Luxman 1500 Service Manual

Page 2

R 1500 CIRCUIT DESCRIPTION

Power Supply

The mains input to the Receiver goes via a 2 pole power
switch to the mains transformer which has four secondary
windings:

1. A 11 volt winding for the panel lamps.

2. A 25 volt winding for the speaker protection circuit.

3. A 75volt centre tapped winding providing after full-
wave rectification ® and G) supplies for the audio power
amp section.

4. A 30-0-30volt winding to provide 45vo|ts DC for the
preamp and tuner sections. A potential divider using a
zener diode which is used to obtain from 45 volts a 12volt
DC supply for the AM and FM tuner sections. The
2501382 transistor is used as a ripple filter providing
40volts to the audio preamps. There are fuses in all the
secondaries for transformer protection. A fuse is used as
well in the preamp.

POWER AMPS

PNP transistors are used as a differential comparator; the
audio input is fed to the first transistor and the feedback to
the other. NPN metal can transistors are used as active
loads for the differential comparator, the one NPN has its
collector to ground, the other being the voltage amplifier
for the entire output stage, which uses a PNP driver and
a PNP output in the negative side connected as a darlington
emitter follower configuration, and an NPN driver and
NPN output is used in the positive side. These 4 transistors
form what is known as a fully complementary symmetrical
output stage.

Their input bases are bias with a 3 diode device (SV03) and
a bias trim pot for idling current set (50 mA), 2 resistors
and a condenser form the boots-trap constant current ioad
for the voltage amplifier to drive the output stage. The
junction of the emitter resistors is as a feed point for the
differential comparator, and at the same time goes via a
relay to the speaker selector switch to the speaker terminals
and headphone socket via 680 ohm.

The headphone socket is always active regardless of speaker
selection. PNP and NPN transistors are used to sense out-
put current and voltage and if the output is short circuited
will limit the drive to the output devices, thereby pro-
tecting the amplifier from overload.

Input sensitivity of the power amp is defined by the ratio
of R811, R813 which = 700 mv for full output, and the
low frequency roll off is determined by the reactance of
C806 to R811 which in this circuit is 10Hz.

PRE AMP
Phono RIAA equalization amplifier 2 phone inputs can
be selected both with the same input sensitivity. The RIAA

amplifier uses a NPN, PNP and NPN direct coupled triplet
of transistors which are selected low noise types, the equali»
zation and DC feedback are taken from the emitter of the
NPN back to the emitter of the input NPN transistor. This
amp has a gain of 36 db (2.3mv - 150mv) at 1 KHZ.
The linearity of this direct coupled combination offers
high overload capabilities (100mv at 1 KHz). The 150mv
nominal level is at the same level as the other functions.
The Mic amplifier uses basically the same circuit without
equalization, being flat from 20 - 50 KHz with 38 dB gain.
The Function Switch selects AM, FM Aux 1 and 2 and
Phone 1 and 2 the output of which can be mixed with
the input from the mic amplifier. This audio then goes to
the tape monitoring function switches.

There are two switches for the tape functions, these are
mounted on the switch printed circuit board. The left
switch enables in the "up" position dubbing from tape
1 ~ 2 and in the lower" position from Tape 2- 1, when
centre", it is off. The right-hand switch enables moni-
toring from Tape 1 in the up position, tape 2 in the lower
and programme source in the centre position.

The switch for the loudness works in conjunction with the
volume control so that its greatest effect is at low volume
settings, the audio then passes on the tone control circuit.

LINEAR EQUALIZER and TONE CONTROL

The required selection will then go to the tone control
printed circuit board which contains 2 NPN transistors

direct coupled linear equalizer amp and 2 NPN transistor

amp for tone control. The income signal is linear equalized
by selecting the feedback components of this amp, The
audio then passes on the tone control circuit. Two direct
coupled NPN transistors are used in the Lux-type active
bass and treble control, operating in a virtual earth mode
with the input at the boost point of the controls and the
base of the transistor fed from the slider of the bass control
via C741 and C742, the emitter being the feedback point
to the controls and providing the output which is at unity
gain with respect to the input, to drive the active filter
circuit. included on the tone control board are two switches
for changing to bass and treble turn over frequency points.

FILTER BOARD

The Sallen Keyes type filter circuit has components select-
able by the front panel low and high cut switches to
provide a 12 dB/oct. roll off. The output is sufficiently
low impeadance to drive the power amp circuit.

SPEAKER PROTECTION

Speaker protection relay is provided in this receiver to
protect speakers from damage. Speaker output terminals
must always have no significant residual DC voltage, but
if some plus or minus voltage appears these terminals, the