McIntosh MR 74 User manual

THEMcINTOSHMR74SOLIDSTATEAMFM/FMSTEREOTUNER
READINGTIME:28MinutesPrice$1.25

YourMR74AMFM/FMSTEREO
TUNERwillgiveyoumanyyears
ofpleasantandsatisfactory
performance.Ifyouhaveany
questionspleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
WARNING:TOPREVENTFIREOR
SHOCKHAZARD,DONOTEXPOSE
THISUNITTORAINORMOISTURE.
TakeAdvantageof3 years
ofFREEFactoryService...
FillintheApplicationNOW.
CONTENTS
SERVICECONTRACT1
INSTALLATION2,3
HOWTOCONNECT4,5
BACKPANELINFORMATION6
FRONTPANELINFORMATION6,7
LISTENINGTOTHEMR748
PERFORMANCELIMITSANDRATINGS9
PERFORMANCECHARTS10,11
TECHNICALDESCRIPTION12,13,14
BLOCKDIAGRAM15
FMSTATIONLOG16
Anapplicationfora FREETHREEYEARFACTORYSERVICECONTRACTisincludedwiththismanual
Thetermsofthecontractare:
1.Mclntoshwillprovideallparts,materialsand
laborneededtoreturnthemeasuredperform-
anceoftheinstrumenttotheoriginalper-
formancelimitsfreeofanycharge.The
SERVICECONTRACTdoesnotcoveranyship-
pingcoststoandfromtheauthorizedservice
agencyorthefactory.
2.AnyMclntoshauthorizedserviceagencywill
repairallMclntoshinstrumentsatnormal
servicerates.Toreceive thefreeserviceunder
thetermsoftheSERVICECONTRACT,the
SERVICECONTRACTCERTIFICATEmustac-
companytheinstrumentwhentakentothe
serviceagency.
3.Alwayshaveservicedonebya Mclntosh
authorizedserviceagency.Iftheinstrument
ismodifiedordamaged,asa resultofun-
authorizedrepairtheSERVICECONTRACT
willbecancelled.Damagebyimproperuse
ormishandlingisnotcoveredbytheSERV-
ICECONTRACT.
4.TheSERVICECONTRACTisissuedtoyouas
theoriginalpurchaser.To protectyoufrom
misrepresentationthiscontractcannotbe
transferredtoa secondowner.
5.ForyourprotectionMclntoshselectsonly
dealerswhohavetechnicalcompetenceto
guidepurchasorsfairly,andprovideservice
whennecessary.ToreceivetheSERVICE
CONTRACTyourpurchasemustbemade
froma Mclntoshfranchiseddealer.
6.Yourcompletelyfilledinapplicationfora
SERVICECONTRACTmustbepostmarked
within30daysofthedateofpurchaseof
theinstrument.
7.ToreceivetheSERVICECONTRACTallin-
formationontheapplicationmustbefilled
in.TheSERVICECONTRACTwillbeissued
whenthecompletelyfilledinapplication
isreceivedatMclntoshLaboratoryIncor-
poratedinBinghamton,NewYork.
Copyright© 1972ByMcIntoshLaboratoryInc.
THREEYEARFACTORYSERVICECONTRACT
1

ItisrecommendedthattheMR74tunerbemount-
edina normalorhorizontalposition.However,with
adequateventilationthetunercanbemountedin
anyposition.
Adequateventilationextendsthetrouble-freelife
ofelectronicinstruments.Itisgenerallyfoundthat
each18°F riseinoperatingtemperaturereducesthe
lifeofelectricalinsulationbyonehalf.Adequate
ventilationisaninexpensiveandeffectivemeansof
preventinginsulationbreakdownthatresultsfrom
unnecessarilyhighoperatingtemperatures.The
directbenefitsofadequateventilationislonger,
trouble-freelife.
TheMR74tunerrequiresa mountingspacethatis
atleast15inchesdeep,17½incheswideand6
incheshigh.Provideadditionalspacefortheairflow
acrossthebottomofthetuneranda meansforwarm
airtoescapeatthetop.
Removethetuner,shelfbrackets,partsbagand
mountingtemplatefromthecarton.Removetuner
fromtheplasticbagandplacethetunerupside
downontheshippingpallet,thenremovethefour
plasticfeetfastenedtothebottomofthechassis.
Theinstallationmaybeaccomplishedinsixsteps:
1.POSITIONTEMPLATEANDMARK
Positiontheplasticmountingtemplateoverthe
areaofthecabinetpanelwheretheMR74isto
installed.Besurethattheedgesofthetemplate
clearanyshelves,partitionsorexistingequip-
mentlocatedbehindthepanel.Withthetem-
plateinplacemarkthesix"A"and"B"holes
andfoursmallholeslocatingthecornersofthe
cutout.Next,jointhefourcornermarkswith
pencillines;theedgeofthetemplatemaybe
usedasa straightedge.
2.DRILLHOLES
Holdinga drillperpendicularlytothepanel,drill
thesix"A"and"B"holesusinga 3/16inchdrill.
THESIXHOLESMUSTBEDRILLEDBEFORE
MAKINGTHECUTOUT.
3.SAWCUTOUT
Usinga sawcarefullycuttherectangularopen-
ingontheinsideofthepencillines.
4.SECUREMOUNTINGSTRIPS
Securemountingstrips(suppliedinthehard-
warepackage)ontheinsideofthecabinet
panel.
Inserttwoscrews(suppliedinthehardware
package)intothecenterholes("B"holesonthe
template).Usethe¾-inchlongscrewsfor
panelsunder½-inchthickor1¼-inchscrews
forpanels½-inchthickorover.Placea mount-
ingstriponthebackofthecabinetpane!as
shown.Alignitwiththethreeholesinthepanel
andtightenthescrew.Thescrewheadshould
pullslightlyintothewoodpanel.Attachtheother
mountingstripbyrepeatingtheprocedure.
5.MOUNTPANLOCSHELFBRACKETS
AttachthePanlocshelfbracketstothecabinet
panelusingfourscrewsoftheproperlength.
Placethetemplateoverthemountingscrews.
Thescrewsshouldbecenteredonthe"A"and
"B"holesinthetemplate.Ifnecessary,loosen
thescrewsandpushthemountingbracketsinto
alignmentthenretighten.
2

