Arcam Alpha 7 Service Manual

ALPHA 7/8 FM/AM TUNER SERVICE MANUAL ALPHA 7/8 FM/AM TUNER SERVICE MANUAL
Arcam Alpha 7/8 FM/AM Tuner Service Manual
Issue 1 Serial Number beginning AT ... -
(Paul Newton Sept 96)
Circuit Description
The Alpha 7 and 8 tuners share the same printed
circuit board and technical specification but the main
functions of the Alpha 8 tuner can also be operated
by remote control.
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 microcontroller
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 backlight on the display PCB.
Z6 is the 5V digital supply for the microcontroller.
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.
The LA1266 (Z9) demodulates the IF at pin 1 into an
audio output at pin 12. The quadrature coil is a
single-tuned device for low-cost and adequate
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 microcontroller which
disables the AM circuitry of Z9 and switches the path
of the audio signal through Z8.
MW/LW switching is performed by the CMOS
switches of Z11. This device is controlled by the 'LW'
signal.
Each band has its own oscillator and aerial tuning
coils, tuned via varicap diodes D23 and D24.
The 450kHz IF signal is filtered by coil and ceramic
filter combination T5.
Birdie Filter
The 'Birdie Filter' based around Q12 is a three-pole
low-pass filter with a cut-off frequency of about 55
kHz. 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 deemphasis network of R1, C1 etc.
The correct deemphasis network is selected by
means of jumpers on SK6. C2 & C102 (220pF) are
selected for 50us and C3 & C103 (560pF) for 75us
deemphasis.
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 microcontroller 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.