McIntosh MR 73 User manual

THEMcINTOSHMR73SOLIDSTATEFM/FMSTEREO-AMTUNER
Price$1.25

YourMR73FM/FMSTEREO-AM
TUNERwillgiveyoumanyyearsof
pleasantandsatisfactoryperformance.
Ifyouhaveanyquestionsconcerning
theoperationormaintenanceof
thisinstrument,pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
TakeAdvantageof3 years
ofFREEFactoryService...
FillintheApplicationNOW.
GUARANTEE...1
IFYOU'REINA HURRY...2
INSTALLATION...3,4
HOWTOCONNECT...5
BACKPANELINFORMATION...6
FRONTPANELINFORMATION...7,8
LISTENINGTOTHEMR77...8
TECHNICALDESCRIPTION...9,10
PERFORMANCELIMITS...11
PERFORMANCECHARTS...12
BLOCKDIAGRAM...13
GUARANTEE
MclntoshLaboratoryIncorporatedguaranteesthis
instrumenttobecapableofperformanceasadver-
tised.Wealsoguaranteethemechanicalandelec-
tricalworkmanshipandcomponentstobefreeof
defectsfora periodof90daysfromdateofpur-
chase.IIsuchdefectsoccur,MclntoshLaboratoryor
THREEYEARFACTORY
Anapplicationfora FREETHREEYEARFAC-
TORYSERVICECONTRACTisincludedwiththis
manual.Thetermsofthecontractare:
ForThreeYearsfromdateofpurchase—
1.Mclntoshwillprovideallparts,materialsand
laborneededtoreturnthemeasuredperform-
anceoftheinstrumenttotheoriginalper-
formancelimitsfreeofanycharge.The
SERVICECONTRACTdoesnotcoverany
shippingcoststoandfromtheauthorized
serviceagencyorthefactory.
2.AnyMclntoshauthorizedserviceagencywill
repairallMclntoshinstrumentsatnormal
servicerates.Toreceivethefreeservice
underthetermsoftheSERVICECONTRACT,
theSERVICECONTRACTCERTIFICATEmust
accompanytheinstrumentwhentakentothe
serviceagency.
3.Alwayshaveservicedonebya Mclntosh
authorizedserviceagency.Iftheinstrument
ismodifiedordamagedasa resultofunau-
thorizedrepairtheSERVICECONTRACTwill
becancelled.
oneofitsauthorizedagencieswillrepairthedefect
atnocosttothepurchaser.Thisguaranteedoes
notextendtocomponentsdamagedbyimproperuse
nordoesitextendtotransportationtoandfromthe
factoryorserviceagency.
SERVICECONTRACT
4.TheSERVICECONTRACTisissuedtoyouas
theoriginalpurchaser.Toprotectyoufrom
misrepresentationthiscontractcannotbe
transferredtoa secondowner.
5.TheSERVICECONTRACTisgiventopur-
chaserswholiveinthe50UnitedStatesor
Canadaonly.
6.ForyourprotectionMclntoshselectsits
dealerscarefully.Onlyonedealerinten
qualifiesfora McIntoshfranchise.Toreceive
theSERVICECONTRACTyourpurchasemust
bemadefroma Mclntoshfranchiseddealer.
7.Yourcompletelyfilledinapplicationfora
SERVICECONTRACTmustbepostmarked
within30daysofthedateofpurchaseof
theinstrument.
8.ToreceivetheSERVICECONTRACTallinfor-
mationontheapplicationmustbefilledin.
TheSERVICECONTRACTwillbeissuedwhen
thecompletelyfilledinapplicationisreceived
atMclntoshLaboratoryIncorporatedinBing-
hamton,NewYork.Iftheapplicationisnot
receivedatMclntoshLaboratory,onlythe
serviceofferedunderthe90-dayguarantee
willapply.
Copyright©1970ByMclntoshLaboratoryInc.
1
CONTENTS

IfYou'reina Hurry
MULTIPATHINDICATOR—
Fluctuatingdisplayindicates
multipathdistortion.Rotate
antennaforminimumfluctuation.FMTUNING—TuneFMuntilthe
indicatorisintheblackareain
thecenterofthemeterface.
STEREOINDICATOR—Lights
whenstationistransmitting
stereopilotcarrierforstereo
broadcasting.SIGNALSTRENGTH—Indicates
relativesignalstrength.
MODESELECTOR—Selectfrom
AMbroadcast,stereoFM
broadcast,withAutomatic
Stereo/Monoswitchesor
monoFM.
MUTING— Eliminatestheusual
noisebetweenFMstations.
VOLUME/OFF—TurnstheAC
powertothetuneronoroffand
adjustsvolumetodesiredlevel.
STEREOFILTER—Reducesnoise
onweakFMstereobroadcasts.
2