6.INSTALLTHEUNIT
Threadthepowercordthroughtheopeningon
thecabinetpanel.Carefullyslidetheinstrument
intotheopeningsotherailsonthebottomen-
gagethetrackonthemountingbrackets.Slide
theinstrumentinuntilitstopsattheadjustpo-
sitionlatches.Pressthelatchesinandcontinue
toslidetheinstrumentuntilitsfrontpanelis
flushwiththecabinetpanel.Atthebottomfront
cornersoftheinstrumentarethePANLOCbut-
tons.DepressingthePANLOCbuttonswilllock
theinstrumentfirmlyinthecabinet.Depres-
singthePANLOCbuttonsa secondtimewill
releasetheinstrument.Youcanthenslidethe
instrumentforwardtotheadjustposition.De-
pressingtheadjustpositionlatcheswillallowthe
instrumenttoberemovedfromthecabinet.
3

HowtoConnect
AUDIOOUTPUTS
UsetheFIXEDOUTPUTjackstoconnecttoastereo
controlpreamplifierorotherequipmentwhichhas
itsownvolumecontrol.TheVOLUMECONTROL
doesnotaffecttheoutputofthetunerattheFIXED
OUTPUTjacks.
TheoutputimpedanceattheFIXEDOUTPUTSis
600ohms.Longercablesthanarenormallysupplied
canbeusedtointerconnecttheMR74withother
equipment.Thelengthofthecableislimitedbythe
capacityofthecable.Thetotalcapacitymustnot
exceed1600pF.Forinstance:cableswitha capacity
of32pFperfootmaybe50feetlong;16pFperfoot
cablemaybe100feetlong.
UsetheFRONTPANELCONTROLLEDjacksto
connecttoequipmentsuchaspoweramplifiersor
taperecorderswherecontrolofvolumeatthetuner
isdesired.TheloadimpedanceconnectedtoFRONT
PANELCONTROLLEDjacksshouldnotbelessthan
10,000ohms.Thereisnodifferenceinthesignal
qualityormaximumoutputlevelsavailableateither
pairofoutputjacks.
CONNECTINGANFMANTENNA
Oneofthreeantennasystemscanbeused:(1)an
outdoorFMantenna,(2)anallchannel(UHF-VHF-
TV)antenna,or(3)theindoordipolesuppliedwith
theMR74.
Anoutdoorantennaisrecommendedforoptimum
performanceinallareas.Infringeareas,bestre-
sultswillbeobtainedwitha highlydirectionalFM
antennausedinconjunctionwitha rotator.Rotate
theantennauntilthebestreceptionisobtained.Con-
nectthe300ohmantennatothe300ANT(red)
terminals.
AnUHF-VHF-TVantennaisofteneffectivebutthe
antennamustbedesignedforbothFMandTVre-
ception.ConnecttheleadsfromtheUHF-VHF-TV
antennatothe300ANT(red)terminals.
CONNECTINGANINDOORDIPOLEANTENNA
Theflexiblefoldeddipoleantenna(300ohm)isfor
useinurbanorhighstrengthsignalareas.
Connectthetwoleadsfromthedipoletothe300
ANT(red)terminals.Theflexibilityofthethinflat
wireassemblypermitsittobeplacedundera rug,
tackedbehindthestereo...or,placedinanyother
convenientlocation.Insomecases,itmaybenec-
essaryto"position"theantennaforbestsignalre-
ception.Thisshouldbedonebeforeitispermanently
located.Avoidlocatingthisantennanexttoother
wiresormetalobjects.Thisantennamaynotprove
effectiveinhouseshavingmetalsidingormetal-clad
insulation.
AMANTENNA
Ahigh-qualityloop-stickantennaisprovided.It
canberotatedthroughnearly180°inalldirections
formaximumperformance,optimumsignalreception
orminimuminterference.Withthismobilityyouwill
notsufferlossofsensitivityregardlessoftheangle
atwhichtheinstrumentismounted.A backpanel
antennajackisprovidedforconnectinganexternal
antennaifdesired.
4
ForbestlongdistanceAMreception,usea copper
antennawire50to150feetinlength.Suspendthe
wireina straightlineashighaspossible.Attachthe
wireateachendwithsuitableglassorceramicin-
sulators.Connecta lead-inwireatanyconvenient
pointontheantenna.Itisrecommendedthata light-
ningarresterbeusedwithanoutdoorAMantenna.
Thearrestershouldbewellgroundedtoa suitable
waterpipeora groundrodsunkintotheground.
Connectthelead-inwiretotheAMANTpushcon-
nectorontheantennaterminalstripontheback
panel.SettheAMANTslideswitchtoEXTposition.
CONNECTINGANAMOUTDOORANTENNA

