PAGE
4
>:+<•*•!
MX99
broadcasts will
come
through
with slightly
less noise
at the MAIN
CH.
position.
For
critical
mono
listening
or recording, selection of the
tuner's regular
output by
the
stereo amplifier is desirable.
5.
Operate your tuner as
you normally would in
either tuning
to a station known to
be broadcasting
stereo at the time,
or in searching
for a station
that
is broadcasting stereo.
(Note: If
your FM tuner has
an
AFC
circuit
and also an
"AFC defeat" switch,
it is
recommended that
you
tune
with
AFC defeated for
greater tuning accuracy
and then switch in
the AFC
afterwards.) When you
are tuned to a station
broad-
casting stereo, the
MX stereo neon lamp indicator
of
the MX-99 will glow steadily.
(NOTE: The neon lamp
will
go out if the station switches
to mono
broadcasting
at any time.) Adjust
your stereo amplifier
controls
for the desired listening level
and best balance
between
channels.
6.
Adjust the SEPARATION control
of the
MX-99
for best
audible
separation
between the stereo
channels.
This setting will
usually be the
same for all stations
broadcasting stereo, although
there
may be one or more
stations in
your area for which different
settings
are
necessary
because of non-standard
practice
on the
part of these particular
stations.
NOTE: The
MX STEREO
lamp
may glow
at some
off-station
points on the dial.
These are
spurious
indications and should be ignored.
SECTION V.
CIRCUIT DESCRIPTION
The composite
stereo signal
as received from
the
ratio
detector is amplified
by VIA.
In order not to
alter the
performance of the ratio
detector, the
input
impedance
of this stage is
made exceptionally
high.
The 19kc pilot signal
is isolated and amplified
by
V4. Frequency doubling is
accomplished by a full-wave
rectifier
(CR1,
CR2)
at
the
plate
ov
V4, and the 38kc
signal thus obtained is
used to synchronize
a 38kc push-
pull
oscillator
(V5)
which drives
a ring modulator
(CR3-CR6,
R12-17).
In addition
to a strong 38kc
com-
ponent, the full-wave
rectifier delivers
a negative
DC
voltage
when the 19kc pilot carrier
is received. This
DC
voltage is
used to cut off switching
tube VlB,
and
thus ignite
the MX
STEREO neon pilot lamp II, indi-
cating that
a stereo broadcast is
in progress.
VIA also delivers
the amplified
composite
stereo
signal to the grid
of a split load phase inverter
(V2A).
V2A provides two
outputs which differ in
phase by
180°
and
which are
alternately
sampled
by the ring
modu-
lator
at a 38kc
rate. The two sampled
outputs are
added and amplified
byV2B. Adding the two alternately
sampled
out of phase signals
effectively
produces an
output which is the input signal
multiplied
by a 38kc
switching
function of zero
average value
and odd sym-
metry. The audible portion
of this signal is
(L-R).
The output
of V2B
(L-R)
is mixed
with the two
outputs of
V2A (L+R,-L-R), available
at the
plate
and
cathode
of V2A, respectively,
to derive L and
-R.
De-
emphasis is accomplished
in the mixing
networks.
Evidently
the phase of one
signal
must
be
reversed
to
duplicate
the original phase relationship,
and this is
accomplished
by V3A. The signal inverted
by V3A
is
fed to the grid
of cathode
follower
V3B and the other
signal (not inverted)
is fed to the grid
of V3C.
Channel
separation is controlled
by varying the
amount
of (L-R) signal which is injected
into the mix-
ing network.
This is effected by a gain control
in the
grid
of
V2B. The output signal level under
the
condition
of optimum
separation is thus determined
by the direct
(L+R) component and is virtually
independent
of the
phase
of the reinjected carrier.
An error in the phase
of the injected carrier will
simply require
a higher
setting
of the separation control
to achieve the
same
output amplitude
and channel
separation that would
be
obtained
under the optimum carrier
phase condition.
Note that the demodulator
is of the balanced
type
so that the potentially
troublesome 38kc
carrier does
not appear in the
adaptor output.
The presence of this
signal might conceivably
introduce
beat notes with tape
recorder bias
oscillators.
Cathode follower
outputs
(V3B
and
V3C) are pro-
vided in
each channel and filtering
of the output wave-
forms
are effected in these stages.
Filtering atten-
uates the high
frequency components which
are intro-
duced in
the demodulation
process, and
which, while
substantially
attenuated by the de-emphasis
networks,
may
become objectionable
in tape recording.
In addi-
tion, the filters are designed
to remove
the 19kc pilot
signal
which is usually
present
to
a noticeable
degree
in the output signals.
SECTION VI. MAINTENANCE
6-1.
GENERAL
Your instrument will
normally
require little
ser-
vice
outside of tube replacement.
The performance
of
the
instrument is not dependent
on tube
selection,
and
all
of the types
employed are available
nationally.
Completed kits
do not require alignment
because
of factory pre-alignment
of all tuned
circuits.
However,
a service
alignment
procedure is given
which does
not
require
the use of
instruments.
Use
this
procedure
on
completed kits only
if there is definite
indication
of
misalignment,
and after checking
that wiring is entirely
correct and that
there are no damaged
or defective
parts.
Trouble-shooting
information
is provided
also.
Included
in this
section are a trouble-shooting
chart
and
a
table
of operating
voltages and resistances.
6-2.
SERVICE
ALIGNMENT
NOTE: This
adjustment procedure
can be
per-
formed only
when
an FM station is
known
to be
broadcasting multiplexed
stereo.
1.
Pull power
cord from outlet.
Remove
the MX-99
cover and
temporarily dismount
(but do not
disconnect)
the printed board.
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