12
Tuner and IF Stages
Tuner
The main chassis is fitted with a voltage synthesis UHF tuner for system I. Control of the tuned frequency is
achieved by a voltage on pin 2 of the tuner. This voltage is derived from integrating a 33 volt PWM switched
waveform. This PWM waveform is derived from pin 1 of the microcontroller I701.
The AFC is sampled by the microcontroller via the I²C bus from I501 and frequency correction is achieved by
microcontroller adjustment of the duty cycle of the PWM which then modifies the tuner frequency.
AGC Adjustment
For most aerial input signal levels the tuner operates at maximum gain. At high signal levels the gain of the
tuner is reduced by an AGC voltage generated in the IF stage. The AGC output from the IF (pin 54 of I501) is
applied to pin 1 of the tuner (H001).
The AGC maintains a maximum IF voltage of 600mV peak to peak. The voltage level can be adjusted using
the following procedure:
Short circuit pins 2 and 3 of P701 to enter service mode. When in service mode use the programme up and
down keys to select service parameter 13. Using a 40MHz or greater oscilloscope, monitor pin 11 of TU001.
Adjust service parameter 13 using the volume up and down keys for 600mV on pin 11 for a single ended
tuner, and 300mV for a differential output tuner. Press the RV key to store the AGC value.
AFC Adjustment
The AFC voltage is not available at any point on the chassis; it is read by the microcontroller via the I²C bus.
Therefore, one of the following methods can be used to correctly align the AFC.
a)
For demodulator tank coil fitted (I501 = TDA884X Mask 1)
Tune a known channel. Using a spectrum analyzer monitor pin 11 of H001. Fine tune the tuner for a
carrier frequency of 38.9MHz. Store the fine tuned frequency (i.e. AFC off). Enter service mode by
applying a short circuit to pins 2 and 3 of P701. When in service mode use the programme up and
down keys to select service parameter 15. Adjust service parameter 15 using the volume up and
down keys until the two bits at the top right of the screen meet the following criteria:
Left hand bit permanently set.
Right hand bit toggles (either 1 to 0 or 0 to 1).
When the AFC value has been set press the TV key to store it.
b)
For demodulator tank coil not fitted (I501 = TDA886X Mask 2)
Adjust service parameter 15 using the volume up and down keys until its value is 50.
When the AFC value has been set press the TV key to store it.
Vision Decoding
The majority of the vision and sound, deflection and colour decoding is performed by I501. The IF signal
passes from the tuner through the SAW filter (X001) to filter unwanted frequencies to I501. It is demodulated
internally and the output at pin 6 is buffered by Q501. The sound and vision components are now separated.
Z501 removes the sound from the vision components and Z601/602 filters the FM sound to pin 1 for
demodulation. Video is then fed to the SCART socket output, pin 19 of P501 via Q502.
