Specifications

CA1 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 51
the base of a control transistor. The collector of this transistor will affect the brightness of the LED inside
the opto coupler and hence the conduction of the photo transistor. Regulation is achieved by varying the
on time of the chopper transistor.
If the main supply were to rise, the voltage on the base of Q705 will also rise, turning it on harder. The
brightness of the LED would increase as the voltage on pin 2 of the opto coupler drops. In turn, this allows
the transistor to conduct harder and hold on Q703 for longer, turning off Q701 for longer and thus reduce
the supply.
If the main supply were to fall, the voltage on the base of Q705 will also fall, reducing its conduction. The
brightness of the LED would decrease as the voltage on pin 2 of the opto coupler rises. In turn, this allows
the transistor to turn off sooner and hold on Q703 for a shorter period, turning off Q701 for a shorter period
and thus increase the supply.
IC201 Supply
It was found that the power feed circuit for IC201 as used in the CS chassis would sometimes fail and cause
IC201 to work erratically. To alleviate this problem, a new regulation circuit has been designed to provide
the +8V supply to pin 53 of IC201. This circuit is shown below.
Figure 44 : IC201 Power Supply Circuit
Q209 is a simple series regulator, whose emitter voltage is dependant upon D212 and D213. The actual
measured voltage the emitter of Q209 is +8.2V ( D213 voltage - D212 voltage + Q209 base/emitter volt drop
). Note that the biasing for D212 and D213 is taken from the +20V supply. This will ensure that if there is
a failure with either the +10V or +20V supplies, IC201 will not generate any vertical or horizontal drive
signals.
D709 also provides the +10V rail that is used by the vertical stage.