McIntosh MR 78 User manual

THEMcINTOSHMR78SOLIDSTATEFM/FMSTEREOTUNER
ReadingTime32MinutesPrice$1.25

YourMR78FM/FMStereoTuner
willgiveyoumanyyearsof
pleasantandsatisfactory
performance.Ifyouhaveany
questionsconcerningtheoperation
ormaintenanceofthisinstrument,
pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
TakeAdvantageof3 years
ofFREEFactoryService. . .
FillintheApplicationNOW.
CONTENTS
GUARANTEE1
INSTALLATION2,3
HOWTOCONNECT4,5
BACKPANELINFORMATION6
FRONTPANELINFORMATION6,7
LISTENINGTOTHEMR78TUNER8
PERFORMANCELIMITS9
TYPICALPERFORMANCECHARTS10,11
TECHNICALDESCRIPTION12,13,14
BLOCKDIAGRAM15,16
MclntoshLaboratoryIncorporatedguaranteesthis
Instrumenttobecapableofperformanceasadver-
tised.Wealsoguaranteethemechanicalandelec-
tricalworkmanshipandcomponentstobefreeof
defectsfora periodof90daysfromdateofpur-
chase.Ifsuchdefectsoccur,MclntoshLaboratory
GUARANTEE
oroneofitsauthorizedagencieswillrepairthe
defectatnocosttothepurchaser.Thisguarantee
doesnotextendtocomponentsdamagedbyim-
properusenordoesitextendtotransportationto
andfromthefactoryorserviceagency.
THREEYEARFACTORYSERVICECONTRACT
Anapplicationfora FREETHREEYEARFACTORY
SERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:
1.Mclntoshwillprovideallparts,materialsand
laborneededtoreturnthemeasuredperformance
oftheinstrumenttotheoriginalperformance
limitsfreeofanycharge.TheSERVICECON-
TRACTdoesnotcoveranyshippingcoststoand
fromtheauthorizedserviceagencyorthefactory.
2.AnyMclntoshauthorizedserviceagencywillre-
pairallMclntoshinstrumentsatnormalservice
rates.Toreceivethefreeserviceundertheterms
oftheSERVICECONTRACT,theSERVICECON-
TRACTCERTIFICATEmustaccompanytheinstru-
mentwhentakentotheserviceagency.
3.Alwayshaveservicedonebya Mclntoshauthor-
izedserviceagency.Iftheinstrumentismodified
ordamaged,asa resultofunauthorizedrepairthe
SERVICECONTRACTwillbecancelled.Damage
byimproperuseormishandlingisnotcoveredby
theSERVICECONTRACT.
4.TheSERVICECONTRACTisissuedtoyouasthe
originalpurchaser.Toprotectyoufrommisrepre-
sentationthiscontractcannotbetransferredtoa
secondowner.
5.TheSERVICECONTRACTisgiventopurchasers
wholiveInthe50UnitedStatesorCanadaonly.
6.ForyourprotectionMclntoshselectsItsdealers
carefully.Onlyonedealerintenqualifiesfora
Mclntoshfranchise.ToreceivetheSERVICE
CONTRACTyourpurchasemustbemadefroma
Mclntoshfranchiseddealer.
7.Yourcompletelyfilledinapplicationfora SERV-
ICECONTRACTmustbepostmarkedwithin30
daysofthedateofpurchaseoftheinstrument.
8.ToreceivetheSERVICECONTRACTallinforma-
tionontheapplicationmustbefilledin.The
SERVICECONTRACTwillbeissuedwhenthe
completelyfilledinapplicationisreceivedat
MclntoshLaboratoryIncorporatedInBinghamton,
NewYork.Iftheapplicationisnotreceivedat
MclntoshLaboratory,onlytheserviceoffered
underthe90-dayguaranteewillapply.
Copyright© 1972ByMclntoshLaboratoryInc.
1

ItisrecommendedthattheMR78tunerbemounted
ina normalorhorizontalposition.However,withade-
quateventilationthetunercanbemountedinany
position.
Adequateventilationextendsthetrouble-freelife
ofelectronicinstruments.Itisgenerallyfoundthat
each18°F riseinoperatingtemperaturereducesthe
lifeofelectricalinsulationbyonehalf.Adequate
ventilationisaninexpensiveandeffectivemeansof
preventinginsulationbreakdownthatresultsfrom
unnecessarilyhighoperatingtemperatures.Thedirect
benefitofadequateventilationislonger,trouble-
freelife.