FMANTENNA
ALTERNATE
AMANTENNA
Ahigh-qualityloopstickantenna
isprovided.Itcanberotatedfor
maximumperformance,optimum
signalreceptionorminimum
interference.
Theantennaisrotatablethrough
nearly180degrees.
LEFTSPEAKERRIGHTSPEAKER
5
PREAMPLIFIER
POWERAMPLIFIER

BackPanelinformationFrontPanelinformation
TP1andTP2
TestpointsTP1andTP2areprovidedfortuner
circuitalignment.Theyarealsousedinconjunction
withtheMclntoshMaximumPerformanceIndicator.
120ACPOWEROUTLET
Provides120voltACpowerupto400wattsfor
additionalequipmentsuchasamplifiers,orother
equipment.Thisoutletisnotfused.Itturnsonand
offwiththefrontpanelACpowerswitchonthe
VOLUMEcontrol.
ACPOWERCORD
ConnecttheACpowercordtoa 120volt,50to60
Hzpowerlinereceptable.Thepowerusedbythe
MR74is30watts.
FUSE
A0.5AMPSLO-BLOfuseprotectsthetunercir-
cuits.Thisfusedoesnotprotectadditionalequip-
mentconnectedtothebackpanelACpoweroutlet.
AMANTSWITCH
AdjusttheMR74AMANTswitchtomatchthepar-
ticulartypeofAMantennaused.EXTpositionis
usedforanoutdoorantenna.LOOPpositionisfor
thebuilt-inferriteloopstick.
ANTENNACONNECTIONSTRIP
ProvideseasypushtypeconnectorsforanAM
antenna,a groundconnection,anda 300ohmFM
antenna.
FMANTENNA— 75OHM
Providesa typeF maleconnectorfora 75ohm
unbalancedFMantenna.
AUDIOOUTPUTS
TheFIXEDOUTPUTpairofAUDIOOUTPUTjacks
providesaudiosignalsunaffectedbytheMR74
frontpanelVOLUMEcontrol.Usetheseoutputjacks
toconnectthetunertoa stereopreamplifierorother
equipmentwhichhasitsownmastervolumecontrol.
TheFRONTPANELCONTROLLEDpairofAUDIO
OUTPUTjacksprovidesaudiosignalcontrolledby
theMR74frontpanelVOLUMEcontrol.Usethese
outputjackstoconnecttoexternalpoweramplifier,
taperecorders,oranyequipmentwhichrequires
frontpanelcontroloftuneroutputvolume.
TUNINGDIAL
TheMR74hasthreedialscales:
1.AM— Marked55to160
2.FM— Marked88to108
3.Loggingscale— Marked0 to100
Theloggingscalecanbeusedtoaccuratelyre-
tuneanystation.Youmayfinditeasiertokeepa
recordofyourfavoritestationsbyuseofthelogging
scale.
Asmallportionofdialpointerhasbeenilluminated
toincreasetheeaseoftuning.
INDICATORS
TheMR74hasfourindicatorsonthedialpanel:
STEREOindicator,MULTIPATHindicator,SIGNAL
STRENGTHmeter,andtheFMTUNINGmeter.
STEREOINDICATOR
TheSTEREOindicatorlightsredwhenthedial
pointeristunedtoorcorssesa stationbroadcasting
the19,000Hzcarrierforstereo.Thespecialcircuit
usedwilllightonlywhenthe19,000Hzmultiplex
carrierispresentinthesignal.Theindicatorwillnot
lightonnoisepulsesorinterference.
MULTIPATHINDICATOR
TheMULTIPATHindicatorisanexclusiveMc-
lntoshdevelopment.
TheproperuseoftheMULTIPATHindicator
makesitpossibletoimproveFMreceptionwith
FMantennapositioning.
Anelectronrayindicatorisusedtoshowmulti-
pathreception.Whenrotatingtheantenna,observa-
tionofthemultipathindicatorwillshowbestorien-
tationfortheFMstationbeingreceived.Multipath
distortioncausesthetwobeamsontheindicatorto
fluctuaterapidly.Whentheantennaisrotatedtothe
bestposition,theindicatorbeamstendtoremain
steady.Thedirectionalantennathenpicksupthe
desiredsignalandrejectsthereflectedmultipath
signals.Insomelocationsitispossibleforthebest
receptiontooccurwhentheantennapicksupa
strongreflectedsignalratherthanthedirectsignal.
Multipathdistortionispracticallyindependentofsig-
nalstrength.
SIGNALSTRENGTHMETER
TheSIGNALSTRENGTHmeterindicatesthe
strengthofthesignalasreceivedfromtheantenna.
Thehighertheindication,thestrongeristhesignal,
FMTUNINGMETER
AnFMstationiscorrectlytunedwhenthemeter
needleisintheblackareaoftheFMTUNINGmeter.
6