Installation
Adequateventilationextendsthetrouble-free
lifeofelectronicinstruments.Itisgenerallyfound
thateach10°centigrade(18°F)riseintempera-
turereducesthelifeofelectricalinsulationbyone
half.Adequateventilationisaninexpensiveand
effectivemeansofpreventinginsulationbreak-
downthatresultsfromunnecessarilyhighoperat-
ingtemperatures.Thedirectbenefitofadequate
ventilationislonger,trouble-freelife.
Allowatleast15inchesdeepx 17½incheswide
x6incheshighformountingtheMR73.Always
allowforairflowbyeitherventilationholesor
spacenexttothebottomofthetuneranda means
fora warmairtoescapeatthetop.
ItisrecommendedthattheMR73bemounted
ina normalorhorizontalposition.However,with
adequateventilationthetunercanbemountedin
anyposition.
TopreparetheMR73forinstallationremovethe
plasticprotectivecovering.TurntheMR73upside
downsothatitrestsonitstopontheshipping
pallet.Removethefourplasticfeetfastenedtothe
bottomofthechassis.
Next,placethemountingbrackets,theparts
bagandthemountingtemplateathand.
ThePANLOCprofessionalmountingdesign
eliminatestheneedforanyshelforbracketto
supporttheMR73.Itiscompletelysupportedby
itsownmountingbrackets.
Thedesignofthemountingtemplateallowsyou
topositionorlocatethecutoutfromthefrontor
rearofthepaneltowhichtheinstrumentistobe
mounted.Positiontheplasticmountingtemplate
overtheareaofthepaneltobecutoutforinstalla-
tion.
Ifthecutoutistobelocatedfromthefrontof
thepanel,beginatStep2.Ifthecutoutistobe
locatedfromtherearofthepanel,beginhere.
Onthebackofthecabinetpanel,scribea
verticalcenterlinethroughtheexactcenterof
theareainwhichthecutoutistobemade.
Placethetemplateagainstthebackofthe
panelandmatchthetemplatecenterlinewith
thecenterlineonthecabinetpanel.
Makesurethatthereisatleast¼ inchclear-
ancebetweenthebottomofthedashedlineof
thecutoutareaonthetemplateandanyshelf
orbracebelowtheproposedcutout.
Markthetwolocatingholes("C"holeson
themountingtemplate).
Drillthetwolocatingholes.Becertainthe
drillisperpendiculartothepanel.
Nowpositionthetemplateonthefrontofthe
panelbyaligningthe"C"locatingholesonthe
templatewiththedrillholes:
3

2.Ifthecutoutistobelocatedfromthefrontof
thepanel:
Withthetemplateinplaceagainstthecabi-
netpanel,markthe"A"and"B"drillholesand
thefoursmallholesthatidentifythecornersof
thecutout.Jointhecornermarkswitha pencil.
Theedgeofthetemplatecanbeusedasa
straightedge.
IMPORTANT:DRILLTHE6 HOLESBEFORE
MAKINGTHECUTOUT.
Accuratelydrillthethreeholesoneachsideof
thecutoutareawitha3/16inchdrill.
WiththesawontheINSIDEOFTHEPENCIL
LINEScarefullycutouttherectangularopening.
Securethemountingstripstotherearofthe
cabinetpanelusingtwoscrewsfromthehardware
package.
Insertthescrewsinthecenterholesofthecabi-
netpanel("B"holesonthetemplate)andtighten.
Thescrewheadshouldpullintothewoodslightly.
(Usetwo¾ inchlongscrewsforpanelsunder½
inch,ortwoVAinchlongscrewsforpanels½ inch
thickandlarger.)
Attachthemountingbracketstothecabinet
panelusingfourscrews.
Placethetemplateoverthemountingscrews.
Themountingscrewsshouldbecenteredinthe
"A"and"B"holesonthetemplate.Thesidesof
themountingbracketsshouldmatchthevertical
dashlinesonthetemplate.Ifnecessary,loosen
thescrewsandpushthebracketsintoalignment
andretighten.
Insertthepowercordthroughtheopening.Care-
fullyslidetheMR73intotheopeningsotherails
onthebottomoftheequipmentslideinthetrack
ofthemountingbrackets.Slidetheinstrumentin
untilitstopsattheadjustpositionlatches.Press
thelatchesinandcontinuetoslidetheinstrument
inuntilthefrontpanelisagainstthecabinetpanel.
AtthebottomfrontcornersofthePANLOCinstru-
mentsarethePANLOCbuttons.Depressingthe
PANLOCbuttonswilllocktheinstrumentfirmlyin
theinstallation. DepressingthePANLOCbuttonsa
secondtime(aswitha ballpointpen)willrelease
theinstrument.Youcanthenslidetheinstrument
forwardtotheinspection-adustmentposition.De-
pressingtheinspection-adjustmentpositionlatches
willallowtheinstrumenttobeslidcompletelyout
oftheinstallation.
VERTICALINSTALLATION
Forverticalinstallationfollowtheabovepro-
cedures.Youmayinstalltwoassistspringsonthe
PANLOCbrackets.
Inthehardwarepacketaretwohelicalsprings.
Fastenthespringstothesmallflangesattherear
ofthePANLOCbrackets.Theflangehasa notch
anda holetomountthespring.Thespringsassist
intheremovalofverticallymountedPANLOC
equipment.
DONOTUSETHESPRINGSONHORIZONTALLY
MOUNTEDEQUIPMENT.
4