TheMR78tunerrequiresa mountingspacethatis
atleast15inchesdeep,17½incheswideand6 inches
high.Provideadditionalspacefortheairflowacross
thebottomofthetuneranda meansforwarmairto
escapeatthetop.
Removethetuner,shelfbrackets,partsbagandthe
mountingtemplatefromthecarton.Removetuner
fromtheplasticbagandplacethetunerupsidedown
ontheshippingpallet,thenremovethefourplastic
feetfastenedtothebottomofthechassis.
Theinstallationmaybeaccomplishedinsixeasysteps.
1.POSITIONTEMPLATEANDMARK
Positiontheplasticmountingtemplateoverthe
areaofthecabinetpanelwheretheMR78istobe
installed.Besurethattheedgesofthetemplate
clearanyshelves,partitionsorexistingequipment
locatedbehindthepanel.Withthetemplateinplace
markthesix"A"and"B"holesandfoursmallholes
locatingthecornersofthecutout.Next,jointhe
fourcornermarkswithpencillines;theedgeofthe
templatemaybeusedasa straightedge.
2.DRILLHOLES
Holdinga drillperpendicularlytothepanel,drill
thesix"A"and"B"holesusinga 3/16inchdrill.
THESIXHOLESMUSTBEDRILLEDBEFORE
MAKINGTHECUTOUT.
3.SAWCUTOUT
Usinga sawcarefullycuttherectangularopening
ontheinsideofthepencilledrectangle.
4.SECUREMOUNTINGSTRIPS
Securemountingstrips(suppliedinthehardware
package)ontheinsideofthecabinetpanel.
Inserttwoscrews(suppliedinthehardwarepack-
age)intothecenterholes("B"holesonthetem-
plate).Usethe¾-inchlongscrewsforpanelsunder
½-inchor1¼-inchscrewsforpanels½-inchthick
orover.Placea mountingstriponthebackofthe
cabinetpanelasshown.Alignitwiththethreeholes
inthepanelandtightenthescrew.Thescrewhead
shouldpullslightlyintothewoodpanel.Attachthe
othermountingstripbyrepeatingtheprocedure.
5.MOUNTPANLOCSHELFBRACKETS
AttachthePanlocshelfbracketstothecabinetpanel
usingfourscrewsoftheproperlength.Placethe
templateoverthemountingscrews.Thescrews
shouldbecenteredonthe"A"and"B"holesinthe
template.Ifnecessary,loosenthescrewsandpush
themountingbracketsintoalignmentthenretighten.
6.INSTALLTHEUNIT
Threadthepowercordthroughtheopeningonthe
cabinetpanel.Carefullyslidetheinstrumentinto
theopeningsotherailsonthebottomengagethe
2

trackonthemountingbrackets.Slidetheinstru-
mentinuntilitstopsattheadjustpositionlatches.
Pressthelatchesinandcontinuetoslidetheinstru-
mentuntilitsfrontpanelisflushwiththecabinet
panel.Atthebottomfrontcornersoftheinstrument
arethePANLOCbuttons.DepressingthePANLOC
buttonswilllocktheinstrumentfirmilyinthecab-
inet.DepressingthePANLOCbuttonsa second
time{aswitha ballpointpen)willreleasethein-
strument.Youcanthenslidetheinstrumentforward
totheadjustposition.Depressingtheadjustposition
latcheswillallowtheinstrumenttoberemoved
fromthecabinet.
3

HowtoConnect
AUDIOOUTPUTS
UsetheFIXEDOUTPUTjackstoconnecttoa stereo
controlpreamplifierorotherequipmentwhichhas
itsownvolumecontrol.TheVOLUMECONTROLdoes
notaffecttheoutputofthetunerattheFIXEDOUT-
PUTjacks.
TheoutputimpedanceattheFIXEDOUTPUTSis
600ohms.Longercablesthanarenormallysupplied
canbeusedtointerconnecttheMR78withother
equipment.Thelengthofthecableislimitedbythe
capacityofthecable.Thetotalcapacitymustnotex-
ceed1600pF.Forinstance:cableswitha capacityof
32pFperfootmaybe50feetlong;16pFperfootcable
maybe100feetlong.
UsetheFRONTPANELCONTROLLEDjackstocon-
necttoequipmentsuchaspoweramplifiersortape
recorderswherecontrolofvolumeatthetunerisde-
sired.TheloadimpedanceconnectedtotheFRONT
PANELCONTROLLEDjacksshouldnotbelessthan
47,000ohms.Thereisnodifferenceinthesignal
qualityormaximumoutputlevelsavailableateither
pairofoutputjacks.