TheactionoftheTUNINGmeterisindependentof
stationsignalstrength.
MODESELECTOR
Selectsanyoneofthreesources:
AM—ConnectstheAMtunersectionoftheMR
74totheoutputjacks.
FM—TheFMpositionprovidesmonophonicFM
orFMstereoattheleftandrightchannel
audiooutputjacksautomatically.Withthe
MODESELECTORintheFMpositiona
stationbroadcastingmonophonicprogram
willbeheardinmono.Whenthestation
switchestostereobroadcast,thestereoin-
dicatorwilllightandtheMR74willauto-
maticallyswitchtostereooperation.Ifmono
broadcastingisresumedtheMR74willau-
tomaticallyswitchtomono.
FM-MONO— ConnectstheFMtunersectionsof
theMR74totheoutputjacks.This
positionbypassestheautomatic
stereoswitching.Thestereoindica-
torwilllightwhena stationistrans-
mittingstereo,butallprogramsare
heardmonophonically.
STEREOFILTER
TheSTEREOFILTERreducesnoiseonweak
stereostations.
Position1 reducesnoisebyapproximately10dB.
Position2 reducesnoisebyapproximately20dB.
SELECTIVITY
InNORMAL,a verylowdistortion10-polefilter
isconnectedtotheIFsystem.Usethispositionof
theSELECTIVITYswitchfornormalreceptioncon-
ditions.
InNARROW,a sharp6-po!efilterisaddedtothe
low-distortion10-po!efilter.Thisyieldsa low-distor-
tion,highlyselective16-polecompositefilter.Use
thisswitchpositiontoreduceinterferenceondistant
stations.
FMMUTING
Mutingsuppressesthebackgroundnoiseandhiss
normallyheardbetweenstations.Turnthecontrolto
eitherLOCALorDISTANTpositionformuting.Weak
stationsthatmaynotoverridenoiseandinterference
arealsosuppressedbythemuting.IntheOUTposi-
tion,themutingisturnedofftoallowFMtuningwith
thenoiseandinterferencepresent.
VOLUME
TheVOLUMEcontrolhasbeenprecisiontracked
throughoutthelisteningrange(0to—65dB)forac-
curatestereobalance.Itadjuststheoutputlevelat
thetunerFRONTPANELCONTROLLEDAUDIOOUT-
PUTjacks.TheFIXEDOUTPUTjacksarenotaf-
fectedbythevolumecontrol.
ThePOWERON/OFFswitchispartoftheVOL-
UMEcontrol.TurningtheVOLUMEcontrolfully
counterclockwiseturnstheACpowertotheMR74
OFF.
PANLOCBUTTONS
Atthebottomofthefrontpanelcornersarethe
PANLOCbuttons.Aftera tunerisinstalledonthe
PANLOCbrackets,pressingthePANLOCbuttons
willlockthetunerfirmlyinposition.Depressingthe
PANLOCbuttonswillreleasethetuner.Thetuner
canthenbeslidforwardtotheinspectionandadjust-
mentposition.ThePANLOCsystemgivesabsolute
easeofinstallation,operationandmaintenance.
SECONDARYCONTROLS
Onthetopofthechassisbehindthefrontpanel
istheDIALSCALEINTENSITYswitch.Adjustthe
brightnessofthediallightsbymeansofthisswitch.
SettheswitchtoBRIGHTformaximumdiallight;set
theswitchtoDIMforsubdueddiallight.
7

