THE McINTOSH MR 74 SOLID STATE AM FM/FM STEREO TUNER

READING TIME: 28 Minutes Price $1.25

Your MR 74 AM FM/FM STEREOTUNER will give you many yearsof pleasant and satisfactoryperformance. If you have anyquestions please contact:

CUSTOMER SERVICEMclntosh Laboratory Inc.2 Chambers StreetBinghamton, New York 13903Phone: 607-723-3512

WARNING: TO PREVENT FIRE ORSHOCK HAZARD, DO NOT EXPOSETHIS UNIT TO RAIN OR MOISTURE.

Take Advantage of 3 yearsof FREE Factory Service . . .Fill in the Application NOW.

CONTENTS

SERVICE CONTRACT 1INSTALLATION 2, 3

HOW TO CONNECT 4,5BACK PANEL INFORMATION 6

FRONT PANEL INFORMATION 6, 7LISTENING TO THE MR 74 8

PERFORMANCE LIMITS AND RATINGS 9PERFORMANCE CHARTS 10, 11

TECHNICAL DESCRIPTION 12, 13, 14BLOCK DIAGRAM 15FM STATION LOG 16

An application for a FREE THREE YEAR FACTORY SERVICE CONTRACT is included with this manual

The terms of the contract are:

1. Mclntosh will provide all parts, materials andlabor needed to return the measured perform-ance of the instrument to the original per-formance limits free of any charge. TheSERVICE CONTRACT does not cover any ship-ping costs to and from the authorized serviceagency or the factory.

2. Any Mclntosh authorized service agency willrepair all Mclntosh instruments at normalservice rates. To receive the free service underthe terms of the SERVICE CONTRACT, theSERVICE CONTRACT CERTIFICATE must ac-company the instrument when taken to theservice agency.

3. Always have service done by a Mclntoshauthorized service agency. If the instrumentis modified or damaged, as a result of un-authorized repair the SERVICE CONTRACTwill be cancelled. Damage by improper use

or mishandling is not covered by the SERV-ICE CONTRACT.

4. The SERVICE CONTRACT is issued to you asthe original purchaser. To protect you frommisrepresentation this contract cannot betransferred to a second owner.

5. For your protection Mclntosh selects onlydealers who have technical competence toguide purchasors fairly, and provide servicewhen necessary. To receive the SERVICECONTRACT your purchase must be madefrom a Mclntosh franchised dealer.

6. Your completely filled in application for aSERVICE CONTRACT must be postmarkedwithin 30 days of the date of purchase ofthe instrument.

7. To receive the SERVICE CONTRACT all in-formation on the application must be filledin. The SERVICE CONTRACT will be issuedwhen the completely filled in applicationis received at Mclntosh Laboratory Incor-porated in Binghamton, New York.

Copyright © 1972 By McIntosh Laboratory Inc.

THREE YEAR FACTORY SERVICE CONTRACT

1

It is recommended that the MR 74 tuner be mount-ed in a normal or horizontal position. However, withadequate ventilation the tuner can be mounted inany position.

Adequate ventilation extends the trouble-free lifeof electronic instruments. It is generally found thateach 18° F rise in operating temperature reduces thelife of electrical insulation by one half. Adequateventilation is an inexpensive and effective means ofpreventing insulation breakdown that results fromunnecessarily high operating temperatures. Thedirect benefits of adequate ventilation is longer,trouble-free life.

The MR 74 tuner requires a mounting space that isat least 15 inches deep, 17½ inches wide and 6inches high. Provide additional space for the air flowacross the bottom of the tuner and a means for warmair to escape at the top.

Remove the tuner, shelf brackets, parts bag andmounting template from the carton. Remove tunerfrom the plastic bag and place the tuner upsidedown on the shipping pallet, then remove the fourplastic feet fastened to the bottom of the chassis.

The installation may be accomplished in six steps:

1. POSITION TEMPLATE AND MARKPosition the plastic mounting template over thearea of the cabinet panel where the MR 74 is toinstalled. Be sure that the edges of the templateclear any shelves, partitions or existing equip-

ment located behind the panel. With the tem-plate in place mark the six "A" and " B " holesand four small holes locating the corners of thecutout. Next, join the four corner marks withpencil lines; the edge of the template may beused as a straight edge.