CONNECTINGANFMANTENNA
Oneofthreeantennasystemscanbeused:(1)an
outdoorFMantenna,(2)anallchannel(UHF-VHF-FM)
antenna,or(3)theindoordipolesuppliedwiththe
MR78.
Anoutdoorantennaisrecommendedforoptimum
performanceinallareas.Infringe(outlying)areas,
bestresultswillbeobtainedwitha highlydirectional
FMantennausedinconjunctionwitha rotator.Rotate
theantennauntilthebestreceptionisobtained.Con-
nectthe300 ohmantennatothe300 ANT(red)
terminals.A UHF-VHF-FMantennaisofteneffective
buttheantennamustbedesignedforbothFMandTV
reception.ConnecttheleadsfromtheUHF-VHF-TV
antennatothe300 ANT(red)terminals.
CONNECTINGANINDOORDIPOLEANTENNA
Theflexiblefoldeddipoleantenna(300ohm)isfor
useinurbanorhighstrengthsignalareas.
Connectthetwoleadsfromthedipoletothe300
ANT(red)terminals.Theflexibilityofthethinflat
wireassemblypermitsittobeplacedundera rug,
tackedbehindthestereo...or,placedinanyother
convenientlocation.Insomecases,itmaybeneces-
saryto"position"theantennaforbestsignalrecep-
tion.Thisshouldbedonebeforeitispermanentlylo-
cated.Avoidlocatingthisantennanexttootherwires
ormetalobjects.Thisantennamaynotproveeffective
inhouseshavingmetalsidingormetal-cladinsulation.
CONNECTINGA 75OHMANTENNA
Anunbalanced75ohmantennacanbeconnectedto
theMR78.A "typeF"connectorisusedtoconnect
the75ohmcoaxialcabletothebackpanel75ANT
input.
4

FMANTENNA
OPTIONAL
75
ANTENNA
LEAD-IN
LEFTSPEAKER
POWERAMPLIFIER PREAMPLIFIER
RIGHTSPEAKER
5

BackPanelInformation
TP1andTP2
TestpointsTP1andTP2areusedwiththeMclntosh
MaximumPerformanceIndicator.
120V ACPOWEROUTLET
Provides120voltACpowerupto400wattsforaddi-
tionalequipmentsuchasamplifiers,etc.Thisoutlet
isnotfused.ItturnsonandoffwiththefrontpanelAC
powerswitchontheVOLUMEcontrol.
ACPOWERCORD
ConnecttheACpowercordtoa 120volt,50to60
Hzpowerlinereceptacle.Thepowerusedbythe
MR78is35watts.
FUSE
A0.5AMPfuseprotectsthetunercircuits.Thisfuse
doesnotprotectadditionalequipmentconnectedto
thebackpanelACpoweroutlet.
ANTENNACONNECTIONSTRIP
Provideseasypushtypeconnectorsfor300ohmFM
antennaanda groundconnection.
FMANTENNA-75OHM
Providesa "typeF"connectorfora 75ohmunbal-
ancedFMantenna.
AUDIOOUTPUTS
Theleft-handpairofAUDIOOUTPUTjackspro-
videsFIXEDOUTPUTaudiosignals.Theprogramat
thesejacksisnotaffectedbytheMR78frontpanel
VOLUMEcontrol.Usetheseoutputjackstoconnect
thetunertoa stereocontrolpreamplifierorother
equipmentwhichhasitsownvolumecontrol.
TherighthandpairofAUDIOOUTPUTjackspro-
videsa FRONTPANELCONTROLLEDaudiosignal.
TheprogramatthesejacksiscontrolledbytheMR78
frontpanelVOLUMEcontrol.Usetheseoutputjacks
toconnecttoexternalpoweramplifier,taperecorders,
oranyequipmentwherecontinuousfrontpanelcon-
troloftuneroutputvolumeisdesirable.
FrontPanelInformation
TheMR78Tunerhasthemostflexiblecontrolsys-
temeverdesignedina stereoFMtuner.Useofthese
controlswillgiveyoua higherlevelofperformance.
VOLUMECONTROLANDACPOWERSWITCH
TheVOLUMEcontrolhasbeenprecisiontracked
throughoutthelisteningrange(0to-65dB)foraccu-
ratestereobalance.
ItsetstheoutputlevelofthetunerFRONTPANEL
CONTROLLEDAUDIOOUTPUTjacks.TheFIXED
OUTPUTjacksarenotaffectedbytheVOLUMEcon-
trol.
