FM STEREOTUNER MR65B

INTRODUCTION

TABLE OF CONTENTS

TECHNICAL DESCRIPTION.

FRONT PANEL INFORMATION

BACK PANEL INFORMATION

INSTALLATION

CONNECTING

Stereophonic FM Multiplex

Monophonic FM

Remote Amplifiers

Off-The-Air Recording

Antenna Connections

OPERATING INSTRUCTIONS.

Stereophonic FM Multiplex

Monophonic FM

External Stereo

ADJUSTMENTS

Muting Threshold

FM Signal Strength Meter

FM Tuning Meter

Dial Panel Lights.

Output Adjust

GUARANTEE

1

1

3

5

6888888

10

11

111111

121212

12

1313

14

MR65B FM STEREO TUNERINTRODUCTION

The Mclntosh Model MR65B is a precision-engineered, sensitive FM multiplex stereotuner. It offers the finest possible receptionof FM monophonic and FM multiplex stereo-phonic broadcasts. Every desirable featurefor your FM listening pleasure is included inthe MR65B tuner. It is perfect for use in astereo music system where nothing less than"THE Best" will do.

Once you have enjoyed the outstandingperformance of the MR65B, you will under-stand why Mclntosh products have earnedtheir reputation as "THE Best."

Your Mclntosh MR65B tuner will give youyears of the finest possible FM reception, andwill become a highly valued part of yourhome music system.

TECHNICAL DESCRIPTION

The MR65B tuner has a 6DS4 high-gainNuvistor for the first RF amplifier. The Nuvis-tor (triode) operates into a triode tube to forma cascode amplifier. Careful design of theoperating characteristics allows the cascodeamplifier to handle a wide dynamic range ofsignals with excellent signal-to-noise ratio.

A double-tuned RF input circuit combinedwith the Nuvistor-tube cascode amplifierreduces spurious signals to an extremely lowvalue. Sensitivity is increased to the highestpossible rating, while still retaining highselectivity.

A newly developed Automatic FrequencyControl (AFC) circuit greatly simplifies tuning.As the tuning knob is moved, the AFC circuitautomatically cuts off for sharpest tuning.After the station is tuned in, the AFC circuitgradually comes into operation over a 3 sec-ond interval. The AFC action automaticallybrings the electrical tuning to the correctpoint for minimum distortion. This auto-matic tuning repeats its action each time thetuning dial is moved to a different station.The amount of AFC action is fully adjustablewith a front panel control.

The AFC circuit is quite unique. It uses asilicon diode in place of a conventional tube.The silicon diode improves the overall AFCperformance since it is unaffected by tem-perature changes, has no warm-up drift andeliminates the chance of tube filament hum.A special temperature-compensated narrowband detector supplies operating signals tothe AFC circuit. This insures positive AFCaction. The narrow band detector alsooperates the FM tuning meter amplifier andthe ultrasonic muting circuit.

- The Mclntosh-developed ultrasonicmutingcircuit automatically suppresses all inter-station noise, including the noises usuallyheard tuning in and out of a station. Weak ordistant stations that don't over-ride the back-ground noise and interference are also sup-pressed by the muting. A switch on the frontpanel allows muting to be cut off for listeningto these weak and noisy stations. A backpanel adjust control is factory-set to theproper muting threshold level.

Three flat-topped response IF amplifiersreject adjacent channel interference. Morethan adequate gain is provided to operateboth limiters on the weakest signals. Twocomplete limiters are used to insure the bestpossible signal-to-noise ratio.

A separate wide band detector handlesexclusively 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 theFM oscillator and IF frequencies.

A unique multiplex stereo (MPX) indicatorlight is included on the front dial panel. TheMPX indicator lights whenever the dialpointer crosses a station broadcasting FMmultiplex stereo. The indicator uses a specialnoise-rejecting bridge circuit with two ger-manium diodes. The indicator lights ONLY onthe 19KC carrier present in a multiplex stereobroadcast. It will not light on noise or inter-ference signals.