2. DRILL HOLESHolding a drill perpendicularly to the panel, drillthe six "A" and " B " holes using a 3/16 inch drill.

THE SIX HOLES MUST BE DRILLED BEFOREMAKING THE CUTOUT.

3. SAW CUTOUTUsing a saw carefully cut the rectangular open-ing on the inside of the pencil lines.

4. SECURE MOUNTING STRIPSSecure mounting strips (supplied in the hard-ware package) on the inside of the cabinetpanel.Insert two screws (supplied in the hardwarepackage) into the center holes ("B" holes on thetemplate). Use the ¾-inch long screws forpanels under ½-inch thick or 1¼-inch screwsfor panels ½-inch thick or over. Place a mount-ing strip on the back of the cabinet pane! asshown. Align it with the three holes in the panel

and tighten the screw. The screw head shouldpull slightly into the wood panel. Attach the othermounting strip by repeating the procedure.

5. MOUNT PANLOC SHELF BRACKETSAttach the Panloc shelf brackets to the cabinetpanel using four screws of the proper length.Place the template over the mounting screws.The screws should be centered on the "A" and" B " holes in the template. If necessary, loosenthe screws and push the mounting brackets intoalignment then retighten.

2

6. INSTALL THE UNITThread the power cord through the opening onthe cabinet panel. Carefully slide the instrumentinto the opening so the rails on the bottom en-gage the track on the mounting brackets. Slidethe instrument in until it stops at the adjust po-sition latches. Press the latches in and continueto slide the instrument until its front panel isflush with the cabinet panel. At the bottom frontcorners of the instrument are the PANLOC but-tons. Depressing the PANLOC buttons will lockthe instrument firmly in the cabinet. Depres-sing the PANLOC buttons a second time willrelease the instrument. You can then slide theinstrument forward to the adjust position. De-pressing the adjust position latches will allow theinstrument to be removed from the cabinet.

3

How to Connect

AUDIO OUTPUTS

Use the FIXED OUTPUT jacks to connect to a stereocontrol preamplifier or other equipment which hasits own volume control. The VOLUME CONTROLdoes not affect the output of the tuner at the FIXEDOUTPUT jacks.

The output impedance at the FIXED OUTPUTS is600 ohms. Longer cables than are normally suppliedcan be used to interconnect the MR 74 with otherequipment. The length of the cable is limited by thecapacity of the cable. The total capacity must notexceed 1600 pF. For instance: cables with a capacityof 32 pF per foot may be 50 feet long; 16 pF per footcable may be 100 feet long.

Use the FRONT PANEL CONTROLLED jacks toconnect to equipment such as power amplifiers ortape recorders where control of volume at the tuneris desired. The load impedance connected to FRONTPANEL CONTROLLED jacks should not be less than10,000 ohms. There is no difference in the signalquality or maximum output levels available at eitherpair of output jacks.

CONNECTING AN FM ANTENNA

One of three antenna systems can be used: (1) anoutdoor FM antenna, (2) an all channel (UHF-VHF-TV) antenna, or (3) the indoor dipole supplied withthe MR 74.

An outdoor antenna is recommended for optimumperformance in all areas. In fringe areas, best re-sults will be obtained with a highly directional FMantenna used in conjunction with a rotator. Rotatethe antenna until the best reception is obtained. Con-nect the 300 ohm antenna to the 300 ANT (red)terminals.

An UHF-VHF-TV antenna is often effective but theantenna must be designed for both FM and TV re-ception. Connect the leads from the UHF-VHF-TVantenna to the 300 ANT (red) terminals.

CONNECTING AN INDOOR DIPOLE ANTENNA

The flexible folded dipole antenna (300 ohm) is foruse in urban or high strength signal areas.

Connect the two leads from the dipole to the 300ANT (red) terminals. The flexibility of the thin flatwire assembly permits it to be placed under a rug,tacked behind the stereo . . . or, placed in any otherconvenient location. In some cases, it may be nec-essary to "position" the antenna for best signal re-ception. This should be done before it is permanentlylocated. Avoid locating this antenna next to otherwires or metal objects. This antenna may not proveeffective in houses having metal siding or metal-cladinsulation.