TheACpowerswitchispartoftheVOLUMEcontrol.
Turningthevolumecontrolfullycounterclockwise
turnstheACpoweroff.
TUNINGDIALANDMETER
TheMR78hastwodialscales:
1.FMscale- marked88to108MHz
2.Loggingscale- marked0 to100
Theloggingscalecanbeusedtoaccuratelyretune
anystation.Youmayfinditeasiertokeepa record
ofyourfavoritestationsbyuseoftheloggingscale.
Asmallportionofthedialpointerhasbeenilluminated
toincreasetheeaseoftuning.AnFMstationistuned
correctlywhentheTUNINGmeterpointerisinthe
blackareaofthemeterscale.Thetuningindicationis
independentofthestationsignalstrength.Maximum
leftandrightpointerswing,whichoccursasyoutune
througha station,isdependentonthesettingofthe
SELECTIVITYswitch.
STEREOFUNCTIONINDICATOR
TheSTEREOFUNCTIONindicatorlightsredwhen
thedialpointeristunedtoorcrossesa stationbroad-
castingthe19,000Hzpilotsignalforstereo.Thespecial
circuitusedwilllightonlywhenthe19,000Hzmulti-
plexpilotispresent.Theindicatorwillnotlighton
noisepulsesorinterference.TheSTEREOFUNCTION
indicatorwilllightregardlessoftheMODEswitch
setting.
MODESWITCH
TheMODEswitchhasthreesettings.
STEREOONLY- Onlystationsbroadcastingthe
19,000Hzpilotsignalforstereocanbeheardinthis
position.Allmonophonicstationsandbackground
noisesaresuppressed.TheTUNINGmeterandSIGNAL
STRENGTHmeterfunctionwillindicatemonophonic
broadcastswhentuningbutnoprogramwillbeheard
fromtheloudspeakers.
MONO-InthispositionoftheMODEswitch,allbroad-
castswillbeheardmonophonically.Theautomatic
stereoswitchingisbypassed.TheSTEREOFUNCTION
indicatorwilllightwhena stationistransmitting
stereo,buttheprogramheardfromtheloudspeakers
willbemono.
STEREOAUTO- InthispositionoftheMODEswitch
bothmonoandstereobroadcastsareheard.Thetuner
automaticallyswitchesfrommonotostereomodeof
operation,dependingonthetypeofbroadcastbeing
received.
MUTINGSWITCHANDFUNCTIONINDICATOR
Mutingsuppressesthebackgroundnoiseandhiss
normallyheardbetweenstations.Turnthecontrol
6

toeitherLOCALorDISTANTpositionformuting.
Weakstationsthatmaynotoverridenoiseandinter-
ferencearealsosuppressedbythemuting.Inthe
OUTposition,themutingisturnedoff.Thisallows
tuningofweakstations,butthenoiseandinterference
willbepresent.
TheMUTINGFUNCTIONindicatorlightsamber
whenevertheMUTINGswitchissettoDISTANTor
LOCALItwillextinguishwhentheMUTINGswitch
issettoOUT.
FILTERSWITCHandFUNCTIONINDICATOR
TheFILTERSWITCHreducesnoiseonweakstereo
stations.
Position1 reducesnoisebyapproximately10dB.
Position2 reducesnoisebyapproximately20dB.
TheFILTERFUNCTIONindicatorlightsgreenwhen-
evertheFILTERswitchissetto1 or2.Itdoesnotlight
whentheFILTERswitchissettoOUT.
SIGNALSTRENGTH/MULTIPATHSWITCHAND
METER
TheSIGNALSTRENGTH/MULTIPATHmetercan
beswitchedtoindicateeitherrelativesignalstrength
orrelativeamountofmultipathinterference.With
METERswitchsettoSIGNALSTRENGTH,greater
pointerdeflectionindicatesa strongersignal.
WiththeMETERswitchsettoMULTIPATH,there
islittleornomultipathinterferencewhenthemeter
indicationremainssteadyandattheleftendofthe
scale.Ifthereisseveremultipathinterference,the
meterindicationwillfollowtheaudiblesignalinmuch
thesamemannerasa VUmeter.Whenthereceiving
antennaisrotatedtominimizemultipathinterference,
themeterindicationwilldecreaseandbecomesteady.
Ifthemultipathinterferenceiseliminated,themeter
indicationwillbenearzeroandsteady.
