Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support GroupPage 26
The signal on pin 22 is a pulse ( at vertical rate ) sitting on a DC pedestal of three volts ( nominal value
as this varies anywhere between +1V and +4V ).
Vertical circuit failure - CRT protection
If the vertical drive were to stop, a bright white vertical line will appear across the centre of the CRT. This
is obviously undesirable as it may burn the phosphor coating if left on for too long. To alleviate this
possibility a feed is taken from the vertical output ( top end of the vertical scanning coils ) via D515. D515
ensures that only signals greater than 8.2V pass to pin 22. D514 provides isolation and D520 clamps
pulse to a maximum level of five volts.
Under normal conditions this has no effect on the operation of the beam current limiter. When the vertical
drive signal falls below 8.2V ( flyback pulse ) the pulses developed at the cathode of D514 will disappear
and this will cause the screen to go ultra black, ensuring that no damage is done to the CRT. In this
situation, if the beam current is increased by turning up the G2 control on T601, a thin horizontal white line
will be seen momentarily unit the beam current limiter circuit compensates and turns the current down
again.
EHT input of IC201
If the beam current increases significantly, then there will be a risk of forward X rays being generated from
the CRT face. To prevent this condition, pin 50 ( EHT IN ) is used to monitor for excessive beam current.
Although the B LIM pin will reduce the risk of forward emissions, as it only changes the video signal, the
EHT IN pin will override this operation. When active, i.e. X ray emission may be possible, pin 50 will return
the set to standby. This pin is nominally 2VDC, but this will change slightly with beam current.