AM ANTENNA

A high-quality loop-stick antenna is provided. Itcan be rotated through nearly 180° in all directionsfor maximum performance, optimum signal receptionor minimum interference. With this mobility you willnot suffer loss of sensitivity regardless of the angleat which the instrument is mounted. A back panelantenna jack is provided for connecting an externalantenna if desired.

4

For best long distance AM reception, use a copperantenna wire 50 to 150 feet in length. Suspend thewire in a straight line as high as possible. Attach thewire at each end with suitable glass or ceramic in-sulators. Connect a lead-in wire at any convenientpoint on the antenna. It is recommended that a light-ning arrester be used with an outdoor AM antenna.The arrester should be well grounded to a suitablewater pipe or a ground rod sunk into the ground.

Connect the lead-in wire to the AM ANT push con-nector on the antenna terminal strip on the backpanel. Set the AM ANT slide switch to EXT position.

CONNECTING AN AM OUTDOOR ANTENNA

FM ANTENNA

ALTERNATEAM ANTENNA

A high-quality loopstick antennais provided. It can be rotated formaximum performance, optimumsignal reception or minimuminterference.

The antenna is rotatable throughnearly 180 degrees.

LEFT SPEAKER RIGHT SPEAKER

5

PREAMPLIFIERPOWER AMPLIFIER

Back Panel information Front Panel informationTP1 and TP2

Test points TP1 and TP2 are provided for tunercircuit alignment. They are also used in conjunctionwith the Mclntosh Maximum Performance Indicator.

120 AC POWER OUTLET

Provides 120 volt AC power up to 400 watts foradditional equipment such as amplifiers, or otherequipment. This outlet is not fused. It turns on andoff with the front panel AC power switch on theVOLUME control.

AC POWER CORD

Connect the AC power cord to a 120 volt, 50 to 60Hz power line receptable. The power used by theMR 74 is 30 watts.

FUSE

A 0.5 AMP SLO-BLO fuse protects the tuner cir-cuits. This fuse does not protect additional equip-ment connected to the back panel AC power outlet.

AM ANT SWITCH

Adjust the MR 74 AM ANT switch to match the par-ticular type of AM antenna used. EXT position isused for an outdoor antenna. LOOP position is forthe built-in ferrite loopstick.

ANTENNA CONNECTION STRIP

Provides easy push type connectors for an AMantenna, a ground connection, and a 300 ohm FMantenna.

FM ANTENNA — 75 OHM

Provides a type F male connector for a 75 ohmunbalanced FM antenna.

AUDIO OUTPUTS

The FIXED OUTPUT pair of AUDIO OUTPUT jacksprovides audio signals unaffected by the MR 74front panel VOLUME control. Use these output jacksto connect the tuner to a stereo preamplifier or otherequipment which has its own master volume control.

The FRONT PANEL CONTROLLED pair of AUDIOOUTPUT jacks provides audio signal controlled bythe MR 74 front panel VOLUME control. Use theseoutput jacks to connect to external power amplifier,tape recorders, or any equipment which requiresfront panel control of tuner output volume.

TUNING DIAL

The MR 74 has three dial scales:1. AM — Marked 55 to 1602. FM — Marked 88 to 1083. Logging scale — Marked 0 to 100The logging scale can be used to accurately re-

tune any station. You may find it easier to keep arecord of your favorite stations by use of the loggingscale.

A small portion of dial pointer has been illuminatedto increase the ease of tuning.

INDICATORS

The MR 74 has four indicators on the dial panel:STEREO indicator, MULTIPATH indicator, SIGNALSTRENGTH meter, and the FM TUNING meter.

STEREO INDICATOR

The STEREO indicator lights red when the dialpointer is tuned to or corsses a station broadcastingthe 19,000 Hz carrier for stereo. The special circuitused will light only when the 19,000 Hz multiplexcarrier is present in the signal. The indicator will notlight on noise pulses or interference.

MULTIPATH INDICATOR

The MULTIPATH indicator is an exclusive Mc-lntosh development.