SELECTIVITYSWITCHANDINDICATOR
TheSELECTIVITYswitchallowsstereoreception
evenunderseverereceivingconditions.Settingthe
SELECTIVITYswitchtoNORMALconnectsa verylow
distortion8-poleRimofilterintheIFcircuit.Usethe
NORMALpositionoftheSELECTIVITYswitchfor
listeningtolocalbroadcasts.
SettingtheSELECTIVITYswitchtoNARROWadds
asharp8-poleRimofiltertotheNORMALIFfilterto
yielda lowdistortion(lessthan0.2%),highlyselective
16-polecompositeIFfilter.UsetheNARROWposi-
tionoftheSELECTIVITYswitchtoreduceanyinter-
ferenceduringthereceptionofdistantstations.
SettingtheSELECTIVITYswitchtoSUPERNAR-
ROWaddsa 4-pole4-zerocrystalfiltertothetwo
previouslymentionedfilters.UsetheSUPERNARROW
positiontoreceivestationsfromdistantcitieswhich
areonchannelsadjacenttolocalstations.Theremay
beusablesignalsneverheardbeforewithordinary
FMtuners.
TheSELECTIVITYindicatorlightsblue.TheIHF
AdjacentChannelSelectivitycharacteristicsare:
NORMAL7 dB;NARROW22dB;andSUPERNA-
ROW55dB.
SECONDARYCONTROLS
Onthetopofthechassisbehindthefrontpanelis
theDIALSCALEINTENSITYSwitchandANTENNA
MATCHSELECTORswitch.
DIALSCALEINTENSITYSWITCH
Adjustthebrightnessofthediallightsbymeansof
thisswitch.SettheswitchtoBRIGHTformaximum
panellight;settheswitchtoDIMforsubdueddial
light.
ANTENNAMATCHSELECTORSWITCH
UndermostconditionstheANTENNAMATCH
SELECTORswitchshouldbesettoHIGHGAINANT.
position.Onlyinextremefringeareasorwhenusinga
low-gain(indoor)antennashouldthisswitchbeset
toLOWGAINANT.position.WhensettoLOWGAIN
ANT.position,backgroundnoiseonweakstations
maybereduced.
7

ListeningtotheMR78Tuner
MONOPHONICFMPROGRAMS
ToreceivemonophonicFMbroadcastssetthe:
MODEswitchtoMONO
MUTINGswitchtoDISTANT
FILTERswitchtoOUT
SELECTIVITYswitchtoNORMAL
Adjustthetuningknobtothedesiredstation.The
stationisproperlytunedwhentheTUNINGmeter
pointercomestorestanywhereovertheblackarea
onthemeterscale.Whiletuningacrossthedialyou
maynoticemovementofthetuningmeterpointer
withouthearinga station.Thisiscausedbya station
soweakordistantthatitdoesnotover-ridetheback-
groundnoise.Extremelyweakstationsmaybeheard
bysettingtheMUTINGswitchtoOUT.
Rotatethedirectionalantennaforbestreceptionas
indicatedbya maximumpointerdeflectiononthe
SIGNALSTRENGTHmeteranda minimumpointer
deflectionontheMULTIPATHmeter.Thenadjustthe
VOLUMEcontroltothedesiredlisteninglevel.
STEREOFMPROGRAMS
Ifyouwishthetunertoautomaticallyswitchbe-
tweenstereoandmonobroadcasts,setthe:
MODEswitchtoSTEREOAUTO
MUTINGswitchtoDISTANT
SELECTIVITYswitchtoNORMAL
Inthismodeofoperation,settingtheFILTERswitch
toposition1 or2 willreducenoiseonweakstereo
stations.
WhentheSTEREOindicatorislighted,thestationis
broadcastinga 19,000Hzpilotsignalforstereoandthe
MR78willautomaticallyswitchtostereo.Ifa station
isnottransmittingthe19,000Hzpilotforstereo,
theSTEREOindicatorwillremainoffandthetuner
willautomaticallyswitchtomono.
WhentheMODEswitchissettoSTEREOONLY,
monophonicbroadcastsandbackgroundnoisebe-
tweenstationswillbemuted.Broadcastswiththe
19,000Hzpilotsignalwillactivateanautomaticstereo
sensingcircuit.Thestereoindicatorwilllightandthe
broadcastwillbeheard.
USINGVARIABLESELECTIVITY
Withtheincreasingnumberofstationscrowding
theFMbroadcastband,anFMtunerwithvariable
selectivityhasbecomea necessity.Localstationsare
receivedbestwiththeSELECTIVITYswitchinthe
NORMALposition.Thispositiongivesthelowest
distortion.Usethispositionforcriticaltapingoflive
localbroadcasts.
