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MR65B FM STEREO TUNER
INTRODUCTION
The Mclntosh Model MR65B is a precision-
engineered, sensitive FM multiplex stereo
tuner. It offers the finest possible reception
of FM monophonic and FM multiplex stereo-
phonic broadcasts. Every desirable feature
for your FM listening pleasure is included in
the MR65B tuner. It is perfect for use in a
stereo music system where nothing less than
"THE Best" will do.
Once you have enjoyed the outstanding
performance of the MR65B, you will under-
stand why Mclntosh products have earned
their reputation as "THE Best."
Your Mclntosh MR65B tuner will give you
years of the finest possible FM reception, and
will become a highly valued part of your
home music system.
TECHNICAL DESCRIPTION
The MR65B tuner has a 6DS4 high-gain
Nuvistor for the first RF amplifier. The Nuvis-
tor (triode) operates into a triode tube to form
a cascode amplifier. Careful design of the
operating characteristics allows the cascode
amplifier to handle a wide dynamic range of
signals with excellent signal-to-noise ratio.
A double-tuned RF input circuit combined
with the Nuvistor-tube cascode amplifier
reduces spurious signals to an extremely low
value. Sensitivity is increased to the highest
possible rating, while still retaining high
selectivity.
A newly developed Automatic Frequency
Control (AFC) circuit greatly simplifies tuning.
As the tuning knob is moved, the AFC circuit
automatically cuts off for sharpest tuning.
After the station is tuned in, the AFC circuit
gradually comes into operation over a 3 sec-
ond interval. The AFC action automatically
brings the electrical tuning to the correct
point for minimum distortion. This auto-
matic tuning repeats its action each time the
tuning dial is moved to a different station.
The amount of AFC action is fully adjustable
with a front panel control.
The AFC circuit is quite unique. It uses a
silicon diode in place of a conventional tube.
The silicon diode improves the overall AFC
performance since it is unaffected by tem-
perature changes, has no warm-up drift and
eliminates the chance of tube filament hum.
A special temperature-compensated narrow
band detector supplies operating signals to
the AFC circuit. This insures positive AFC
action. The narrow band detector also
operates the FM tuning meter amplifier and
the ultrasonic muting circuit.
- The Mclntosh-developed ultrasonicmuting
circuit automatically suppresses all inter-
station noise, including the noises usually
heard tuning in and out of a station. Weak or
distant stations that don't over-ride the back-
ground noise and interference are also sup-
pressed by the muting. A switch on the front
panel allows muting to be cut off for listening
to these weak and noisy stations. A back
panel adjust control is factory-set to the
proper muting threshold level.
Three flat-topped response IF amplifiers
reject adjacent channel interference. More
than adequate gain is provided to operate
both limiters on the weakest signals. Two
complete limiters are used to insure the best
possible signal-to-noise ratio.
A separate wide band detector handles
exclusively the audio frequencies for low dis-
tortion and wide frequency response.
Both the RF and IF circuits are com-
pletely shielded and exceed the FCC require-
ments for suppression of radiation from the
FM oscillator and IF frequencies.
A unique multiplex stereo (MPX) indicator
light is included on the front dial panel. The
MPX indicator lights whenever the dial
pointer crosses a station broadcasting FM
multiplex stereo. The indicator uses a special
noise-rejecting bridge circuit with two ger-
manium diodes. The indicator lights ONLY on
the 19KC carrier present in a multiplex stereo
broadcast. It will not light on noise or inter-
ference signals.
The MR65B multiplex decoder circuit
uses a special peak-switching, self-matrixing
detector. Stereo channel separation is main-
tained at better than 30db.
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Two pairs of audio output jacks are on the
back panel. One pair of jacks is controlled
only by the front panel volume control. The
other pair of jacks is controlled only by the
back panel output adjust controls. Two stage
feedback type preamplifiers on both chan-
nels provide low impedance outputs. More
than sufficient output volume is available to
feed power amplifiers as well as preamplifiers.
A new type of mechanical tuning assembly
gives the MR65B an extremely smooth fly-
wheel tuning action. The tuning capacitor is
driven directly, and in turn drives the pointer.
Backlash is practically eliminated with this
method of design. A teflon lined pointer
carriage and nylon pulleys reduce friction
and wear to give an unusually smooth and
quiet dial action.
SPECIFICATIONS
Useable Sensitivity
2.5 microvolts at 100% modulation (±75KC
deviation) for less than 3% total noise and
harmonic distortion in accordance with IHFM
standards.
Audio Frequency Response
Within ½db from 20 to 20,000 cycles.
Distortion
Less than 0.5% at 100% modulation ±75KC
deviation.
Capture Ratio
1.5db at 100% modulation.
Muting
IF injected ultrasonic muting: at least 60db
noise reduction between stations.
Oscillator Drift
Less than 25KC with AFC disabled; negligible
with AFC in operation.
Image Rejection
Better than 80db at 90MC; better than 70db
at 105MC.
Hum
Better than 70db below 100% modulation.
Output
Approximately 2.5 volts; low impedance.
Antenna Inputs
300 ohms balanced; 75 ohms unbalanced.
RF Amplifier
Cascode with 6DS4 Nuvistor in first stage.
IF Amplifiers
Four, with 200KC bandwidth, flat top
response
Limiters
Two
Radiation
Substantially below FCC requirements.
Multiplex Channel Separation
Better than 30db at 1000 cycles.
Multiplex Filter
Greater than 48db suppression of 19KC
pilot and 38KC carrier.
Multiplex Indicator
Front panel multiplex stereo light activated
by 19KC carrier-only.
Multiplex Type
Peak-detecting, self-matrixing detector.
Tube and Semiconductor Complement
1-6DS4 Nuvistor, 1st RF.
1-12AT7, 2nd RF and Mixer.
1-6BN4A, Oscillator.
1-6AU6, 1st IF.
1-6AU6, 2nd IF.
1-6AU6, 3rd IF.
1-6AU6, 4th IF and 1st Limiter.
1-6CS6, 2nd Limiter and Muting.
1-6BN8, Muting Amplifier, Muting Detector,
AVC Clamper.
2-6BL8, Left and Right, 1st and 2nd Audio
Amplifiers.
1-12AU7, Balanced Tuning Meter Amplifier.
1-6U8, MPX Amplifier, MPX Indicator Con-
trol.
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