The proper use of the MULTIPATH indicatormakes it possible to improve FM reception withFM antenna positioning.

An electron ray indicator is used to show multi-path reception. When rotating the antenna, observa-tion of the multipath indicator will show best orien-tation for the FM station being received. Multipathdistortion causes the two beams on the indicator tofluctuate rapidly. When the antenna is rotated to thebest position, the indicator beams tend to remainsteady. The directional antenna then picks up thedesired signal and rejects the reflected multipathsignals. In some locations it is possible for the bestreception to occur when the antenna picks up astrong reflected signal rather than the direct signal.Multipath distortion is practically independent of sig-nal strength.

SIGNAL STRENGTH METER

The SIGNAL STRENGTH meter indicates thestrength of the signal as received from the antenna.The higher the indication, the stronger is the signal,

FM TUNING METER

An FM station is correctly tuned when the meterneedle is in the black area of the FM TUNING meter.

6

The action of the TUNING meter is independent ofstation signal strength.MODE SELECTOR

Selects any one of three sources:AM— Connects the AM tuner section of the MR

74 to the output jacks.FM— The FM position provides monophonic FM

or FM stereo at the left and right channelaudio output jacks automatically. With theMODE SELECTOR in the FM position astation broadcasting monophonic programwill be heard in mono. When the stationswitches to stereo broadcast, the stereo in-dicator will light and the MR 74 will auto-matically switch to stereo operation. If monobroadcasting is resumed the MR 74 will au-tomatically switch to mono.

FM-MONO — Connects the FM tuner sections ofthe MR 74 to the output jacks. Thisposition bypasses the automaticstereo switching. The stereo indica-tor will light when a station is trans-mitting stereo, but all programs areheard monophonically.

STEREO FILTER

The STEREO FILTER reduces noise on weakstereo stations.

Position 1 reduces noise by approximately 10 dB.Position 2 reduces noise by approximately 20 dB.

SELECTIVITYIn NORMAL, a very low distortion 10-pole filter

is connected to the IF system. Use this position ofthe SELECTIVITY switch for normal reception con-ditions.

In NARROW, a sharp 6-po!e filter is added to thelow-distortion 10-po!e filter. This yields a low-distor-tion, highly selective 16-pole composite filter. Usethis switch position to reduce interference on distantstations.

FM MUTING

Muting suppresses the background noise and hissnormally heard between stations. Turn the control toeither LOCAL or DISTANT position for muting. Weakstations that may not override noise and interferenceare also suppressed by the muting. In the OUT posi-tion, the muting is turned off to allow FM tuning withthe noise and interference present.

VOLUME

The VOLUME control has been precision trackedthroughout the listening range (0 to —65 dB) for ac-curate stereo balance. It adjusts the output level atthe tuner FRONT PANEL CONTROLLED AUDIO OUT-PUT jacks. The FIXED OUTPUT jacks are not af-fected by the volume control.

The POWER ON/OFF switch is part of the VOL-UME control. Turning the VOLUME control fullycounterclockwise turns the AC power to the MR 74OFF.

PANLOC BUTTONSAt the bottom of the front panel corners are the

PANLOC buttons. After a tuner is installed on thePANLOC brackets, pressing the PANLOC buttonswill lock the tuner firmly in position. Depressing thePANLOC buttons will release the tuner. The tunercan then be slid forward to the inspection and adjust-ment position. The PANLOC system gives absoluteease of installation, operation and maintenance.

SECONDARY CONTROLS

On the top of the chassis behind the front panelis the DIAL SCALE INTENSITY switch. Adjust thebrightness of the dial lights by means of this switch.Set the switch to BRIGHT for maximum dial light; setthe switch to DIM for subdued dial light.

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Listening to Your MR 74LISTENING TO AM

Turn the MODE SELECTOR switch to AM and theSELECTIVITY switch to NORMAL. Turn the tuningknob to the desired station. The SIGNAL STRENGTHmeter will indicate the relative strength of the par-ticular AM station being received.

Adjust the VOLUME control to desired listeninglevel. If the FIXED AUDIO OUTPUT jacks are used,the tuner VOLUME control will not affect volume.