Ifyouexperience"crosstalk"or"splatter"ona sig-
nalfroma strongadjacentoralternatechannel,use
theNARROWpositionoftheSELECTIVITYswitch
toreducetheinterference.
The"SUPERNARROW"SELECTIVITYpositionis
usedtofindclearsignalsfromdistantstationsthat
cannotbeheardfromordinarytuners.
WhenyouuseSUPERNARROWSELECTIVITY,
youmayfindthatreceptionofdistantstationsadja-
centtoa channeltransmittingSCA(backgroundmusic
programs)isdifficult.StationstransmittingSCA
oftenhavewidesidebands.Thesesidebandsspillover
ontoadjacentchannelstations.IfyoutunetheMR78
tooneofthesidebandsoftheSCAtransmission,you
maybeabletohearthebackgroundmusicprogram
simultaneouslywiththeregularprogram.
8

PerformanceLimits
PERFORMANCEGUARANTEE- Performancelimits
arethemaximumdeviationfromperfectionpermitted
fora Mclntoshinstrument.Wepromiseyouthatthe
MR78youbuymustbecapableofperformanceat
orexceedingtheselimitsoryougetyourmoneyback.
Mclntoshistheonlymanufacturerthatmakesthis
guarantee.
TUNINGRANGE:88to108MHz
ANTENNAINPUTS:300ohmsbalanced;75ohms
unbalanced
INTERMEDIATEFREQUENCY(IF):10.7MHz
SENSITIVITY:2µVfor35dBofquieting;2.5µVat
100%modulation(±75KHzdeviation)for3%total
noiseandharmonicdistortion.
SIGNALTONOISERATIO:75dBbelow100%modu-
lation.
HARMONICDISTORTION:0.2%monoorstereoat
100%modulationto15,000Hz.Typically,0.05%at
1,000Hz
DRIFT:25,000Hzforthefirsttwominutes;thereafter
5,000Hzat25°Cin24hours.
FREQUENCYRESPONSE:MONO- ±1dB20Hzto
20,000Hzwithstandardde-emphasis,(75µS);
STEREO- ±1dB20Hzto15,000Hzwithstandard
de-emphasis<75µS)
CAPTURERATIO:0.25dBdetectoronly;2.5dBcom-
pletetuner
SELECTIVITY:IHF
SELECTIVITYSWITCHSETTING
SUPER
NORMALNARROWNARROW
AdjacentChannel7 dB22dB55dB
AlternateChannel55dB80dB80dB
SUPURIOUSREJECTION:100dBIHF
IMAGEREJECTION:100dB,88to108MHz.IHF
INTERMODULATIONDISTORTION:0.2%monoor
stereoforanycombinationoffrequenciesfrom20Hz
to15,000Hzwithpeakmodulationequalto100%or
less.Typically0.1%
9
MAXIMUMSIGNALINPUT:12voltsacross300
ohmantennaterminalswillnotincreaseharmonic
orintermodulationdistortion
AUDIOHUM:70dBdownfrom100%modulation
MUTING:70dBnoisereductionbetweenstations
MUTINGTHRESHOLD(Typical):Distantposition
5µV;Localposition20µV
SCAFILTER:50dBdownfrom67KHzto74kHz;
275dBperoctaveslope
STEREOSEPARATION:40dBat1,000Hz
STEREOFILTER(Typical):10dBnoisereductionin
Position1.
20dBnoisereductioninPosition2.
AUDIOOUTPUT:FrontPanelControlled:2.5volts
into47,000ohms.
FixedOutput:2.5voltsinto47,000ohms;1.0volt
into600ohms.
Alltunerperformancelimitsweremeasuredwith
SELECTIVITYswitchsetatNORMAL,unlessotherwise
stated.
GENERALINFORMATION
POWERREQUIREMENT:120volts,50/60Hz35watts
SEMICONDUCTORCOMPLEMENT:3 JFET's,2
MOSFET's,17BipolarTransistors,43Diodes,4 Inte-
gratedCircuits
MECHANICALINFORMATION
SIZE:FrontPanel:16incheswide(40.65cm)by5-7/16
incheshigh(13.8cm);Chassis:15incheswide(38.1
cm)by13inchesdeep(33.1cm),includingPANLOC
shelfandbackpanelconnectors;Knobclearance"
1-1/2inches(3.85cm)infrontofmountingpanel.
WEIGHT:27pounds(12.25kg)net,39pounds(17.69
kg)inshippingcarton.
