SONY SPPS2430
Service Manual

With AFC enabled the tuner frequency is corrected by the AE'C circuit (via pin 5, 1101 and pin 12, 1001) whilst the tuner frequency remains within the AFC hold range, and CITAC operates almost independently of the Tuning Microcomputer (IROl). If the tuner frequency should move outside the AFC hold-range however, CITAC can no longer track the signal and the Tuning Microcomputer (IROl) must regain control of the tuning system. In practice, during normal operation, the tuner frequency is continuously checked against the programmed frequency and if a mismatch is found the programmed frequency is incremented/ decremented in 5OkHz steps to track the signal. This ensures that the AFC hold range is maintained symmetrical about the tuner frequency whilst tracking a drifting signal. The remote control and local keypads give a variety of methods for tuning a particular frequency. However, the basic tuning method is always the same, although the means by which the channel number is calculated may change. Refer to Fig. 3.5 for a full Tuning System Flowchart. In-channel Search: Once the required vision-carrier frequency is known the relevent band is calculated by IROl from the channel number and the band is switched via the IIC bus and 1001 pins 18,19 and 20. The carrier frequency is then transmitted to 1002 (from the Tuning Microcomputer) as data on the IIC bus and 1001 tunes to this frequency. When the tuner frequency matches the programmed frequency of the Tuning Microcomputer, IROl enables the AX and checks for an A.FC lock condition. If AE'C lock occurs then a further check is made for coincidence detection in the synchronisation circuit (Via 1401 pin 13 and 1001 pin 10). This enables the microcomputer to detect that a valid television signal has been tuned. If AX lock fails or coincidence is not achieved by 1501 the Microcomputer initiates an 'in-channel search' for a valid TV'signal. The channel is split into six discrete sections or 'microsteps' and each microstep is searched in turn (lowest frequency first) by setting the tuner frequency to the middle of the microstep, enabling the XX, and checking for AFC lock and coincidence of the synchronisation circuit. If a valid signal is not found the search continues in the next microstep. The process is repeated in up to six micro steps until a valid TV signal is found. The action that follows is dependent on the tuning selection mode used.
?u�ote :

_An in-channel search may be forced at any time by entering again the currently selected programme nmher.

Direct Programme Selection: In normal viewing when a programme selection is made the channel frequency associated with this programme is read, by IROl, from the Non-Volatile-Memory (IR02) where it has previously been stored. Once the frequency is known IROl initiates and 'in-channel search' (see previous paragraph). If a valid signal is not found within this channel then the stored frequency is re-tuned (after the microstep search) and tuning is aborted.