LISTENING TO MONOPHONIC FM

Turn the MODE SELECTOR to FM MONO to listento monophonic FM. In this position all programs willbe mono. The stereo light will be seen on stereo sta-tions but the program heard will be mono.

Set the STEREO FILTER switch to OUTSet the MUTING switch to OUTSet the SELECTIVITY switch to NORMALTurn the tuning knob to the desired station. The

Station is properly tuned when the FM TUNINGmeter pointer comes to rest anywhere in the blackarea of the meter scale. While tuning across the dialyou may notice movement of the tuning meter with-out hearing a station. This is cuased by a weak sta-tion that does not over-ride the background noise.With muting in operation signals that are marginalare automatically suppressed. To hear these weakerstations, set the FM MUTING switch to OUT. Usually

the listening quality will be rather poor due to thebackground noise.

If an antenna rotator is used, rotate the directionalantenna for best reception as shown by the SIGNALSTRENGTH and MULTIPATH indicators.

Adjust the VOLUME control to desired listeninglevel. If the FIXED AUDIO OUTPUT jacks are used,the tuner VOLUME control will not affect volume.

LISTENING TO STEREO FM

Turn the MODE SELECTOR to FMSet the STEREO FILTER switch to OUTSet the MUTING switch to OUTSet the SELECTIVITY switch to NORMALTurn the tuning knob to the desired station. The

station is properly tuned when the FM TUNING meterpointer comes to rest anywhere in the black area ofthe meter scale.

When the STEREO indicator is lighted, the stationis broadcasting a 19,000 Hz carrier for stereo and theMR 74 will automatically switch to stereo. If a stationis broadcasting a monophonic FM program, theSTEREO indicator will remain off and the tuner willautomatically switch to mono.

Adjust the VOLUME control to the desired listeninglevel. If the FIXED AUDIO OUTPUT jacks are used,the tuner VOLUME control will not affect volume.

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PERFORMANCE GUARANTEE — Performance limitsare the maximum deviation from perfection permittedfor a Mclntosh instrument. We promise you that theMR 74 you buy must be capable of performance at orexceeding these limits or you get your money back.Mclntosh is the only manufacturer that makes thisguarantee.

FM SECTION

TUNING RANGE: 88 to 108 MHzANTENNA INPUTS: 300 ohms balanced; 75 ohms

unbalancedINTERMEDIATE FREQUENCY (IF): 10.7 MHzSENSITIVITY: 2.5 µV at 100% modulation (± kHz

deviation) for 3% total noise and harmonic dis-tortion

SIGNAL TO NOISE RATIO: 70 dB below 100% mod-ulation

HARMONIC DISTORTION: MONO — 0.3% at 100%modulation + 75 kHz deviation; STEREO — 0.5%at 100% modulation

DRIFT: 25,000 Hz for the first two minutes; there-after 5,000 Hz at 25°C in 24 hours

FREQUENCY RESPONSE: ± 1 dB 20 Hz to 15,000 Hzwith standard de-emphasis (75 µ sec.)

CAPTURE RATIO: 1.5 dB minimum

SELECTIVITY:Switch Setting NORMAL NARROWAdjacent Channel: 6 dB 15 dBAlternate Channel: 68 dB 88 dB

SPURIOUS REJECTION: 90 dBIMAGE REJECTION: 95 dB; 88 to 108 MHz (IHF)MUTING: 50 dB noise reduction in LOCAL positionMUTING THRESHOLD: DISTANT position 3 µ V ; LO-

CAL position 10 µVSCA FILTER: 50 dB down from 67 kHz; 275 dB per

octave slopeSTEREO SEPARATION: 35 dB at 1,000 Hz

STEREO FILTER: 10 dB noise reduction in Position 120 dB noise reduction in Position 2

AM SECTION

SENSITIVITY: 75 µV (external ant.)SIGNAL TO NOISE RATIO: 55 dB at 100% modula-

tion; 45 dB minimumHARMONIC DISTORTION: 1% at 30% modulation

SELECTIVITY, ADJACENT CHANNEL: 35 dB inNORMAL position; 45 dB in NARROW position

IMAGE REJECTION: 65 dB 540 kHz to 1600 kHzFREQUENCY RESPONSE: SELECTIVITY SWITCH IN

NORMAL position 3500 Hz, 6 dB downNARROW position 2100 Hz, 6 dB down

All tuner performance limits were measured with se-lectivity switch set at normal; unless otherwise stated