HowtoConnect
AUDIOOUTPUTS
UsetheFIXEDOUTPUTjackstoconnecttoa
conventionalcontrolpreamplifierwhichhasits
ownvolumecontrol.TheVOLUMECONTROLdoes
notaffecttheoutputofthetunerattheFIXED
OUTPUTjacks.
TheoutputimpedanceattheFIXEDOUTPUTS
is600ohms.Longercablesthanarenormallysup-
pliedcanbeusedtointerconnecttheMR73with
otherequipment.Thelengthofthecableislimited
bythecapacityofthecable.Thetotalcapacity
mustnotexceed1,000pF.Forinstance:cables
witha capacityof25pFperfootmaybe40feet
long;13.5pFperfootcablemaybe75feetlong.
UsetheFRONTPANELCONTROLLEDjacksto
connecttoa conventionalcontrolpreamplifier
whenuseofthetunervolumecontrolisdesired.
Thesejacksmaybeusedtoconnecttoexternal
equipmentsuchaspoweramplifiersortapere-
corderswherecontrolofvolumeatthetuneris
necessary.Theloadimpedanceconnectedtothe
FRONTPANELCONTROLLEDjacksshouldbenot
lessthan47,000ohms.Thereisnodifferencein
thesignalqualityormaximumoutputlevelsavail-
ableateachpairofoutputjacks.
CONNECTINGANFMANTENNA
Oneofthreeantennasystemscanbeused:(1)
anoutdoorFMantenna,or(2)a VHF-TVantenna,
or(3)theindoordipolesuppliedwiththeMR73.
Anoutdoorantennaisrecommendedforopti-
mumperformanceinallareas.Infringe(outlying)
areas,bestresultswillbeobtainedwitha highly
directionalFMantennausedinconjunctionwitha
rotator.Rotatetheantennauntilthebestreception
isobtained.Connectthe300ohmantennatothe
FMANT(red)terminals.
CONNECTINGA 75OHMANTENNA
Anunbalanced75ohmantennacanbecon-
nectedtotheMR73.A "typeF"connectorisused
toconnectthe75ohmcoaxialcabletotheback
panelFMANT75input.
AVHF-TVantennaisofteneffectivewhenitis
designedforbothFMandTVreception.Connect
thetwoleadsfromtheVHF-TVantennatothe
300ANT(red)terminals.
CONNECTINGANINDOORDIPOLEANTENNA
Theflexiblefoldeddipoleantenna(300ohm)is
foruseinurbanorhighstrengthsignalareas.
Connectthetwoleadsfromthedipoletothe
FMANT(red)terminals.Theflexibilityofthethin
flatwireassemblypermitsittobeplacedundera
rug,tackedbehindthestereo...or,placedinany
otherconvenientlocation.Insomecases,itmay
benecessaryto"position"theantennaforbest
signalreception.Thisshouldbedonebeforeitis
permanentlylocated.
Avoidlocatingtheantennanexttootherwires
ormetalobjects.Thisantennamaynotprove
effectiveinhouseshavingmetalsidingormetal
foilinsulation.
AMBUILT-INANTENNA
FormostlocalandmoderatelydistantAMrecep-
tionthebuilt-inferriteloopstickantennamaybe
used.SettheAMANTslideswitchontheMR73
backpaneltotheLOOPposition.
AMOUTDOORANTENNA
ForbestlongdistanceAMreception,usea cop-
perantennawire50to150feetinlength.Suspend
thewireina straightlineashighaspossible.At-
tachthewireateachendwithsuitableglassor
ceramicinsulators.Connecta lead-inwireatany
convenientpointontheantenna.Itisrecom-
mendedthata lightningarresterbeusedwithan
outdoorAMantenna.Thearrestershouldbewell
groundedtoa suitablewaterpipeorcopperor
aluminumrodsunkintotheground.
Connectthelead-inwiretotheAMANTpush
connectorontheantennaterminalstriponthe
backpanel.SettheAMANTSlideSwitchtothe
EXTposition.
5

