Luxman 600 Service Manual

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SERVICE MANUAL

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R600 CIRCUIT DESCRIPTION

Power Supply

The mains input to the receiver goes via 2 pole power

switch to the mains transformer which has two secondary

windings:

1) A 7-volt winding for the panel lamps.

2) A 500-50 volt winding to provide 55-volt DC for the
audio power amp, pre-amp and tuner sections.

The 0608 (ZSCIEOQ) transistor is used as a ripple filter

providing 43-volt to the preamp section and the 0610

(2801509) transistor is used as part of the over load pro-

tection as well as a ripple filter for the early stage of the

power amp at the voltage of 52»volt. A potential divider

using a zenor diode is used to obtain from {SS-volt a 12-

volt DC supply for AM and FM tuner sections. There are

fuses in the secondaries for transformer protection.

POWER AMPS

The circuit is basically a 6 transistor direct coupled quasi
complementary design, since only one power supply voltage
with respect to ground is used, the audio from the proceed-
ing ore-amp tone control stages is capacitor coupled to the
base of the first PNP transitor, this base is also biased to
approximately half supply volts so therefore is used as a
reference to set the output stage to half supply, D.C.
negative feedback from the output is applied to the emitter,
so that the base and emitter form a simple type of voltage
comparitor.

The collector is coupled to the base of the second transistor
which is the voltage driver for the power transistor emitter
followers.

Two NPN transistors form the positive power emitter
followers and a PNP, NPN compound form the negative
power emitter followers. Their bases are biased by a 2 diode
device (SV-02) and a bias trim pot to set the output idling
current at 35mA.

The output from the junction of these four transistors is
coupled by a. 2200MF condenser to the loudspeaker
terminals. A condenser and two resistors form a bootstrap
constant current collector load for the voltage amplifier
transistor.

Overall AC feed back is taken from the loud speaker side of
the output condenser and applied back to the emitter of
the first transistor.

Audio is _also taken from the speaker side of the condenser
via a 470 ohms resistor to the headphone socket.

PRE AMP

The RlAA amplifier uses an NPN and PNP direct coupled
pair of transistors which are selected low noise types, the
equalization and DC feedback are taken from the collector
of the PNP back to the emitter of the input NPN transistor.
This amp has a gain of 36 db (2.5 mV - 150 mV) at 1 KHZ.
The linearity of this direct coupled combination offers high
overload capabilities (100 mV at 1 KHZ). The 150 mV.
nominal level is at the same level as the other functions.
The Function Switch selects AM, FM, AUX and Phono.
This audio then goes to the tape monitoring function
switches.

The Tape Monitor Switch allows selection of either the
direct programme source as selected by the Function
Selector or play back from a tape recorder, if the .tape
recorder has a three head configuration, then in the monitor
position it is possible to compare the recorded signal with
the direct signal almost immediately.

The required selection will then go to the 2 transistor
NPN, PNP direct coupled flat response amp which drive
succeeding tone control circuit with low impedance. A NPN
transistor is used in a Baxandale-type active bass and
treble control, operating in a virtual earth mode with the
input audio at the boost point of the controls and the base
of the transistor fed from the slider of the bass control via
C908, the collector being the feedback point to the controls
and providing the output which is at unity gain with respect
to the input to drive the filters and the power amplifier.
The filters are constructed by CR type 6 db/oct hi and low
cut filters with hi cut 7 KHz and off, and low cut 70 Hz
and off.

AM SECTION

A superhet design incorporates an integrated circuit for all
stages, 99, the mixer, local oscillator and two stages of IF am-
plification, the IF being,455 KHZ. A ferrite rod antenna
with three winding is used, the first winding is connected
to the External Antenna Terminal, the second to the first
section of two gang tuning condenser, the last winding then
feeds-the input of the |.C..The LC. which includes all the
necessary stages produces an audio signal at its output suit-
able for directly feeding the Function Selector.