GENERAL

AUDIO OUTPUT: front panel controlled: 2.5 volts into47,000 ohms. Fixed Output: 2.5 volts into 47,000ohms; 1.0 volt into 600 ohms

POWER REQUIREMENTS: 120 volts, 50/60 Hz, 30watts

SEMICONDUCTOR COMPLEMENT: 5 FETs, 17 tran-sistors, 2 ICs, 30 diodes, 1 indicator tube

MECHANICAL INFORMATION

SIZE: Front panel: 16 inches wide (40.64 cm) by5-7/16 inches high (13.81 cm); Chassis: 15 incheswide (38.1 cm) by 13 inches deep (33.02 cm), in-cluding PANLOC shelf and back panel connectors;Knob clearance: 1½ inches (3.81 cm) in front ofmounting panel

WEIGHT: 25 pounds (11.34 kg) net; 37 pounds (16.78kg) in shipping carton

FINISH: Front Panel: Anodized gold and black withspecial gold/teal panel nomenclature illumination;Chassis: Chrome and black

MOUNTING: Mclntosh developed professional PAN-LOC

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Performance Limits

FM SIGNAL PERFORMANCE

PerformanceCharts

0.1 1.0 10 100 1000 10,000

RF INPUT SIGNAL IN MICROVOLTS ACROSS 300 OHMS

10,000

FM - IF CHARACTERISTIC

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

FM HARMONIC DISTORTION

FREQUENCY IN Hz

10 100 1000 10000 20000

-100 0 100 200 300 400

10

FREQUENCY IN kHz

AM

PLI

TU

DE

RE

SP

ON

SE

IN

dB

PE

R

CE

NT

HA

RM

ON

IC

DIS

TO

RT

ION

3

2

1

0

0

10

20

30

40

50

60

70

RE

LA

TIV

E O

UT

PU

T I

N d

B

AM FREQUENCY RESPONSE

20 100 1000 10000 20000

FREQUENCY IN Hz

AM SELECTIVITY AT 1000 kHz

0

-20

-40

-60

-80-85

-1001030 1020 1010 1000 990 980 970

11

RE

LA

TIV

E

OU

TP

UT

IN d

B

FREQUENCY IN kHz

0

-10

-20

-30

-40RE

LA

TIV

E

OU

TP

UT

IN d

B

TUNING MECHANISM AND DIAL DRIVE

In the MR 74, unique design and careful manufac-ture of the mechanical dial drive assembly givessmooth flywheel tuning.

By controlling the relationship of mass and me-chanical resistance, and by dividing the workloadsin the dial drive system, it becomes nearly impossi-ble to detect any backlash. Yet, the entire dial driveis a model of mechanical stability.

For added ease and increased tuning accuracy, asection of the dial pointer is illuminated.

FM

The Radio Frequency (RF} section houses thecomplete FM-RF front-end and part of the AM-RFcircuit.

A seven-section variable capacitor is the heart ofthe RF section. Four sections of the variable capaci-tor are in the FM front end and the remaining three

are in the AM section. By interleaving the sections(FM-AM-FM-etc.) spurious responses and oscillatorpulling are significantly reduced. The four FM sec-tions of the variable tuning capacitor provide a highdegree of RF selectivity and excellent spurious re-jection. Use of the latest "state of the art" fieldeffect transistors with a well-designed variable tun-ing capacitor provides an excellent RF front end.

All of the RF circuits, including the selectivity cir-cuit and the AM sections of the variable capacitorare encased in a metal module. Each FM-RF sectionis isolated in a separate compartment by metalshielding. Careful design and manufacturing increasethe protection against radiation and interference.The MR 74 exceeds the FCC requirements for sup-pression of local oscillator radiation.