BackPanelInformation
TP1andTP2
TestpointTP1andTP2isprovidedfortuner
circuitalignment.Itisalsousedinconjunction
withtheMclntoshMaximumPerformanceIndi-
cator.
117VACOUTLET
Provides117voltACpowerupto350wattsfor
additionalequipmentsuchasamplifiers,orother
equipment.Thisoutletisnotfused.Itturnsonand
offwiththefrontpanelACpowerswitchonthe
VOLUMEcontrol.
ACPOWERCORD
ConnecttheACpowercordtoa 117volt,50to
60Hzpowerlinereceptacle.Thepowerusedby
theMR73is20watts.
FUSE
A0.5AMPfuseprotectsthetunercircuits.This
fusedoesnotprotectadditionalequipmentcon-
nectedtothebackpanelACoutlet.
AMANTSWITCH
AdjusttheMR73AMANTswitchtomatchthe
particulartypeofAMantennaused.EXT.position
isfora conventionalantenna,usuallyoftheout-
doortype,from50to150feetinlength.LOOP
positionisforthebuilt-inFerriteLoopstick.
ANTENNACONNECTIONSTRIP
ProvideseasypushtypeconnectorsforanAM
antenna,a groundconnection,anda 300ohmFM
antenna.
FMANTENNA—75OHM
Providesa typeF maleconnectorfora 75ohm
unbalancedFMantenna.
AUDIOOUTPUTS
Theleft-handpairofAUDIOOUTPUTjackspro-
videsaudiosignalsunaffectedbytheMR73front
panelVOLUMEcontrol.Usetheseoutputjacksto
connectthetunertoa stereocontrolpreamplifier
whichhasitsownmastervolumecontrol.
TherighthandpairofAUDIOOUTPUTjacks
providesaudiosignalcontrolledbytheMR73
frontpanelVOLUMEcontrol.Usetheseoutput
jackstoconnecttoexternalpoweramplifier,tape
recorders,oranyequipmentwhichrequirescon-
tinuousfrontpanelcontroloftuneroutputvolume.
6

FrontPanelInformation
TUNINGDIAL
TheMR73hasthreedialscales:
1.AM— Marked55to160kHz
2.FM—Marked88to108MHz
3. Loggingscale— Marked0 to 100
Theloggingscalecanbeusedtoaccuratelyre-
tuneanystation.Youmayfinditeasiertokeepa
recordofyourfavoritestationsbyuseofthelog-
gingscale.
Asmallportionofdialpointerhasbeenillumi-
natedtoincreasetheeaseoftuning.
INDICATORS
TheMR73hasfourindicatorsonthedialpanel:
STEREOindicator,MULTIPATHindicator,SIGNAL
STRENGTHmeter,andtheTUNINGmeter.
STEREOINDICATOR
TheSTEREOindicatorlightsredwhenthedial
pointeristunedtoorcrossesa stationbroadcast-
ingthe19,000Hzcarrierforstereo.Thespecial
circuitusedwilllightonlywhenthe19,000Hz
multiplexcarrierispresentinthesignal.Theindi-
catorwillnotlightonnoisepulsesorinterference.
MULTIPATHINDICATOR
TheMULTIPATHindicatorisanexclusiveMc-
lntoshdevelopment.
TheproperuseoftheMULTIPATHindicator
makesitpossibletoimproveFMreceptionwith
preciseFMantennapositioning.
Anelectronrayindicatorisusedtoshowmulti-
pathreception.Itoperatesbymovementofelec-
tronbeamsinsidea vacuumtube.Whenrotating
theantenna,observationofthemultipathindicator
willshowbestorientationfortheFMstationbeing
received.Multipathdistortioncausesthetwo
beamsontheindicatortofluctuaterapidly.When
theantennaisrotatedtothebestposition,theindi-
catorbeamstendtoremainsteady.Thedirectional
antennathenpicksupthedesiredsignalandre-
jectsthereflectedmultipathsignals.Insomeloca-
tionsitispossibleforthebestreceptiontooccur
whentheantennapicksupa strongreflectedsig-
nalratherthanthedirectsignal.Multipathdistor-
tionispracticallyindependentofsignalstrength.
SIGNALSTRENGTHMETER
TheSIGNALSTRENGTHmeterindicatesthe
strengthofthesignalasreceivedfromthean-
tenna.Thehighertheindication,thestrongeris
thesignal.
FMTUNINGMETER
AnFMstationiscorrectlytunedwhenthemeter
needleisintheblackareaoftheFMTUNING
meter.TheactionoftheTUNINGindicatorisinde-
pendentofstationsignalstrength.
MODESELECTOR
Selectsanyoneofthreeprogramsources.
AM— ConnectstheAMtunersectionofthe
MR73totheoutputjacks.
FM— TheFMpositionprovidesmonophonicFM
orFMstereoattheleftandrightchannel
audiooutputjacksautomatically.Withthe
MODESELECTORintheFMpositiona
stationbroadcastingmonophonicpro-
gramwillbeheardinmono.Whenthe
stationswitchestostereobroadcast,the
stereoindicatorwilllightandtheMR73
willautomaticallyswitchtostereoopera-
tion.Ifmonobroadcastingisresumedthe
MR73willautomaticallyswitchtomono.
FM-Mono— ConnectstheFMtunersectionsofthe
MR73totheoutputjacks.Thisposition
bypassestheautomaticstereoswitching.
Thestereoindicatorwilllightwhena sta-
tionistransmittingstereo,butthepro-
gramheardfromtheloudspeakerswillbe
mono.
STEREOFILTER
TheSTEREOFILTERreducesnoiseonweak
stereostations.
TurntheswitchtotheINpositiononnoisy
stereostations.
FMMUTING
Mutingsuppressesthebackgroundnoiseand
hissnormallyheardbetweenstations.Turnthe
controltotheINpositionformuting.Weaksta-
tionsthatmaynotoverridenoiseandinterference
arealsosuppressedbythemuting.
IntheOUTposition,themutingisturned-off.
ThisallowsconventionalFMtuningwiththenoise
andinterferencepresent.TheOUTpositionisused
totuneweakornoisystations.
VOLUME
TheVOLUMEcontrolhasbeenprecisiontracked
throughoutthelisteningrange(0to—65dB)for
accuratestereobalance.Itadjuststheoutputlevel
atthetunerFRONTPANELCONTROLLEDAUDIO
OUTPUTjacks.TheFIXEDOUTPUTjacksarenot
affectedbythevolumecontrol.
ThePOWERON/OFFswitchispartoftheVOL-
UMEcontrol.TurningtheVOLUMEcontroltotally
counterclockwiseturnstheACpowertotheMR73
OFF.
7