ListeningtoYourMR74
LISTENINGTOAM
TurntheMODESELECTORswitchtoAMandthe
SELECTIVITYswitchtoNORMAL.Turnthetuning
knobtothedesiredstation.TheSIGNALSTRENGTH
meterwillindicatetherelativestrengthofthepar-
ticularAMstationbeingreceived.
AdjusttheVOLUMEcontroltodesiredlistening
level.IftheFIXEDAUDIOOUTPUTjacksareused,
thetunerVOLUMEcontrolwillnotaffectvolume.
LISTENINGTOMONOPHONICFM
TurntheMODESELECTORtoFMMONOtolisten
tomonophonicFM.Inthispositionallprogramswill
bemono.Thestereolightwillbeseenonstereosta-
tionsbuttheprogramheardwillbemono.
SettheSTEREOFILTERswitchtoOUT
SettheMUTINGswitchtoOUT
SettheSELECTIVITYswitchtoNORMAL
Turnthetuningknobtothedesiredstation.The
StationisproperlytunedwhentheFMTUNING
meterpointercomestorestanywhereintheblack
areaofthemeterscale.Whiletuningacrossthedial
youmaynoticemovementofthetuningmeterwith-
outhearinga station.Thisiscuasedbya weaksta-
tionthatdoesnotover-ridethebackgroundnoise.
Withmutinginoperationsignalsthataremarginal
areautomaticallysuppressed.Toheartheseweaker
stations,settheFMMUTINGswitchtoOUT.Usually
thelisteningqualitywillberatherpoorduetothe
backgroundnoise.
Ifanantennarotatorisused,rotatethedirectional
antennaforbestreceptionasshownbytheSIGNAL
STRENGTHandMULTIPATHindicators.
AdjusttheVOLUMEcontroltodesiredlistening
level.IftheFIXEDAUDIOOUTPUTjacksareused,
thetunerVOLUMEcontrolwillnotaffectvolume.
LISTENINGTOSTEREOFM
TurntheMODESELECTORtoFM
SettheSTEREOFILTERswitchtoOUT
SettheMUTINGswitchtoOUT
SettheSELECTIVITYswitchtoNORMAL
Turnthetuningknobtothedesiredstation.The
stationisproperlytunedwhentheFMTUNINGmeter
pointercomestorestanywhereintheblackareaof
themeterscale.
WhentheSTEREOindicatorislighted,thestation
isbroadcastinga 19,000Hzcarrierforstereoandthe
MR74willautomaticallyswitchtostereo.Ifa station
isbroadcastinga monophonicFMprogram,the
STEREOindicatorwillremainoffandthetunerwill
automaticallyswitchtomono.
AdjusttheVOLUMEcontroltothedesiredlistening
level.IftheFIXEDAUDIOOUTPUTjacksareused,
thetunerVOLUMEcontrolwillnotaffectvolume.
8

PERFORMANCEGUARANTEE— Performancelimits
arethemaximumdeviationfromperfectionpermitted
fora Mclntoshinstrument.Wepromiseyouthatthe
MR74youbuymustbecapableofperformanceator
exceedingtheselimitsoryougetyourmoneyback.
Mclntoshistheonlymanufacturerthatmakesthis
guarantee.
FMSECTION
TUNINGRANGE:88to108MHz
ANTENNAINPUTS:300ohmsbalanced;75ohms
unbalanced
INTERMEDIATEFREQUENCY(IF):10.7MHz
SENSITIVITY:2.5µVat100%modulation(±kHz
deviation)for3%totalnoiseandharmonicdis-
tortion
SIGNALTONOISERATIO:70dBbelow100%mod-
ulation
HARMONICDISTORTION:MONO— 0.3%at100%
modulation+ 75kHzdeviation;STEREO— 0.5%
at100%modulation
DRIFT:25,000Hzforthefirsttwominutes;there-
after5,000Hzat25°Cin24hours
FREQUENCYRESPONSE:± 1 dB20Hzto15,000Hz
withstandardde-emphasis(75µsec.)
CAPTURERATIO:1.5dBminimum
SELECTIVITY:
SwitchSettingNORMALNARROW
AdjacentChannel:6 dB15dB
AlternateChannel:68dB88dB
SPURIOUSREJECTION:90dB
IMAGEREJECTION:95dB;88to108MHz(IHF)
MUTING:50dBnoisereductioninLOCALposition
MUTINGTHRESHOLD:DISTANTposition3µV;LO-
CALposition10µV
SCAFILTER:50dBdownfrom67kHz;275dBper
octaveslope
STEREOSEPARATION:35dBat1,000Hz
STEREOFILTER:10dBnoisereductioninPosition1
20dBnoisereductioninPosition2
AMSECTION
SENSITIVITY:75µV(externalant.)
SIGNALTONOISERATIO:55dBat100%modula-
tion;45dBminimum
HARMONICDISTORTION:1%at30%modulation
SELECTIVITY,ADJACENTCHANNEL:35dBin
NORMALposition;45dBinNARROWposition
IMAGEREJECTION:65dB540kHzto1600kHz
FREQUENCYRESPONSE:SELECTIVITYSWITCHIN
NORMALposition3500Hz,6 dBdown
NARROWposition2100Hz,6 dBdown
Alltunerperformancelimitsweremeasuredwithse-
lectivityswitchsetatnormal;unlessotherwisestated
GENERAL
AUDIOOUTPUT:frontpanelcontrolled:2.5voltsinto
47,000ohms.FixedOutput:2.5voltsinto47,000
ohms;1.0voltinto600ohms
POWERREQUIREMENTS:120volts,50/60Hz,30
watts
SEMICONDUCTORCOMPLEMENT:5 FETs,17tran-
sistors,2 ICs,30diodes,1 indicatortube
MECHANICALINFORMATION
SIZE:Frontpanel:16incheswide(40.64cm)by
5-7/16incheshigh(13.81cm);Chassis:15inches
wide(38.1cm)by13inchesdeep(33.02cm),in-
cludingPANLOCshelfandbackpanelconnectors;
Knobclearance:1½inches(3.81cm)infrontof
mountingpanel
WEIGHT:25pounds(11.34kg)net;37pounds(16.78
kg)inshippingcarton
FINISH:FrontPanel:Anodizedgoldandblackwith
specialgold/tealpanelnomenclatureillumination;
Chassis:Chromeandblack
MOUNTING:MclntoshdevelopedprofessionalPAN-
LOC
9
PerformanceLimits

