Specifications

CA1 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support GroupPage 52
CA1 Reset
Whenever the receiver is switched on from the mains ( on/off button pressed ) it is necessary to reset the
main microprocessor, IC1001. This process is achieved with the use of the circuit outlined below.
Figure 45 : CA1 Reset Circuit
When the power supply starts up, a permanent +5V line , designated +5Va, will be generated via D712 and
series regulator Q704. This supply, along with that produced by D712 ( +7V ), is used to produce the reset
pulse that is fed to pin 15 of IC1001.
At start up D714 ( a 5.1 volt zener diode ) will start to conduct when the +7V supply reaches 5.1V, allowing
the +5Va line to come up and supply the microprocessor, IC1001. This will allow C712 to charge via R724.
When this voltage rises to 0.65V, Q704 will turn on and its collector voltage will drop, which in turn will turn
off Q1012 allowing its collector to rise. IC1001 is now reset.
R725 will discharge C712 quickly should the set be turned off then on again quickly, thus preventing
corruption of the reset pulse and possibly IC1001 hanging. Capacitor C1033 will sped up the switching
action of Q1012.