PANLOCBUTTONS
Atthebottomofthefrontpanelcornersarethe
PANLOCbuttons.Aftera tunerisinstalledonthe
PANLOCbracket,pressingthePANLOCbuttons
willlockthetunerfirmlyinposition.Depressing
thePANLOCbuttons(aswitha ballpointpen)will
releasethetuner.Thetunercanthenbeslidfor-
wardtotheinspectionandadjustmentposition.
ThePANLOCsystemgivesabsoluteeaseofin-
stallation,operationandmaintenance.
SECONDARYCONTROLS
Onthetopofthechassisbehindthefrontpanel
arethreesecondarycontrols.TheyareMUTING
adjust,AMSENSITIVITY,andDIALSCALEIN-
TENSITY.
MUTING
TheMR73ultrasonicmutingcircuitsuppresses
allnoisebetweenstations.Itsuppressesallweaker
stationsnotstrongenoughtooverridetheback-
groundnoise.
Themutingthresholdsettingdeterminesthe
strengthofthesignalwhichcanbeheardwith
mutinginoperation.Themutingthresholdiscare-
fullyadjustedtooptimumatthefactoryusingpre-
cisiontestinstruments.
Ifitisfoundnecessarytoadjustthemuting
threshold,turntheMUTINGADJtotheLEFT
(counterclockwise)tolowerthemutingthreshold.
Thisallowsweakernoisierstationstobeheard
attheMUTINGINsetting.Turnthecontroltothe
RIGHT(clockwise)toraisethemutingthreshold.
Thisallowsonlythemorepowerfulstationstobe
heardattheMUTINGINsetting.
AMSENSITIVITY
WhentheAMSENSITIVITYswitchisinthe
LOWpositionthenoisebetweenstationsisre-
duced.TheRFgainisnotasgreatinthelow
position.
DIALSCALEINTENSITY
Adjustthebrightnessofthedialpanellightsby
meansofthisswitch.SettheswitchtoBRIGHT
formaximumpanellight.SettheswitchtoDIM
forlowerdiallight.
ListeningtotheMR73
LISTENINGTOAM
TurntheMODESELECTORswitchtoAM.Turn
thetuningknobtothedesiredstation.TheSIGNAL
STRENGTHmeterwillindicatetherelativestrength
oftheparticularAMstationbeingreceived.
AdjusttheVOLUMEcontroltodesiredlistening
level.IfthefixedAUDIOOUTPUTjacksareused,
thetunerVOLUMEcontrolwillnotaffectvolume.
LISTENINGTOMONOPHONICFM
TurntheMODESELECTORtoFMMONOto
listenonlytomonophonicFM.
IfyouwishtohavetheMR73automatically
switchtoa stereobroadcast,settheMODESE-
LECTORtoFM.
TurntheSTEREO-FILTERtoOUT.
TurntheMUTINGtoIN.
Turnthetuningknobtothedesiredstation.The
stationisproperlytunedwhentheFMTUNING
meterpointercomestorestanywhereintheblack
areaofthemeterscale.Whiletuningacrossthe
dialyoumaynoticemovementofthetuningmeter,
withouthearinga station.Thisiscausedbya sta-
tionsoweakthatisdoesnotover-ridetheback-
groundnoise.Withultrasonicmutinginoperation
signalsthataremarginalareautomaticallysup-
pressed.Toheartheseweakerstations,turnthe
MUTINGswitchtoOUT.Usuallythelistening
qualitywillberatherpoorduetothebackground
noise.
Rotatethedirectionalantennaforbestreception
asshownbytheSIGNALSTRENGTHandMULTI-
PATHindicators.
AdjusttheVOLUMEcontroltodesiredlistening
level.IfthefixedAUDIOOUTPUTjacksareused,
thetunerVOLUMEcontrolwillnotaffectvolume.
LISTENINGTOSTEREOFM
Repeatthesameprocedureasformonophonic
FMbutturntheMODESELECTORtoFM.
WhentheSTEREOindicatorislighted,thesta-
tionisbroadcastinga 19,000Hzcarrier forstereo
andtheMR73willautomaticallyswitchtostereo.
Ifa stationisbroadcastingmonophonicFMpro-
gram,theSTEREOindicatorwillremainoffand
thetunerwillautomaticallyswitchtomono.
AdjusttheVOLUMEcontroltodesiredlistening
level.IfthefixedAUDIOOUTPUTjacksareused,
thetunerVOLUMEcontrolwillnotaffectvolume.
TheSTEREOFILTERreducesnoiseonweaksta-
tions.
8

