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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 This has been
replaced by a series regulator circuit, which is supplied from one side of the mains, with an earth return via the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 This has been
replaced by a series regulator circuit, which is supplied from one side of the mains, with an earth return via the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
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