A dual insulated gate metal oxide silicon field ef-fect transistor (MOSFET) is used as first and secondRF amplifier. Each gate of the transistor is internallyprotected by back-to-back diodes against incomingtransients. Use of MOSFET's greatly reduces thecross-modulation products over a wide dynamicrange. A wide dynamic range permits the input cir-cuits to accept extremely strong signals withoutoverload. Since both RF amplifiers have insulatedgate configurations, external neutralization is notrequired. This design results in a very stable RFamplifier circuit.

Low temperature coefficient components for theFM local oscillator prevent frequency drift. The fre-quency stability inherent in the local oscillator makesautomatic frequency control (AFC) unnecessary. Therate of drift of the local oscillator is less than tenparts per million per degree centigrade.

The mixer design uses a junction field effect tran-sistor (JFET) for high sensitivity and freedom fromoverload. The mixer delivers the composite FM sig-nal at the 10.7 MHz intermediate frequency. The pathof the IF signal is controlled by the front panelSELECTIVITY selector switch.

At the NORMAL position, the SELECTIVITY switchdirects the signal through an IF preamplifier stagethat uses a JFET and a double-tuned IF transformer.The signal then goes to the FM-IF and discriminatormodule for further amplification. Setting the SELEC-TIVITY selector switch to NARROW routes the signalthrough two double-tuned transformers, a ceramicfilter network, and a single-gate MOSFET. The sidesof the IF curve are compressed by this circuit nar-rowing the IF bandpass. In this mode of operationweak stations adjacent to strong stations can betuned.

Antenna connections for either 300 ohm twin leadtransmission line or 75 ohm coaxial cable are pro-vided on the back panel. The normal input imped-ance of the RF amplifier is 75 ohms. Impedancematch to 300 ohms is provided by a negligible lossbalun transformer designed by Mclntosh. Connec-

12

tions for both 300 ohm twin lead and 75 ohm co-axial cable are made with push type terminals.

FM-IF AND DISCRIMINATOR

The MR 74 uses linear-phase IF filters. The IF fil-ters have equal time delay in their pass band region.Any error in time delay causes FM distortion. Allother IF filters have delay distortion, some as muchas 100% of the 10.7 MHz transit delay. The MR 74has less than 1.0% delay distortion from antennainput to discriminator output. This makes possiblethe overall low distortion performance limit for theFM tuner and multiplex section.

Amplification of the IF signal is provided by twohigh gain integrated circuits, each containing 16transistors, 3 zener diodes, 5 diodes, and 23 resis-tors, all on a single monolithic silicon chip. The ex-ceptionally high gain of the integrated circuit assures"hard limiting" at very low levels of input signals.

A "phase" or "Foster Seeley" discriminator hasbeen designed to complement the integrated circuitIF section. The detected output signal of the dis-criminator is extremely low in distortion content.

Paralleling the main signal path through the FM-IFand detector module, a secondary amplifying andde-modulating process is used to activate the signalstrength meter and to provide test point TP1 with asignal to be used with the Mclntosh Maximum Per-formance Indicator.

FM STEREO MULTIPLEX

The 19,000 Hz pilot signal, broadcast by an FMstation, is filtered from the composite stereo inputsignal, amplified by a special limiting amplifier,doubled to the 38,000 Hz carrier frequency, and thenamplified again by a limiting amplifier. The compos-ite signal minus the 19,000 Hz pilot is combined withthe 38,000 Hz carrier signal. The new combinationof signals is fed to the special detector circuit men-tioned above. Balanced full wave detectors are usedto cancel the 38,000 Hz components in the output.The SCA (Subsidiary Communication Authorization)signal is removed from the composite output. This isaccomplished by the use of a new "Image Param-eter" band elimination filter that has been computerdesigned. The SCA filter rejects SCA signals with-out impairing stereo performance.

When the 19,000 Hz carrier of a stereo signal isreceived, the automatic FM stereo switching circuitactivates the multiplex decoding circuit. This lightsthe stereo indicator. The circuit switching is all doneelectronically. The automatic stereo switching canbe defeated by setting the mode selector switch toFM MONO. (In this position the stereo indicator willstill light to indicate the presence of a stereo signal.)On monophonic transmissions the stereo switchingis inactive, assuring optimum signal to noise ratio.The stereo switching circuit has been designed sothat noise will not activate it.

