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T51 Tuner Circuit Description
Power Supply
Mains voltage selection is made by placing a fuse, of
appropriate value, in the 115V or 230V position.
The single secondary is rectified and regulated and
supplies four regulators.
Z7 is always powered when the mains is connected and
keeps the preset memory of the micro controller intact.
The high-value capacitor (C84) will keep this data safe, for
a limited time, in case of mains power failure.
Z5 provides the main 12V supply rail for the RF and audio
circuits. This is always connected, except for the supply to
the FM front end, which is disabled by Q2 when the unit is
turned 'off'.
Z4 powers the LED back light on the display PCB.
Z6 is the 5V digital supply for the micro controller.
FM Tuner & IF
The FM aerial input is mixed in the front end FE1 which
provides a 10.7MHz IF output. This is filtered and
amplified by ceramic filters CF1 & CF2 and the circuit
based around Q9 & Q10. A third filter CF3 can be put in
circuit by fitting jumpers JP1 & JP2 across pins 2 & 4 and
1 & 3 of SK6 respectively. This effectively narrows the IF
band to improve selectivity. Transistors Q17 and Q21 are
part of the AGC circuit. The position of the jumper link
JP7 to either SK10 or SK11 determines from which point
the AGC circuit is triggered. The default is for the jumper
to be fitted to SK10 which minimizes cross modulation
and prevents the IF amp becoming saturated.
The LA1266 (Z9) demodulates the IF at pin 1 into an
audio output at pin 12. The quadrature coil is a
double-tuned device for best distortion performance.
When a station is tuned correctly the 'On Tune' signal goes
low and drives a green LED on the display PCB. This
signal is also used by the auto-stop detection and muting
circuits.
AM Tuner & IF
The LA1266 (Z9) contains the AM RF amplifier, oscillator
and mixer circuitry.
Switching between FM and AM modes is controlled by the
'FM' signal from the micro controller which disables the
AM circuitry of Z9 and switches the path of the audio
signal through Z8.
MW/LW switching is performed by switching transistors
Q12, Q14 & Q15.
Each band has its own oscillator and aerial tuning coils,
tuned by a varicap diode all contained within the AM
module L5.
The 450kHz IF signal is filtered by coil and ceramic filter
combination T5.
Birdie Filter
The Birdie Filter (L2) is a four pole low pass filter, flat to
about 53kHz, then with a very steep roll off from then on..
This is designed to reduce co-channel interference in FM
Stereo and cut down on irritating birdie noises.
Stereo Decoder
Stereo decoding is performed by LA3401 (Z8). The 38kHz
reference signal is accurately created by ceramic resonator
X3 and does not require adjustment.
Stereo separation is adjusted for maximum by preset RV1.
This IC also contains FM/AM audio switching and muting
functions.
Audio Filters & Output
The left and right channel audio outputs from Z8 are
filtered by the de-emphasis network of R1, C1 etc. The
correct de-emphasis network is selected by the position of
jumpers JP3 & JP4 on the pins of SK8. C2 & C102
(220pF) are selected for 50us and C3 & C103 (560pF) for
75us de-emphasis.
The final filter, based on Z3, is a two-pole low-pass circuit
with a cut-off frequency of around 20kHz. Also included
is a 19kHz notch filter, L1 & C4, that removes most of the
residual pilot tone.
Muting
When the power switch is turned off Q5 is switched off,
enabling the mute circuitry of the stereo decoder (Z8) via
diode D9. This removes the need for output muting
transistors and eliminates switching clicks and pops.
At switch-on the mute is enabled while C21 is charged via
Z8.
In normal use the mute inside Z8 is enabled in three other
ways:
The AMUTE signal from the micro controller is active at
power-on and for a short time when a preset is changed.
When the band is changed the output is muted by charging
C52.
The audio output must be muted whenever the tuner is in
stereo mode and is also off-tune. This is to eliminate noise
during tuning and is provided by Q8 and the MONO and
OFF TUNE control signals.
In Mono mode this mute is disabled so that weak stations
can be heard, despite the noise.
Micro controller
Control of the DiVA T51 tuner is performed by a single
micro controller Z10, LA7230. This also provides keypad
input and drives a LCD directly.
At power-on a matrix of diodes is scanned to set-up the
controller for the correct options:
D7 selects the correct frequency ranges.
D10 enables the use of Auto tuning as well as manual
tuning.
D8 allows the Auto-tuning mode to use the IF Count
method of searching.
The position of jumpers JP 5 & JP6 fitted to SK9 are used
to disable or enable LW and also whether the AM band
steps are 9 kHz (UK) or 10 kHz (USA).
Z10 provides control signal outputs for the following
functions:
MONO - To defeat the stereo decoder
