Sharp 66FW-54H Service Manual ▷ View online
DA100 (50Hz) and DA50W Chassis – Repair Notes
Page 17 of 80
Standby Power Supply
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 5.1V 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 This has been
replaced by a series regulator circuit, which is supplied from one side of the mains, with an earth return via the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. One of the main reasons for this circuit is power consumption. Coupled with the degauss
control circuit, standby wattage is reduced to less than 3W.
D708 and D707 form 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 half cycles at
mains potential. Each time this signal exceeds 47v D729 will conduct via R774 and R775, turning on Q717.
bridge rectifier with two additional 68k resistors and a 5.1V 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 This has been
replaced by a series regulator circuit, which is supplied from one side of the mains, with an earth return via the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. One of the main reasons for this circuit is power consumption. Coupled with the degauss
control circuit, standby wattage is reduced to less than 3W.
D708 and D707 form 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 half cycles at
mains potential. Each time this signal exceeds 47v D729 will conduct via R774 and R775, turning on Q717.
Figure 7: Standby Power Supply
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 – IC702 pin 8. However, before it can be used the signal is passed through a Schmidt Trigger
circuit (Q708 and Q709) to ensure that the signal is free from noise.
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 – IC702 pin 8. However, before it can be used the signal is passed through a Schmidt Trigger
circuit (Q708 and Q709) to ensure that the signal is free from noise.
Sharp Electronics (UK) Limited - May 2003
Revision 3
DA100 (50Hz) and DA50W Chassis – Repair Notes
Page 18 of 80
Degauss Circuit
This circuit is configured to ensure that the degauss circuit will not operate unless the switch mode power supply
is running and pin 14 of IC702 is low. A positive bias is applied to the emitter of Q714 from T701 pin 15
(rectified by D727 and limited by R762, R768 and D728) via the optical diac; IC708. Since Q714 is a pnp device,
once the base is taken low compared to the emitter, Q714 will turn on allowing current to flow through IC708
(LED section of this device), turning on the triac T702, allowing current to flow through the degauss coils.
is running and pin 14 of IC702 is low. A positive bias is applied to the emitter of Q714 from T701 pin 15
(rectified by D727 and limited by R762, R768 and D728) via the optical diac; IC708. Since Q714 is a pnp device,
once the base is taken low compared to the emitter, Q714 will turn on allowing current to flow through IC708
(LED section of this device), turning on the triac T702, allowing current to flow through the degauss coils.
Figure 8: Degauss Circuit
Current will continue to flow through the degauss coils until either the PTC (POR701) has increased in resistance
sufficiently to significantly reduce the current flow, or IC702 pin 14 goes high, turning of Q714, preventing
current flow through IC708, turning the triac T702 off. Since the default condition for IC702 pin 14 is low, it
can be seen that the degauss circuit should now operate each time the television is turned on from the main
on/off switch or switched from standby to normal operation. This is the case if the main processor has set up
communication with the slave processor or not.
sufficiently to significantly reduce the current flow, or IC702 pin 14 goes high, turning of Q714, preventing
current flow through IC708, turning the triac T702 off. Since the default condition for IC702 pin 14 is low, it
can be seen that the degauss circuit should now operate each time the television is turned on from the main
on/off switch or switched from standby to normal operation. This is the case if the main processor has set up
communication with the slave processor or not.
Sharp Electronics (UK) Limited - May 2003
Revision 3
DA100 (50Hz) and DA50W Chassis – Repair Notes
Page 19 of 80
+5V Supply Generation
This DC-DC converter will operate from a supply between 8V to 55V and produces an output voltage of between
3.3V and 50V depending on the value of R747. For an output of 5V, R747 must be 2k7Ω. C741 connected to pin 3
provides a soft-start routine. If pin 3 is held low then the device will not operate. The frequency of the internal
oscillator is fixed by the value of R706 and C735. Over-current and over-voltage protection are incorporated
into the design of this device.
3.3V and 50V depending on the value of R747. For an output of 5V, R747 must be 2k7Ω. C741 connected to pin 3
provides a soft-start routine. If pin 3 is held low then the device will not operate. The frequency of the internal
oscillator is fixed by the value of R706 and C735. Over-current and over-voltage protection are incorporated
into the design of this device.
Figure 9: +5V Generation Circuit
+3.3V Supply Generation
As the processor in this chassis runs at 3.3V (pins 7, 28, 38, 49 and 78), it is necessary to change the 5V supply.
A series regulation circuit comprising of Q710, Q711 and Q712 provides this function.
A series regulation circuit comprising of Q710, Q711 and Q712 provides this function.
Figure 10: 3.3V Regulation Circuit
Sharp Electronics (UK) Limited - May 2003
Revision 3
DA100 (50Hz) and DA50W Chassis – Repair Notes
Page 20 of 80
Overriding the Power Supply Control
It is possible to ascertain if the power supply is working by removing the standby control voltage from the base
of Q702. If R792 is disconnected, the power supply should start if there are no other problems with it or the
supply lines. It is good practice to disconnect R623 when undertaking this exercise, as this will ensure that the
horizontal output stage does not become active. If R623 is not disconnected, and there is a fault in the
horizontal stage, the power supply may not start. When the power supply starts with R792 disconnected, it
should regulate correctly. Note that supplies can be disconnected, but the integrity of the feedback loop must
be maintained, failure to do so would result in very high HT and subsequent component faults.
of Q702. If R792 is disconnected, the power supply should start if there are no other problems with it or the
supply lines. It is good practice to disconnect R623 when undertaking this exercise, as this will ensure that the
horizontal output stage does not become active. If R623 is not disconnected, and there is a fault in the
horizontal stage, the power supply may not start. When the power supply starts with R792 disconnected, it
should regulate correctly. Note that supplies can be disconnected, but the integrity of the feedback loop must
be maintained, failure to do so would result in very high HT and subsequent component faults.
Figure 11: Location of R792
Figure 12: Location of R623
Sharp Electronics (UK) Limited - May 2003
Revision 3
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