7
TUNER and IF STAGES
TUNER and IF STAGES
Tuner
The main chassis is fitted with a voltage synthesis UHF/VHF
tuner (TU001) for system B/G. 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 which is produced on pin 1 of the microcontroller
IC701. Band selection is by three control lines to pins 3, 4
and 5 of the tuner.
The AFC is sampled by the microcontroller via the I²C bus
from IC504 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 IC504) is applied to pin 1
of the tuner (TU001).
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 1 and 2 of PL701 to enter service mode.
When in service mode use the programme up and down
keys to select service parameter 13. Using a 40MHz
Bandwidth 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 TV 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 demodulator tank coil.
a) Tune to a known channel. Using a spectrum
analyser monitor pin 11 of TU001. 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 1 and 2 of
PL701. 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 key until the two bits at the top right of
the screen meet the following criteria.
Left hand bit permanently set to 1.
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) Inject a 600mV peak to peak signal at 38.9 MHz
into pin 1 of FL003, ground pin 2 of FL003. Enter
service mode by applying a short circuit to pins 1
and 2 of PL701. When in service mode select and
adjust parameter 15 as in a) above to meet the
following criteria.
Left hand bit permanently set to 1.
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.
Vision Decoding
The majority of the vision, deflection and colour decoding is
performed by IC504. The IF signal passes from the tuner
through the SAW filter (FL003) to filter unwanted frequencies
to IC504. It is demodulated internally and the output at pin 6
is buffered by TR502. The sound and vision components are
now separated. FL601 removes the sound from the vision
components with the unfiltered demodulated signal
containing the FM sound carrier(s) to pin 1 for demodulation
on a daughter board. Video is fed to the SCART socket
output, pin 19 of SK501 via TR503.
