McIntosh MR-65-B Owners manual

FM STEREO

TABLE OF CONTENTS

TUNER

INTRODUCTION

TECHNICAL DESCRIPTION.

FRONT PANEL INFORMATION
BACK PANEL INFORMATION
INSTALLATION

CONNECTING

OPERATING INSTRUCTIONS.

ADJUSTMENTS

MR65B

Stereophonic FM Multiplex
Monophonic FM
Remote Amplifiers
Off-The-Air Recording
Antenna Connections

Stereophonic FM Multiplex
Monophonic FM
External Stereo

Muting Threshold
FM Signal Strength Meter
FM Tuning Meter
Dial Panel Lights.
Output Adjust

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GUARANTEE

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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.

TECHNICAL DESCRIPTION

The MR65B tuner has a 6DS4 high-gain

Nuvistor for the first RF amplifier. The Nuvis­tor (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 sec­ond 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.

Once you have enjoyed the outstanding

performance of the MR65B, you will under­stand 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.

- The Mclntosh-developed ultrasonicmuting

circuit automatically suppresses all inter­station 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 com­pletely shielded and exceed the FCC require­ments 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 ger­manium diodes. The indicator lights ONLY on

the 19KC carrier present in a multiplex stereo

broadcast. It will not light on noise or inter­ference signals.

The MR65B multiplex decoder circuit

uses a special peak-switching, self-matrixing

detector. Stereo channel separation is main­tained at better than 30db.

1

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 chan­nels provide low impedance outputs. More
than sufficient output volume is available to
feed power amplifiers as well as preamplifiers.

SPECIFICATIONS

A new type of mechanical tuning assembly
gives the MR65B an extremely smooth fly­wheel 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.

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.

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.

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.

2

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.

1-12AU7, MPX Oscillator.
1—ST2-275, Voltage Reference.
1—MA113 (Transistor), MPX Indicator Lamp

Switch.
2—Diodes, Wide Band Discriminator
2—Diodes, Narrow Band Discriminator.
4—Diodes, Balanced MPX Detector.
2—Diodes, Balanced Detector for MPX Indi-

cator.
2—Rectifiers, High Voltage Power Supply.

Power Consumption

70 watts, 105 to 125 volts, 50 to 60 cycles.

FRONT PANEL INFORMATION

Dimensions

Front panel 15-3/8inches x 5-1/8 inches; overall

depth of chassis behind front panel, 12-3/16

inches; clearance in front of mounting panel
including knobs, 1-1/2 inches.

Weight

Chassis only, 22 pounds.

In shipping carton, 31 pounds.

Figure 1. MR65B Front Panel.

TUNING DIAL

The tuning dial is calibrated in both

megacycles and a 0-100 logging scale. Tun­ing back to a particular station is much easier
by keeping a record of the exact station loca­tion on the logging scale.

METERS

Figure 2. FM Signal Strength Meter.

At the left side of the dial is the FM SIG­NAL STRENGTH meter. The amount of meter
pointer deflection indicates the relative sig­nal strength of the station being received.
Use the signal strength meter to correctly
position a directional FM antenna. Maximum
meter deflection for a particular station
occurs when the antenna is pointed in the
direction for best signal pickup.

Figure 3. FM Tuning Meter.

At the right side of the dial is the FM TUN-

ING meter. An FM station is correctly tuned
when the meter pointer is within the black
area at the center of the meter scale. Both

meters can be adjusted for proper zero set­tings if necessary. (See section titled AD-

JUSTMENTS.)

MULTIPLEX STEREO INDICATOR

At the center of the tuning dial is the
multiplex stereo (MPX) indicator. The indi­cator lights (red) MPX whenever the dial

pointer crosses a station broadcasting multi­plex stereo. The indicator lights ONLY on the
19KC multiplex carrier present in a multi­plex stereo broadcast. The indicator will
NOT light on noise or interference signals.

3

VOLUME

Controls the output volume level of the

tuner at the right hand pair of AUDIO OUT-

PUT jacks on the back panel. This pair of
jacks is marked FRONT PANEL CONTROLLED.
The other pair of audio output jacks, located
to the left, is not affected by the front panel
volume control. The output volume level at

these jacks is controlled by the dual con­centric OUTPUT ADJ controls to their left.

(See section

titled

ADJUSTMENTS.)

AUTO FREQ CONTROL

MODE SELECTOR

Figure 4. Mode Selector.

Power Off

Turns off the tuner AC power and also turns
off the AC outlet on the tuner hack panel.

If a TV antenna is used for FM in the man-

ner described under ANTENNA CONNEC­TIONS, it will automatically be switched back
to the TV set in the POWER OFF position.

FM

Provides monophonic FM at both pairs of

left and right channel audio output jacks.

MPX Stereo

Provides FM multiplex stereo at the respec-

tive left and right channel audio output jacks.

Figure 5. Auto Freq Control.

The Automatic Frequency Control (AFC)
circuit makes FM tuning easier. It also elimi­nates the possibility of the tuner drifting off

the station. The AFC circuit in the MR65B is

electronically delayed in its action. This is an
exclusive Mclntosh development. As the

tuning dial is moved, the AFC circuit is tem-

porarily cut off. After the dial pointer has

come to rest on a particular station, the AFC
gradually comes into operation over a three
second interval. The AFC automatically brings
the electrical tuning to the correct point for
minimum distortion. This automatic tuning

repeats itself each time the tuning dial is

moved to a different station.

The degree of AFC action is adjustable

with the AUTO FREQ CONTROL. Full left
(MIN) position of the control cuts off AFC
action. Full right (MAX) position gives maxi­mum AFC action.

Normally, the best tuning occurs with the
Auto Freq. Control set at or near MAX posi­tion. If several stations are very close to­gether on the dial, or if a desired station is
very close together on the dial, or if a desired
station is very close to a much stronger sta­tion, a lower AFC setting is usually necessary
for best tuning.

MUTING

EXT Stereo

Provides monophonic FM at the RIGHT

channel audio output jacks. The back panel

EXT STEREO INPUT jack is also connected
through the tuner preamplifier to the LEFT
channel audio output jacks in this switch

position. A monophonic AM tuner, for exam­ple, may be connected to the EXT STEREO

INPUT for AM-FM stereo reception.

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Figure 6. Muting Control.