this device.
During the period when Q717 is turned off Q715 will turn on allowing C739 to charge to approximately
7v which is limited to 5v by R777 and D710.
It can now be seen that the period of Q715 conduction is very short (it is only turned on when the
100Hz signal at the junction of D707 and D708 is less than 47v). Therefore, a top-up supply from the
bridge rectifier circuit comprising of D734 and D731 is added to the cathode of D710 via R721 and
R703.
The 100Hz signal at the junction Q717 and D733 is also used as a timing signal for the real time clock
within the slave processor. However, before it can be used the signal is passed through a Schmitt
Trigger circuit (Q708 and Q709) to ensure that the signal free from noise.
HT
+5v
Clock
R721
150k
R703
150k
D734
D731
D708
D707
R771
470k
D729
47v
R774
470k
R775
470k
R776
82k
Q717
Q715
D733
D732
15v
R777
47
C739
220
C724
220
D710
5v1
R739
47k
R738
47k
R737
180k
R735
470k
Q708
Q709
R736
470k
R770
220
R787
47k
C725
4n7
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