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FM muting in the MR 74 operates by detectingultra-sonic noise. When noise is present the mutingcircuit can be activated or defeated by the use ofthe muting switch on the front panel.

The STEREO FILTER is provided to reduce noisewhen listening to weak stereo stations. Careful de-sign permits an ideal compromise between channelseparation and noise rejection.

AM

The AM-RF amplifier circuit includes a three sec-tion variable tuning capacitor in the metal enclosedshielded RF module which also houses the FM-RFfront end. A three section variable capacitor is usedfor greatest spurious rejection. The RF amplifier isunique. The circuit has constant sensitivity, constantselectivity, and high image rejection across the com-plete AM band. Ordinary AM-RF circuits cannot doall of these simultaneously. The circuit designachieves high sensitivity even at the low end of theband. Spurious, image, and intermediate frequencyrejection are all superior. The same circuit deliversequal selectivity across the entire band.

In addition, there is no loss of audio frequencyresponse at the low end of the band, common in AMreceivers. Another advantage of the Mclntosh circuitis freedom from cross modulation and overloadingby strong local stations.

A high-quality loopstick antenna is provided. It canbe rotated for maximum performance, optimum sig-nal reception or minimum interference. In eachMR 74, the loopsticks are individually tuned for opti-mum performance. After tuning, the loopstick issealed. This custom matching of the loopstick to theAM-RF front end maximizes the performance of theloopstick antenna. The antenna is rotatable throughnearly 180 degrees. With this mobility you will notsuffer loss of sensitivity regardless of the angle atwhich the instrument is mounted. A back panel con-nector is provided for an external antenna. The an-tenna is coupled to the tuner through a carefullydesigned matching transformer which provides opti-mum performance regardless of antenna length.

To maintain the excellent image rejection and lackof spurious cross modulation of the AM-RF amplifieran autodyne converter circuit was used in producingthe AM-IF signal.

AM-IF uses two double tuned IF transformers de-signed to obtain a high degree of selectivity yet al-lowing good audio fidelity. With the SELECTIVITYswitch in NARROW, a narrow band ceramic filter isadded to the AM-IF amplifier.

A 10,000 Hz active filter eliminates the 10,000 Hzwhistle and irritating noise caused by an adjacentstation. Because an active filter is used, the outputlevel at 10,000 Hz, or the whistle frequency, is downover 20 dB or one hundredth of what it would be

without filtering. With the SELECTIVITY switch inNARROW position, the active filter cutoff frequencyis lowered. The filter then effectively suppresses the5,000 Hz whistle from nearby television receivers.

The AVC (automatic volume control) system wasdesigned to prevent blasting when the AM is tunedthrough a strong signal. Distortion at low audio fre-quencies is minimized by using two AVC filter sec-tions instead of the conventional one.

AUDIO AMPLIFIER

The audio amplifier increases the level of the pro-gram adequate to drive a preamplifier or other ac-cessory equipment. It consists of two, two-transistoramplifiers, one for each channel. The design usesconsiderable negative feedback to help achieve lowdistortion, wide frequency response, and excellentstability. Each audio amplifier delivers 2.5 volts tothe FIXED OUTPUT jacks. A second pair of outputsare available where level can be varied by the VOL-UME control.

POWER SUPPLY

Special design attention has been given to thepower supply section. Three separate power supplycircuits are used. The first is a 24 volt regulated sup-ply. This 24 volt regulator is very elaborate in design,using a specially selected transistor and associatedcircuit. The regulator uses electronic filtering to in-sure the lowest possible background hum level,maximum stability and extremely good regulation. Allsignal stages are powered from this regulator.

The second circuit is a half wave rectifier andfilter for the DC high voltage needed for the anodeof the multipath indicator. The third power circuit isa half wave rectifier which supplies DC to the AMsection.

14

BlockDiagram

FM STATION LOG

CALLLETTERS FREQUENCY LOCATION

ANTENNADIRECTION

CALLLETTERS FREQUENCY LOCATION

ANTENNADIRECTION

16

McINTOSH LABORATORY INC.

2 CHAMBERS ST., BINGHAMTON, N. Y. 13903607-723-3512

Design subject to change without notice.

Printed in U.S.A.

038-839

BE112003