FMSECTION:Thisisdividedintotwoseparate
modularsections:
A.The100MHzRadioFrequency(RF)Section.
ThissectionhousesthecompleteFM-RFfront-
endandpartoftheAM-RFcircuits.A special,
foursection,variabletuningcapacitorprovides
ahighdegreeofRFselectivityandexcellent
spuriousresponserejection.Theproblemof
imagerejectionhasbeengreatlyreducedin
theRFsectionoftheMR73.
Thetwostageparallel-fed-cascodejunction
fieldeffecttransistor(JFET)RFamplifiergives
bettersensitivityandhighergainthanconven-
tionalone-stageamplifiers.TheuseofJFET's
helpstofurtherreducetheproblemoffrontend
overloadinstrongsignalareas.
AmixerusingJFEThasbeendesignedfor
highsensitivityandfreedomfromoverload.
Lowtemperaturecoefficientcomponentshave
beendesignedintotheFMlocaloscillatorto
preventfrequencydrift.Thefrequencystability
inherentinthelocaloscillatormakesautomatic
frequencycontrol(AFC)unnecessary.Therate
ofdriftofthelocaloscillatorislessthanten
partspermillionperdegreecentigrade.
BoththeFMandAM-RFfrontendshave
beendesignedinthesamecompletelyencased
metalmodules.Thisdesigngivesprotection
againstradiationorinterference.TheRFcir-
cuitsoftheMR73exceedtheFCCrequire-
mentsforsuppressionofoscillatorradiation.
TheMR73hasantennaconnectionsfor
either300ohmtwinleadtransmissionlineor
75ohmcoaxialcable.Thenormalinputimped-
anceofthefirstRFamplifieris75ohms.Im-
pedancematchto300ohmsisprovidedbya
Mclntoshdesignedbaluntransformerwhich
hasnegligiblelosses.Connectionsfora 300
ohmlinearemadewithnewpushtypetermi-
nals.Notoolsarerequired.A typeF malecon-
nectorisfurnishedfor75ohmcoaxialcable.
Forgreatersignaltransferandlowerdistor-
tion,a specialmatchingtransformerhasbeen
designedtointerfacetheFM-RFtotheFM-IF
amplifier.Thismatchingtransformerconsider-
ablyenhancesthelinearphasecharacteristics
oftheIFamplifier.
B.TheFM-IFandDetectorSection.TheFM-IFand
Detectordesignrequiredextendedengineering
timeandtestingandusesthelatestconcepts
inintegratedcircuitsandcrystalfilters.The
FM-IFconsistsoftwointegratedcircuitsand
twophaselinearcrystalfilters.Theycombine
togivea totalgainofover120dB(thesignal
isamplifiedtoover1,000,000timesitsoriginal
level).Theresponsecurvehasa nearlyflattop
withlinearphasecharacteristics.Theskirtsof
theresponsecurveareverysteep.Themaxi-
mumwidthis240kHzat-3.0dBand400kHz
at-60dB.Theresponsecurveissymmetrical
eachsideofthecenterfrequency.Thecrystal
filtersarepermanentlysealedanddonotre-
quireadjusting.TheIFcannotdriftnorvibrate
outofadjustment.
Eachofthetwointegratedcircuitsusedin
theFM-IFoftheMR73contain16transistors,
3zenerdiodes,5 diodesand23resistors,all
ona singlemonolithicsiliconchip.
Theexceptionallyhighgainofthetwointe-
gratedcircuitsassumes"hardlimiting"atvery
lowlevelsofinputsignals.
A"phase"or"FosterSeeley"discriminator
hasbeendesignedtocomplementtheinte-
gratedcircuitIFsection.TheIFsectionhas
exceptionallyhighgainwithhardlimitingchar-
acteristics.Itdevelopsa captureratiothatis
verylow.Thedetectedoutputsignalofthe
discriminatorisextremelylowindistortioncon-
tent.De-emphasisofthediscriminatoroutput
restoresthefrequencyamplitudecharacter-
isticstothesameleveltheywerebeforetrans-
mission.
C.FMStereoMultiplexSection.MclntoshLabora-
toryhasdevelopeda specialdetectingcircuit
inthemultiplexsection.A particularadvantage
ofthiscircuitistheeliminationofthecritical
adjustmentsnecessarywithcommonlyused
matrixingcircuits.ThecircuitdetectstheL-R
9

