Revox A76 Service Manual

T(fis)
Fig.
3.4-1
amplifier
Q405
and
Q406
produces
a
76
kHz
frequency
which,
after
pulse
shaping, passes
I
C
401
operated as a
fre-
quency
divider at the
ratio
of 1
:2:4.
The
resulting
38
kHz
(subcarrier)
is
then used
to operate
the
switching
demodula-
tor.
The 19
kHz signal
from
the
frequency
divider is fed to
the
phase
comparator Q403/Q404
where
it
is
compared with
the
pilot-frequency
arriving
from the
bandpass L401/
L402.
The 15 Hz
low-pass
filter formed by
R409,
C408,
R410
prevents any
interference
components
in the
pilot
signal
from reaching the
varicap
diodes D402
in the 76
kHz oscilla-
tor.
This
arrangement
operates
in the
manner
of
a
narrow
pilot frequency
bandpass
with
a
±
15
Hz
bandwidth. The bias
voltage
for
the
varicap-diodes
is
obtained
from the
Zener-
diode D401.
3.7.
Mulitplex
decoder
1.076.220
(schematic
E)
The
Multiplex-Decoder
operates
on the
switching
principle.
Separation
of
the
multiplexed
signal
takes place
in the
low-
pass
network
R508, C504,
R509
for the
main
channel
and
in the
38
kHz
bandpass L501,
C501,
R502
for the
sub-carrier
or
difference
channel.
Amplification
of the
sub-carrier
takes
place
in Q501
whereas
Q503
and Q504
operate as
the
switching
demodulator.
The
switching
principle was
selected
because
in contrast to
envelope-detection
it transposes
only
those
frequencies
in
the audible
range which
are uneven
multiples
of
the sub-
carrier.
Thus
the lowest
critical
interference
frequencies
occur around 114
kHz and they
are effectively
suppressed
by
the 1
14
kHz
filter
L505
-
C518
and
the
38
kHz
band-
pass
in
the
difference channel.
Across the
load resistors R518/R519
the
signals L-R
and
—
(
L-R
)
are being formed,
with the
main
signal appearing
in
phase
across those load
resistors.
Any
residual compo-
nents are
suppressed
by
the in-phase
operation of the
differen-
tial
amplifier
Q505
and
Q506,
which
works
at
the same
time as the
matrix
by
forming the L
and R
signals from the
difference channel
and from
the main
channel, which
arrives
from
the
Q502
amplifier.
8
7
6
X
/
5
\
200
100
kHz
UK)
2CK)
1
pV
2
5
10
20 50
100
2(K}
5(K)
1 mV
2
5 10
Vanteon
Fig.
3.4-3
A
low
pass 15
kHz filter to
suppress
multiplex
residuals is
situated between
the
impedance
matching
stages Q507/Q508
and
the matrix
output.
The
audio
level at
the low
impedance
outputs is
adjustable
from 50
mV
to
1
V,
referenced
to
an
FM deviation
of
75
kHz.
3.8.
Indicator
circuits
1.076.200
(schematic
F)
This
portion
of the
tuner
contains the
auxiliary
circuits
for
tuning
indication
and
for
various
automatic
functions.
The
IF-signal
arriving
from the
fourth IF stage
is
amplified
by
Q601
and then
passed
to
a
narrow
band
detector whose
small
band-width
produces
the sensitive
response
of the
"CENTER TUNING"
(44)
meter.
As
already
mentioned
in
a
previous
chapter,
a
level
depen-
dent
current
for signal
strength
indication
is
derived
from
the
first three
I
F
stages
and
fed
to
the
"SIGNALSTRENGTH"
meter
after
amplification
in the Q602
amplifier.
The
level
dependent
triggering circuit
formed by Q604
and
Q605
delivers a
gating
signal
to
the base
of Q510
in
the
stereo
decoder
for
mono-stereo
changeover
and
also biases
the
last IF
stage
for noise squelch
and
silent
tuning. By
de-
pressing
the
button "TRIGGER
LEVEL
VARIATION"
(48)
R702
in
the
collector leg
of
Q602,
which
normally
provides
maximum
sensitivity, is
replaced by
the potentiometer
"TRIGGER
LEVEL"
(
39
).
This
allows
a
wide
variation
of
the
triggering
point
with
consequent muting
even
of
rather
powerful stations.
With the
button
"MUTING ON"
(
49
)
released, the
triggering circuit
will respond
for
automatic
mono-stereo
changeover
only.
In
the
multiplex
decoder the
circuit
formed
by
0510/051
1
and
0512
analyses
whether
or not the
following
conditions
are met
for automatic
changeover to
stereo
operation:
1.
signal
from the
level dependent
trigger (input base
0510)
2.
presence
of
a
pilot
signal. (This
signal is derived
from
the
band-pass
in the
sub-carrier
regenerating
module.
It is
amplified
by
0509
and
then
rectified
by
the
diode D502).
7

Revox A76 Specifications

General

Brand	Revox
Model	A76
Category	Tuner
Language	English