FMSIGNALPERFORMANCE
Performance
Charts
0.11.0101001000
RFINPUTSIGNALINMICROVOLTSACROSS300 OHMS
10,000
FM- IFCHARACTERISTIC
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
FMHARMONICDISTORTION
FREQUENCYINHz
1010010001000020000
-1000 100200 300 400
10
FREQUENCYINkHz
AMPLITUDERESPONSEINdB
PERCENTHARMONICDISTORTION
3
2
1
0
0
10
20
30
40
50
60
70
RELATIVEOUTPUTINdB

AMFREQUENCYRESPONSE
2010010001000020000
FREQUENCYINHz
AMSELECTIVITYAT1000kHz
0
-20
-40
-60
-80
-85
-1001030102010101000990980970
11
RELATIVEOUTPUTINdB
FREQUENCYINkHz
0
-10
-20
-30
-40
RELATIVEOUTPUTINdB

TUNINGMECHANISMANDDIALDRIVE
IntheMR74,uniquedesignandcarefulmanufac-
tureofthemechanicaldialdriveassemblygives
smoothflywheeltuning.
Bycontrollingtherelationshipofmassandme-
chanicalresistance,andbydividingtheworkloads
inthedialdrivesystem,itbecomesnearlyimpossi-
bletodetectanybacklash.Yet,theentiredialdrive
isa modelofmechanicalstability.
Foraddedeaseandincreasedtuningaccuracy,a
sectionofthedialpointerisilluminated.
FM
TheRadioFrequency(RF}sectionhousesthe
completeFM-RFfront-endandpartoftheAM-RF
circuit.
Aseven-sectionvariablecapacitoristheheartof
theRFsection.Foursectionsofthevariablecapaci-
torareintheFMfrontendandtheremainingthree
areintheAMsection.Byinterleavingthesections
(FM-AM-FM-etc.)spuriousresponsesandoscillator
pullingaresignificantlyreduced.ThefourFMsec-
tionsofthevariabletuningcapacitorprovidea high
degreeofRFselectivityandexcellentspuriousre-
jection.Useofthelatest"stateoftheart"field
effecttransistorswitha well-designedvariabletun-
ingcapacitorprovidesanexcellentRFfrontend.
AlloftheRFcircuits,includingtheselectivitycir-
cuitandtheAMsectionsofthevariablecapacitor
areencasedina metalmodule.EachFM-RFsection
isisolatedina separatecompartmentbymetal
shielding.Carefuldesignandmanufacturingincrease
theprotectionagainstradiationandinterference.
TheMR74exceedstheFCCrequirementsforsup-
pressionoflocaloscillatorradiation.
Adualinsulatedgatemetaloxidesiliconfieldef-
fecttransistor(MOSFET)isusedasfirstandsecond
RF amplifier.Eachgateof thetransistoris internally
protectedbyback-to-backdiodesagainstincoming
transients.UseofMOSFET'sgreatlyreducesthe
cross-modulationproductsovera widedynamic
range.A widedynamicrangepermitstheinputcir-
cuitstoacceptextremelystrongsignalswithout
overload.SincebothRFamplifiershaveinsulated
gateconfigurations,externalneutralizationisnot
required.Thisdesignresultsina verystableRF
amplifiercircuit.
Lowtemperaturecoefficientcomponentsforthe
FMlocaloscillatorpreventfrequencydrift.Thefre-
quencystabilityinherentinthelocaloscillatormakes
automaticfrequencycontrol(AFC)unnecessary.The
rateofdriftofthelocaloscillatorislessthanten
partspermillionperdegreecentigrade.
Themixerdesignusesa junctionfieldeffecttran-
sistor(JFET)forhighsensitivityandfreedomfrom
overload.ThemixerdeliversthecompositeFMsig-
nalatthe10.7MHzintermediatefrequency.Thepath
oftheIFsignaliscontrolledbythefrontpanel
SELECTIVITYselectorswitch.
AttheNORMALposition,theSELECTIVITYswitch
directsthesignalthroughanIFpreamplifierstage
thatusesa JFETanda double-tunedIFtransformer.
ThesignalthengoestotheFM-IFanddiscriminator
moduleforfurtheramplification.SettingtheSELEC-
TIVITYselectorswitchtoNARROWroutesthesignal
throughtwodouble-tunedtransformers,a ceramic
filternetwork,anda single-gateMOSFET.Thesides
oftheIFcurvearecompressedbythiscircuitnar-
rowingtheIFbandpass.Inthismodeofoperation
weakstationsadjacenttostrongstationscanbe
tuned.
Antennaconnectionsforeither300ohmtwinlead
transmissionlineor75ohmcoaxialcablearepro-
videdonthebackpanel.Thenormalinputimped-
anceoftheRFamplifieris75ohms.Impedance
matchto300ohmsisprovidedbya negligibleloss
baluntransformerdesignedbyMclntosh.Connec-
12