The MR65B multiplex decoder circuituses a special peak-switching, self-matrixingdetector. Stereo channel separation is main-tained at better than 30db.

1

Two pairs of audio output jacks are on theback panel. One pair of jacks is controlledonly by the front panel volume control. Theother pair of jacks is controlled only by theback panel output adjust controls. Two stagefeedback type preamplifiers on both chan-nels provide low impedance outputs. Morethan sufficient output volume is available tofeed power amplifiers as well as preamplifiers.

A new type of mechanical tuning assemblygives the MR65B an extremely smooth fly-wheel tuning action. The tuning capacitor isdriven directly, and in turn drives the pointer.Backlash is practically eliminated with thismethod of design. A teflon lined pointercarriage and nylon pulleys reduce frictionand wear to give an unusually smooth andquiet dial action.

SPECIFICATIONS

Useable Sensitivity2.5 microvolts at 100% modulation (±75KCdeviation) for less than 3% total noise andharmonic distortion in accordance with IHFMstandards.

Audio Frequency ResponseWithin ½db from 20 to 20,000 cycles.

Distortion

Less than 0.5% at 100% modulation ±75KCdeviation.

Capture Ratio1.5db at 100% modulation.

MutingIF injected ultrasonic muting: at least 60dbnoise reduction between stations.

Oscillator DriftLess than 25KC with AFC disabled; negligiblewith AFC in operation.

Image RejectionBetter than 80db at 90MC; better than 70dbat 105MC.

Hum

Better than 70db below 100% modulation.

OutputApproximately 2.5 volts; low impedance.

Antenna Inputs300 ohms balanced; 75 ohms unbalanced.

RF AmplifierCascode with 6DS4 Nuvistor in first stage.

IF AmplifiersFour, with 200KC bandwidth, flat topresponse

LimitersTwo

RadiationSubstantially below FCC requirements.

Multiplex Channel SeparationBetter than 30db at 1000 cycles.

Multiplex FilterGreater than 48db suppression of 19KCpilot and 38KC carrier.

Multiplex IndicatorFront panel multiplex stereo light activatedby 19KC carrier-only.

Multiplex TypePeak-detecting, self-matrixing detector.

Tube and Semiconductor Complement1—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.

2

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

Switch.2—Diodes, Wide Band Discriminator2—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.

Dimensions

Front panel 15-3/8inches x 5-1/8 inches; overalldepth of chassis behind front panel, 12-3/16inches; clearance in front of mounting panelincluding knobs, 1-1/2 inches.

Weight

Chassis only, 22 pounds.In shipping carton, 31 pounds.

FRONT PANEL INFORMATION

Figure 1. MR65B Front Panel.

TUNING DIAL

The tuning dial is calibrated in bothmegacycles and a 0-100 logging scale. Tun-ing back to a particular station is much easierby keeping a record of the exact station loca-tion on the logging scale.

METERS

Figure 2. FM Signal Strength Meter.

At the left side of the dial is the FM SIG-NAL STRENGTH meter. The amount of meterpointer deflection indicates the relative sig-nal strength of the station being received.Use the signal strength meter to correctlyposition a directional FM antenna. Maximummeter deflection for a particular stationoccurs when the antenna is pointed in thedirection 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 tunedwhen the meter pointer is within the blackarea at the center of the meter scale. Bothmeters can be adjusted for proper zero set-tings if necessary. (See section titled AD-JUSTMENTS.)

MULTIPLEX STEREO INDICATOR

At the center of the tuning dial is themultiplex stereo (MPX) indicator. The indi-cator lights (red) MPX whenever the dialpointer crosses a station broadcasting multi-plex stereo. The indicator lights ONLY on the19KC multiplex carrier present in a multi-plex stereo broadcast. The indicator willNOT light on noise or interference signals.

3