FINISH:Frontpanel:Anodizedgoldandblackwith
specialgold/tealpanelnomenclatureillumination;
Chassis:Chromeandblack
MOUNTING:MclntoshdevelopedprofessionalPAN-
LOC

TechnicalDescription
TUNINGMECHANISMANDDIALDRIVE
IntheMR78,theuniquedesignandcarefulmanu-
factureofthemechanicaldialdriveassemblygives
smoothflywheeltuning.
Bycontrollingtherelationshipofmassandmechan-
icalresistance,andbydividingtheworkloadsinthe
dialdrivesystem,itbecomesnearlyimpossibleto
detectanybacklash.Yet,theentiredialdriveisa
modelofmechanicalstability.
Foraddedeaseandincreasedtuningaccuracy,a
sectionofthedialpointerisilluminated.
FRONTEND
FMsignalsenterthetunerthroughtheantennater-
minals.A coaxialcableconductsthesignaltotheRF
amplifier.Intheamplifierthestationsareseparated,
amplified,andconvertedtoa common10.7MHzIF
frequency.Insteadofusinga conventionaleasily
overloadedtransistororFETasa straightRFampli-
fier,theMR78usesa ruggedJunctionFieldEffect
Transistorasanimpedanceconvertertodrivea 5-watt
powertransistor.Thiscombination(acascodecir-
cuit)makestheRFamplifiervirtuallyimpossibleto
overloadorcross-modulate.Asanexample,ifyou
aretunedtoa 3 microvoltsignalat96.3MHz,theMR
78willrejectsignalselsewhereonthedialwhichare
atleast4,000,000timesstronger.Thus,a 12voltsignal
receivedat104.3MHzwouldnotbotherthesignalat
96.3MHz.Whentunedto104.3MHz,thetunerwill
notoverload.AllMR78tunersmustpassthe12volt
overloadtest.
TheMR78hasanantennamatchingcircuitinthe
RFinput.Infringeareasa broadlow-lossRFcircuit
providesmaximumsensitivityfora low-gainantenna.
Inmetropolitanareascrowdedwithmanystrongsig-
nals,a sharpRFinputcircuitgreatlyreducesspurious
responsesandachievesmaximumsensitivitywhen
connectedtoa high-gainantenna.
TheRFbandpassoftheantennamatchingcircuit
canbemodifiedbyswitchingtheresonatorL1C1in
oroutofthecircuit.Broadbandpassisobtainedbyset-
tingtheANTENNAMATCHINGswitchtotheLOW
GAINANT.position.Thisconnects+24voltstothecir-
cuitturningdiodeD1offanddiodesD2,D3on.Inthis
configurationtheantennaisconnecteddirectlytothe
RFamplifierinputresonatorL2C2.DiodeD3istapped
onL2toyieldoptimumnoise,whileD2shortsreson-
atorL1C1soitslossisnotcoupledintothecircuit.
SharpbandpassisobtainedbysettingtheANTENNA
MATCHINGswitchtotheHIGHGAINANT.position.
Thisconnects-12voltstothecircuit,turningdiode
D1onanddiodesD2,D3off.TheRFinputcircuit
becomesa doubletunedfilterwithhighQ inboth
resonators,whichprovidessharpbandpass.DiodeD1
istappedonL1toyieldoptimumselectivity.Theoscill-
ographbelowshowsthetwobandpasscurvesofthe
RFinputcircuit.
12

AnothernewdesignintheMR78frontendisthe
integratedcircuitbalancedmixer.Thismixerisprac-
ticallyimpossibletooverload.Oscillatorpulling,
crossmodulation,andothertypesofdistortionsocom-
moninordinarytransistororFETmixersisminimized.
Abalancedbifilartransformercouplesthe10.7MHz
outputtotheIFamplifier.
CompletingtheMR78frontendistheoscillator.
Itusesa high-Qceramicformtankcoil.Theoscillator
isfreeformspuriousradiations,andoperatesathigh
efficiency.AFCisnotrequired.
RIMOFILTERIFAMPLIFIER
TheMR78useslinear-phaseIFfilters.TheRimo
filter,designedfroma FORTRANcomputerprogram,
isthemostgeneralclassofminimum-phaseconstant-
delayfilter.Thefilterdesignhasgreatmathematical
complexity.UsingnumericalintegrationintheS-plane,
an"IBM"1130highspeedcomputerspenteighteen
minutesonthemathematicsforthedesignoftheIF
filter.Itwouldhavetakenanengineer,workingtwen-
ty-fourhoursa day,sevendaysa week,andworking
error-freethree-hundredyearstoperformthesame
mathematicalcalculations.
