Luxman T 300 Service Manual

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1306

SERVICE MANUAL

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CIRCUIT DESCRIPTION

GENERAL

LUX T-3OO is all solid state high-fidelity AM/FM stereo
transistorized radio tuner. The circuitry used in this
instrument is mostly integrated in 5 printed circuit boards.
The FM tuner circuit board is completely shielded, and
mounted on a 7-gang AM/FM tuning capacitorr Conventional
wiring is used in mains selector, function switch sections
and front and rear panel assembly parts.

FM TUNER FL517U BOARD

The RF-amplifier is designed to provide stable amplification,
sharp selectivity at PM broadcast frequencies, and an
optimum noise figure. Dual-gate MOS fieldeffect transistors
are ideally suited for this job as they have characteristics
similar to that of a conventional transistor or a junction
field-effect transistor and in addition have wide dynamic
range, The latter characteristic results in very low cross-
modulation products. The advantage of the dual~gate MOS
FET is its superior cross-modulation performance as a
function of AGC. The stage employs dual-gate MOS FET
(3SK40) in a commonrsource configuration. and the RF
signal is applied to gate No. i, and gate No. 2 is used for
gain control. The double-tuned coupling is employed
between HF and mixer stages to provide sharp selectivity.
The mixer stage is designed to minimize the spurious
responses and maximize the noise figure. The 38 K40 mixer
transistor ldualgate MOS FETl operates in the common-
source configuration with the RF signal applied to the gate
1 terminal and local-oscillator signal to the gate 2. The 10.7
MHz IF output from the mixer is coupled to the first
lF-amplifier stage by means of a double-tuned transformer
T101, The SE3001 local oscillator transistor is operated in
a common-collector circuit that generates a clean oscillator
wave-form to minimize the generation of spurious responses.
The oscillator signal is coupled to the mixer gate by means
of 2-picofarad capacitor, which isolates the tuned circuit of
the oscillator from the input circuit of the mixer and thus
minimizes the possibility of oscillator instabilities as a result
of "pulling".

lF-AMPLIFIER

The lFAamplifier uses a 25C381R transistor, two
LM703LH integratedcirciuts, and a TA7061AP integrated-
circuit. The selectivity stages are two block filters,
which consist of five poles. and a double-tuned trans-
former. The LM703LH integrated-circuit is a basic differ-
ential amplifier with constant-current transistor that
provides 35dB power gain. The TA7061 AP integrated-circuit
consists of three differential amplifier in cascade. Each
differential pair is isolated from the succeeding pair by a
common-collector stage used as a buffer. The amplifier
section provides 68dB total power gain at 10. 7MHz, and
features an output stage with exceptionally good limiting

characteristics because of its transistor constant-cu rrent sink.

The selectivity of this IF section is determined by the block
filters in the interstage amplifying paths, The filters provide
extremely sharp skirt selectivity and flat response inside the

pass band. Detector transformer T203 is designed to provide
thewide peak»to-peak separation required for good multiplex

operation. A null-type meter connected across the output of
the ratio detector is'used as a center tuning indicator. L201

and C228 remove the 10.7MH2 component from the output

signal. The IF signal for AGC is extracted from the collector
of 0201 to drive 0.210.

0210 acts as an IF-amplifier to drive the voltage doubler
0206, D207. This DC voltage is applied to 0101 on the FM

TUNER as an AGC BIAS. The lF signal extracted from the
output of the T202, is amplified by 0208 buffer amplifier
to drive the rectifier D210 and associated components. This
DC voltage drives signal meter and muting circuits. VR204
is the calibration adjustment. The amplitude component of
IF signal, extracted from the output of the second block

filter F202, is rectified by D209. The AC component of this
signal is amplified by two transistors 0214, 0215, and feeds
to the multipath check terminal.

MULTIPLEX DEMODULATOR

The composite multiplex signal from the ratio detector is
applied to the base of transistor 0301 through SCA filter
T301. 0301 is an isolation stage which provides a high
impedance load for the ratio detector and a low impedance
source for the demodulator circuit. The composite multiplex
signal is extracted from its emitter circuit and the 19KHZ
pilot signal is taken from a tuned circuit in the collector
circuit. The 19KHz pilot signal, separated by the tuned
coupling circuits T302, is amplified by 0303 to drive the
frequency dou oler. The output of 0303 is fed to a balanced
full-wave rectifier which consists of D301, D302 and the
secondary winding of T303. The output of the rectifier is
unfiltered and develops both DC and 38KHz components.
The 38KHz component is amplified by 0305 and appears at
the secondary winding of T304. The composit signal taken
from the emitter of 0301 is added to the 38KHz sub-carrier
in the secondary winding of T304 through 0302 FET muting
circuit and 0304 emitter follower circuit which drives the
composite signal. The demodulator circuit employs four
diodes in a balanced bridge arrangement. The system cancels
much of the residual RF products. The 38 KHz pulses switch
the composite signal in the switching transformer to produce
L and R signals at the output when the sub-carrier has the
proper phase with respect to the composite signal.
Demodulated Land R signals are applied to buffer amplifiers
0306, 0307 which are used for matching of succeeding
low-pass filter. All components above 16KHz are eliminated
by the low-pass filter LF301, therefore residual sub-carrier
level is less than 66dB. Demodulated L and Fl signals through
the noise-filter and deemphasis networks.

The network that connects the emitters of 0305 and 0308
provides a form of negative feedback between left and right
channels. Any residual L signals in the R channel are
cancelled out by the signals from the L channe|.The same is
true on residual R signals in the L channel. VR301 therefore
provides the maximum channel separation. VR302 is used to
compensate for differences in demodulator effiCiency and
provide equal overall gain in both channels with respect to