Specifications

CA1 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support Group Page 55
CA1 Protection
Figure 48 : CA1 Protection Circuit
Unlike the CA10 chassis, the CA1 incorporates a protection circuit that operates with inputs from the beam
current limiter, frame output stage and CRT drive amplifier supply.
Basic protection is provided should the frame drive or beam current feed back drop below 1.5V on pin 7
of IC1001 ( PROT ). Should this happen, then it is possible to over ride the protection circuit by removing
D1012 from the circuit, this will allow pin 7 to rise up to +5V via R1066. Note that the nominal state of pin
7 is +5V.
Automatic Beam Current Limiting and X-Ray Protection
Pin 34 of IC201 ( ABL ) serves the same function as in the CA10 chassis, which is to blank the screen
should there be excessive beam current or a frame scan problem. If the beam current increases too far,
an ultra black picture will be produced by reducing the black level of the video signal to sync level.
From the vertical stage, the flyback pulse is also monitored so that in the event of a frame collapse or a
malfunction in the scanning coil drive circuit, the CRT emission will be blanked. This is desirable to reduce
the risk of damage to the CRT phosphors.
If the beam current continues to increase, there will be a danger of forward X-ray emissions from the CRT
face. This is avoided by monitoring the voltage level on pin 3 ( EHT IN ) of IC201 and should this rise
above a pre-defined level, the receiver will shut down.