sidebands,thenautomaticallymatrixesthere-
coveredinformationwiththeL + R maincarrier
signal.Thisyieldstheleftandrightprogram
outputwithmaximumseparation.
The19kHzpilotsignalisfilteredfromthe
compositestereoinputsignal,amplifiedbya
speciallimiting amplifier,doubledtothe38
kHzcarrierfrequency,andthenamplifiedagain
bya limiting amplifier.Thecompositesignal
minusthe19kHzpilotiscombinedwiththe
38kHzcarriersignal.Thenewcombinationof
signalsisfedtothespecialdetectorcircuit
mentionedabove.Balancedfullwavedetectors
areusedtocancelthe38kHzcomponentsin
theoutput.
TheSCA(SubsidiaryCommunicationAuthor-
ization)signalmustberemovedfromthecom-
positeoutput.Thisisaccomplishedbytheuse
ofa new"ImageParameter"bandelimination
filterthathasbeencomputerdesigned.The
SCAfilterrejectsSCAsignalswithoutimpair-
ingstereoperformance.FMmutingintheMR73
operatesbydetectingultra-sonicnoisewhich
ispresentwhentuningbetweenstationsor
whenreceivinga weakstation.Themutingcir-
cuitcanbeactivatedordefeatedbytheuseof
themutingswitchonthefrontpanel.Thelevel
ofmutingdesiredcanbeadjustedbythemut-
inglevelcontrolonthetoppanel.Varyingthe
mutingcontroladjuststhethresholdatwhich
themutingtakeseffect.Whenthe19kHzcarrier
ofa stereosignalisreceived,theautomaticFM
stereoswitchingcircuitactivatesthemultiplex
decodingcircuit.Thislightsthestereoindi-
cator.Thecircuitswitchingisalldoneelectron-
icallywithnoclicks.Theautomaticstereo
switchingcanbedefeatedbyturningthemode
selectorswitchtoFMMONO.(Inthisposition
thestereoindicatorwillstilllighttoindicatethe
presenceofa stereosignal.)Onmonophonic
transmissionsthestereoswitchingisinactive
atalltimes,assuringa greatersignaltonoise
ratio.Thestereoswitchingcircuithasbeen
designedsothatnoisewillnotactivateit.
AMSECTION
Metaloxidesiliconfieldeffecttransistorsare
usedintheAMsection.Mclntoshengineeringcon-
tinuouslypushesthelimitof"thestateoftheart"
inelectronictechnology.AnAM-RFamplifiercir-
cuitincludesa threesectionvariabletuningca-
pacitorinthemetalenclosedshieldedmodule,
whichalsohousestheFM-RFfrontend.TheAM-RF
amplifierusesa dual-insulatedgateMOSFETto
obtainmoreoverloadprotectionagainststrong
localstationsthancanbeobtainedwithconven-
tionalbipolartransistors.
TheAM-RFmixerisalsoa dual-insulatedgate
MOSFET.TheuseofMOSFET'sintheRFamplifier
andmixerprovidestheMR73withverylittlecross
modulation(spuriousresponse)andtheimagere-
jectionisverygood.
ThreedoubletunedIFtransformersareusedto
obtaina highdegreeofselectivityyetstillallow
goodAMfidelity.A 10kHzwhistlefilterhasbeen
incorporatedattheoutputoftheAMdetector.Its
purposeistosuppressheterodyningthatoccurs
betweenadjacentAMcarriers.
Toincreasethe"pullingpower"oftheAMsec-
tionoftheMR73,connectanexternalantennaat
therearapronoftheunitbyusinga pushconnec-
tor.Aninternaltransformermatchestheexternal
antennatotheinputimpedanceoftheAMfront
end.Anoptionalhighlysensitiveloopstickantenna
isalsoprovidedforlocalorstrongstations.A
switchonthebackpanelselectseitherloopstick
orexternalantenna.
AnAMsensitivityswitchhasbeenprovidedto
decreaseboththegainoftheRFamplifierandthe
noisebetweenstations.
AUDIOPREAMPLIFIERSECTION
Theaudioamplifierincreasesthelevelofthe
programadequatetodrivea preamplifierorother
accessoryequipment.Itconsistsoftwo,three
transistoramplifiers,oneforeachchannel.The
designusesconsiderablenegativefeedbackto
helpachievelowdistortion,widefrequencyre-
sponse,andexcellentstability.Eachaudioampli-
fierdelivers2.5voltstotheFIXEDOUTPUTjacks
at600ohmsimpedance.A secondpairofoutputs
areavailablewherelevelcanbevariedbythe
VOLUMEcontrol.
TheSTEREOFILTERisconnectedintheaudio
amplifiertoreducenoisewhenlisteningtoweak
stereostations.Carefuldesignpermitsanideal
compromisebetweenchannelseparationandnoise
rejectionwhenusingtheSTEREOFILTER.
POWERSUPPLY
Specialdesignattentionhasbeengiventothe
powersupplysection.Twoseparaterectifiercir-
cuitsareused.Thefirstisa 16voltregulated
supply.The16voltregulatorisveryelaboratein
design,usinga speciallyselectedtransistorand
associatedcircuit.Theregulatoruseselectronic
filteringtoinsurethelowestpossiblebackground
humlevel,maximumstabilityandextremelygood
regulation.Allsignalstagesarepoweredfromthis
regulator.
AhalfwaverectifierandfiltersupplytheDC
highvoltageneededfortheanodeofthemultipath
indicator.Thesecondpowersupplyisa fullwave
rectifierwhichsuppliesDCtothemultiplexindi-
catorandtothevoltageregulator.
10

