Luxman WL 550 Service Manual

Page 2

CIRCUIT DESCRIPTION

GENERAL

LUX WL550 is all solid state high-fidelity AM/FM stereo
transistorized radio tuner. The circuitry used in this
instrument is mostly integrated in 2 printed circuit boards.
The FM tuner circuit board is completely shielded, and
mounted on a 7-gang AM/FM tuning capacitor. Con-
ventional wiring is used in mains selector, function switch
sections and front and rear panel assembly parts.

FM TUNER FL516U BOARD

The FiF-amplifier is designed to provide stable amplifi-
cation, sharp selectivity at FlVl broadcast frequencies, and
an optimum noise figure. The dualgate junction FET is
ideally suited for this job as it has superior cross-modu-
lation performance as a function of AGC, and in addition
has a very wide dynamic range. The stage employs dual-
gate FET (35K30) in a common-source configuration, and
the RF signal and gain control signal are applied to gate
N01 and gate No.2 is grounded to earth potential. The
double~tuned coupling is employed between RF and mixer
stages to provide sharp selectivity. The mixer stage is
designed to minimize the spurious responses and noise
figure. The 25C535 mixer transistor operates in the
common emitter configuration with the RF signal and
local oscillator signal are applied to the base terminal.
The 10.7MHz 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 by means of 1»picofarad capacitor, which isolates
the tuned circuit of the oscillator from the input circuit
of the mixer and this minimizes the possibility of oscillator
instabilities as a result of pulling".

lF-AMPLIFIER

The lF-amplifier uses two 28(2381 transistors, a ppcSSSA
integratedcircuit, and a TA7061P integrated-circuit. The
selectivity stages are two ceramic filters, and a double
tuned transformer. The ypc555A integrated-circuit is a
basic differential amplifier with constant-current transistor
that provides 31-dB power gain. The TA7061P 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 ampli-
fier section provides 68-dB total power gain at 10.7 MHz,
and features an output stage with exceptionally good
limiting characteristics because of its transistor constant-
current sink. The selectivity of this IF section is determined
by the ceramic filters in the interstage amplifying paths.

The filters provide extremely sharp skirt selectivity and
flat response inside the pass band. Detector transformer
T203 and matched pair diodes D206,D207 form a balanced
ratio detector, that transforms the frequency-modulated
signal into an audio signal. The detector transformer T203
is designed to provide the wide peak-topeak 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. The IF signal for AGC is
extracted from the base of 0202 to drive 0205. 0205 acts
as an lF-amplifier to drive the voltage doubler D201, D202.
This DC voltage is applied to 0101 on the FM TUNER as
an AGC BIAS. The IF signal, extracted from the output
of the T202, is amplified by 0206 buffer amplifier to
drive the rectifier D204 and associated components. This
DC voltage drives a signal meter and muting circuits. The
amplitude component of IF signal, extracted from the
output of the second ceramic filter F202 is rectified by
D203. The AC component of this signal feeds to the
multi-path check terminal.

MULTIPLEX DEMODULATOR

The upc554C is a monolithic FM stereo multiplex decorcer
system constructed on a single silicon chip using the planner
epitaxial process. This integrated circuit accomplishes the
demodulation of a stereo multiplex signal into the right
and left audio channels while inherently suppressing SCA
frequency component. Internal provision is made for inter-
station audio muting, stereo/mono mode switching and
driving an external stereo mode indicator lamp.

Stereo demodulation in the ppc554C is achieved by a fully
balanced differential synchronous demodulator (Figure).
In this demodulator, the composite stereo signal is fed to
the base of transistor 011. The resulting AC current which
flows in the collector of 011 is then switched at 38KHz
rate by the differential pair 07 and 08. The amplitude
of the 38KHz switching signal is large enough to instan-
taneously gate the AC current alternately to the left and
If the 38KHZ gating signal is
properly phased with respect to the composite stereo signal,

right audio channels.

this process will separate the stereo signal into its left and
right audio components. A second differentialstage (09,
010 and 013) is added to the stereo demodulator for
two reasons. First, the 38KHz also switches the DC current
flowing in the collector of 011 resulting in a largeundesired
38KHz voltage in both channels. By gating the DC current
in 013 and adding the resulting signals in phaseoppOSlthH
to those from 012, these 38KH2 components are cancelled.
More importantly, however, the second differential stage
provides improved stereo separation. With asingle dif-
ferential demodulator stage, the theoretical Stereo sepa-