tionsforboth300ohmtwinleadand75ohmco-
axialcablearemadewithpushtypeterminals.
FM-IFANDDISCRIMINATOR
TheMR74useslinear-phaseIFfilters.TheIFfil-
tershaveequaltimedelayintheirpassbandregion.
AnyerrorintimedelaycausesFMdistortion.All
otherIFfiltershavedelaydistortion,someasmuch
as100%ofthe10.7MHztransitdelay.TheMR74
haslessthan1.0%delaydistortionfromantenna
inputtodiscriminatoroutput.Thismakespossible
theoveralllowdistortionperformancelimitforthe
FMtunerandmultiplexsection.
AmplificationoftheIFsignalisprovidedbytwo
highgainintegratedcircuits,eachcontaining16
transistors,3 zenerdiodes,5 diodes,and23resis-
tors,allona singlemonolithicsiliconchip.Theex-
ceptionallyhighgainoftheintegratedcircuitassures
"hardlimiting"atverylowlevelsofinputsignals.
A"phase"or"FosterSeeley"discriminatorhas
beendesignedtocomplementtheintegratedcircuit
IFsection.Thedetectedoutputsignalofthedis-
criminatorisextremelylowindistortioncontent.
ParallelingthemainsignalpaththroughtheFM-IF
anddetectormodule,a secondaryamplifyingand
de-modulatingprocessisusedtoactivatethesignal
strengthmeterandtoprovidetestpointTP1witha
signaltobeusedwiththeMclntoshMaximumPer-
formanceIndicator.
FMSTEREOMULTIPLEX
The19,000Hzpilotsignal,broadcastbyanFM
station,isfilteredfromthecompositestereoinput
signal,amplifiedbya speciallimitingamplifier,
doubledtothe38,000Hzcarrierfrequency,andthen
amplifiedagainbya limitingamplifier.Thecompos-
itesignalminusthe19,000Hzpilotiscombinedwith
the38,000Hzcarriersignal.Thenewcombination
ofsignalsisfedtothespecialdetectorcircuitmen-
tionedabove.Balancedfullwavedetectorsareused
tocancelthe38,000Hzcomponentsintheoutput.
TheSCA(SubsidiaryCommunicationAuthorization)
signalisremovedfromthecompositeoutput.Thisis
accomplishedbytheuseofa new"ImageParam-
eter"bandeliminationfilterthathasbeencomputer
designed.TheSCAfilterrejectsSCAsignalswith-
outimpairingstereoperformance.
Whenthe19,000Hzcarrierofa stereosignalis
received,theautomaticFMstereoswitchingcircuit
activatesthemultiplexdecodingcircuit.Thislights
thestereoindicator.Thecircuitswitchingisalldone
electronically.Theautomaticstereoswitchingcan
bedefeatedbysettingthemodeselectorswitchto
FMMONO.(Inthispositionthestereoindicatorwill
stilllighttoindicatethepresenceofa stereosignal.)
Onmonophonictransmissionsthestereoswitching
isinactive,assuringoptimumsignaltonoiseratio.
Thestereoswitchingcircuithasbeendesignedso
thatnoisewillnotactivateit.13
FMmutingintheMR74operatesbydetecting
ultra-sonicnoise.Whennoiseispresentthemuting
circuitcanbeactivatedordefeatedbytheuseof
themutingswitchonthefrontpanel.
TheSTEREOFILTERisprovidedtoreducenoise
whenlisteningtoweakstereostations.Carefulde-
signpermitsanidealcompromisebetweenchannel
separationandnoiserejection.
AM
TheAM-RFamplifiercircuitincludesa threesec-
tionvariabletuningcapacitorinthemetalenclosed
shieldedRFmodulewhichalsohousestheFM-RF
frontend.A threesectionvariablecapacitorisused
forgreatestspuriousrejection.TheRFamplifieris
unique.Thecircuithasconstantsensitivity,constant
selectivity,andhighimagerejectionacrossthecom-
pleteAMband.OrdinaryAM-RFcircuitscannotdo
allofthesesimultaneously.Thecircuitdesign
achieveshighsensitivityevenatthelowendofthe
band.Spurious,image,andintermediatefrequency
rejectionareallsuperior.Thesamecircuitdelivers
equalselectivityacrosstheentireband.
Inaddition,thereisnolossofaudiofrequency
responseatthelowendoftheband,commoninAM
receivers.AnotheradvantageoftheMclntoshcircuit
isfreedomfromcrossmodulationandoverloading
bystronglocalstations.
Ahigh-qualityloopstickantennaisprovided.Itcan
berotatedformaximumperformance,optimumsig-
nalreceptionorminimuminterference.Ineach
MR74,theloopsticksareindividuallytunedforopti-
mumperformance.Aftertuning,theloopstickis
sealed.Thiscustommatchingoftheloopsticktothe
AM-RFfrontendmaximizestheperformanceofthe
loopstickantenna.Theantennaisrotatablethrough
nearly180degrees.Withthismobilityyouwillnot
sufferlossofsensitivityregardlessoftheangleat
whichtheinstrumentismounted.A backpanelcon-
nectorisprovidedforanexternalantenna.Thean-
tennaiscoupledtothetunerthrougha carefully
designedmatchingtransformerwhichprovidesopti-
mumperformanceregardlessofantennalength.
Tomaintaintheexcellentimagerejectionandlack
ofspuriouscrossmodulationoftheAM-RFamplifier
anautodyneconvertercircuitwasusedinproducing
theAM-IFsignal.
AM-IFusestwodoubletunedIFtransformersde-
signedtoobtaina highdegreeofselectivityyetal-
lowinggoodaudiofidelity.WiththeSELECTIVITY
switchinNARROW,a narrowbandceramicfilteris
addedtotheAM-IFamplifier.
A10,000Hzactivefiltereliminatesthe10,000Hz
whistleandirritatingnoisecausedbyanadjacent
station.Becauseanactivefilterisused,theoutput
levelat10,000Hz,orthewhistlefrequency,isdown
over20dBoronehundredthofwhatitwouldbe