PerformanceLimits
PERFORMANCEGUARANTEE
PerformanceLimitsarethemaximumdeviation
fromperfectionpermittedfora McIntoshinstru-
ment.WepromiseyouthattheMR73youbuy
mustbecapableofperformanceatorexceeding
thelimitsoryougetyourmoneyback.McIntosh
istheonlymanufacturerthatmakesthisguarantee.
.
FM
USEABLESENSITIVITY:2.5microvoltsat100%
modulation(±75kHzdeviation)for3%total
noiseandharmonicdistortioninaccordance
withIHFstandards.
AUDIOFREUQENCYRESPONSE:±1dB20Hzto
20,000Hzwithstandardde-emphasis(75
µSEC.)and19,000Hzpilotfilter.
HARMONICDISTORTION:Mono:0.3%at100%
modulation±75kHzdeviation.Stereo:0.7%
orless.
CAPTURERATIO:1.5%dBat100%modulation.
MUTING:AFinjectedultrasonicmuting:60dB
noisereductionbetweenstations.
OSCILLATORDRIFT:25,000Hzinanambienttem-
peratureof25°C.
IMAGEREJECTION:80dBat90MHz;70dBat
100MHz.
HUM:70dBbelow100%modulation.
OUTPUT:2.5volts;lowimpedance.
FMANTENNAINPUTS:300ohmsbalanced;75
ohmsunbalanced.
IF:TwoICsandtwophaselinearcrystalfilters.
RADIATION:FCCrequirements.
STEREOSEPARATION:35dBat1,000Hz.
STEREOFILTER:-dBnoisereduction.
STEREOINDICATOR:Frontpanelmultiplexstereo
lightactivatedby19,000Hzcarrieronly';
SCAFILTER:50dBdownat67kHzto74kHz,275
dBperoctaveslope.
AUTOMATICMONO-STEREOSWITCH:Mclntosh
developed;allelectronicautomaticmono-
stereoswitchingcircuit.
SEMICONDUCTORCOMPLEMENT:24Transistors,
22diodes,2 IntegratedCircuits,1 Indicator
Tube..
AM
SENSITIVITY:12microvoltsat1,000Hz(usingthe
externalantennainput).
FREQUENCYRESPONSE:-6dBat5,000Hz
HARMONICDISTORTION:1%at30%modulation.
SELECTIVITY:-30dBat10,000Hz
IMAGEREJECTION:60dBat1,000kHz.
SIGNALTONOISERATIO:55dB.
AMANTENNAINPUTS:Built-inloopstickorex-
ternalantennaswitchselected.
POWERREQUIREMENTS:117Volts,50/60Hz,25
Watts.
MECHANICALSPECIFICATIONS
SIZE:Frontpanel:16incheswideby51/16inches
high;Chassis:15incheswideby13inches
deep,includingPANLOCshelfandbackpanel
connectors;KnobClearance:1½inchesin
frontofmountingpanel.
WEIGHT:24poundsnet,36poundsinshipping
carton.
FINISH:Frontpanel:Anodizedgoldandblack
withspecialgold/tealpanelnomenclature
illumination.
CHASSIS:Chromeandblack.
MOUNTING:Mclntoshdevelopedprofessional
PANLOC.
11

PerformanceCharts
12
R.F.INPUTSIGNALINVOLTSACROSS300
1µV1µV10µV100µV1mV10mV100mV
-dB
0
10
20
30
40
50
60
70
%HARMONICDISTORTION
1.2
.8
.4
0
2010010001000020000
FREQUENCYINHERTZ
HARMONICDISTORTION
IFAMPLITUDERESPONSE
-10
-20
-30
-40
-50
-60
-70
RESPONSENdB
-300-200-1000 100200300
FREQUENCYINkHz

BlockDiagram
13

McINTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903
607-723-3512
Designsubjecttochangewithoutnotice.
PrimedinU.S.A.
038-448