withoutfiltering.WiththeSELECTIVITYswitchin
NARROWposition,theactivefiltercutofffrequency
islowered.Thefiltertheneffectivelysuppressesthe
5,000Hzwhistlefromnearbytelevisionreceivers.
TheAVC(automaticvolumecontrol)systemwas
designedtopreventblastingwhentheAMistuned
througha strongsignal.Distortionatlowaudiofre-
quenciesisminimizedbyusingtwoAVCfiltersec-
tionsinsteadoftheconventionalone.
AUDIOAMPLIFIER
Theaudioamplifierincreasesthelevelofthepro-
gramadequatetodrivea preamplifierorotherac-
cessoryequipment.Itconsistsoftwo,two-transistor
amplifiers,oneforeachchannel.Thedesignuses
considerablenegativefeedbacktohelpachievelow
distortion,widefrequencyresponse,andexcellent
stability.Eachaudioamplifierdelivers2.5voltsto
theFIXEDOUTPUTjacks.A secondpairofoutputs
areavailablewherelevelcanbevariedbytheVOL-
UMEcontrol.
POWERSUPPLY
Specialdesignattentionhasbeengiventothe
powersupplysection.Threeseparatepowersupply
circuitsareused.Thefirstisa 24voltregulatedsup-
ply.This24voltregulatorisveryelaborateindesign,
usinga speciallyselectedtransistorandassociated
circuit.Theregulatoruseselectronicfilteringtoin-
surethelowestpossiblebackgroundhumlevel,
maximumstabilityandextremelygoodregulation.All
signalstagesarepoweredfromthisregulator.
Thesecondcircuitisa halfwaverectifierand
filterfortheDChighvoltageneededfortheanode
ofthemultipathindicator.Thethirdpowercircuitis
ahalfwaverectifierwhichsuppliesDCtotheAM
section.
14

Block
Diagram

FMSTATIONLOG
CALL
LETTERSFREQUENCYLOCATIONANTENNA
DIRECTIONCALL LETTERSFREQUENCYLOCATIONANTENNA
DIRECTION
16

McINTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903
607-723-3512
Designsubjecttochangewithoutnotice.
PrintedinU.S.A.
038-839