TheRimofilterhasflattimedelayinitspassband
region.AnyerrorintimedelaycausesFMdistortion.
AllotherIFfiltershavedelaydistortion,asmuchas
100%.TheMR78filterhaslessthan1.0%delaydis-
tortionfromantennainputtodiscriminatoroutput.
ConstantdelayintheIFfilterbandpass(whichis
equivalenttolinearphase)isessentialforlowdistor-
tionFMreception.TheMR78tuneruseslinearphase
(Rimo)filtersatNORMALandNARROWpositionsof
theSELECTIVITYswitchsettings.
Foralmostallreceptionconditionsthetunerpro-
videsstereosignalsthathavethelowestpossible
distortion.
Constantdelaydesigntechniquesareusedinthe
4-pole4-zerocrystalfilterusedintheSUPERNARROW
position.Howeverduetotheextremeselectivityof-
fered,delaydistortionisveryslightlyincreasedover
theotherselectivitypositions.
13

TheMR78hasthenarrowestIFbandwidtheverused
ina stereotuner.Itisthecorrectwidthtoletjustone
FMstationthrough.
TheexcellentselectivityoftheMR78{210kHz
wideat60dBdown)permitstuningstationsthatare
impossibletoreceiveonordinarytuners.
LINEAR-PHASEBRIDGEDISCRIMINATOR
TheadvantagesoftheRimofilterwouldbelostif
ithadtoworkintoanordinaryFMdetector.A new
detectorhadtobedeveloped.Itisthelinearphase
bridgediscriminator.Thelinear-phasebridgedis-
criminatorusesa balancedtransmissionlinebridge
inconjunctionwitha differentialvoltage-doubling
rectifiertoacheivenearlydistortionlessdemodulation
ofFMsignals.Mclntoshhasa USpatentonthisnew
FMdetector.
TheresistorsR1andR3forma Wheatstonebridge
incombinationwiththetwotransmissionlinesections.
TheRFvoltagesE1andE2atthejunctionsofthere-
sistorsandthetransmissionlineswillvarywithfre-
quencyinsucha mannerthattheirRMSarithmetic
difference(E1- E2)isa straight-linefunction.Thus
theseRFvoltagesarepeakrectifiedbyanamplitude
detector,andthedifferencetaken,producinga dis-
tortionlessrecoveryoftheFMinformation.
Ordinarystereofiltersusea singlecapacitor.The
MR78usesanelaboratetwin-Tbandpassfilter.This
circuitismuchmoreeffectiveinsuppressingnoise
andstillpreservingthestereoimage.
AUDIOPREAMPLIFIERSECTION
Theaudioamplifierincreasesthelevelofthepro-
gramadequatetodrivea preamplifierorotheracces-
soryequipment.Itconsistsoftwo,two-transistoramp-
ifiers,oneforeachchannel.Thedesignusesconsider-
ablenegativefeedbacktohelpachievelowdistortion,
widefrequencyresponse,andexcellentstability.Each
audioamplifierdelivers2.5voltstotheFIXEDOUT-
PUTjacks.A secondpairofoutputsareavailable
wherelevelcanbevariedbytheVOLUMEcontrol.
POWERSUPPLY
Specialdesignattentionhasbeengiventothepower
supplysection.Twoseparatepowercircuitsareused.
Thefirstisa 24voltregulatedsupply.
The24voltregulatoruseselectronicfilteringtoinsure
thelowestpossiblebackgroundhumlevel,maximum
stabilityandextremelygoodregulation.Allsignal
stagesarepoweredfromthisregulator.
Thesecondisa halfwaverectifierwithelectronic
filterwhichsuppliesDCtothemultiplexdecoder.
BASICLINEARPHASEBRIDGEDISCRIMINATOR
PerformanceoftheMR78bridgediscriminatorhas
reduceddistortiontonearlythetheoreticalzero.In
additiontoitsexcellentdistortion-freeperformance,
thebridgediscriminatoralsoexhibitsa captureratio
closeto0 dB.
STEREODECODER
Anintegratedcircuitaudioamplifierwithover120
dBofnegativefeedbackintwoloopsisusedtodrive
thestereodemodulator.Thisnewcircuitkeepsthe
distortionlowinthestereomodeofoperation.Highly
stablepilotrecoverycircuitsareusedtominimize
19,000Hzintermodulationbeats.MUTING,STEREO
ONLY,andAUTOMATICSTEREO/MONOswitching
arefunctionsperformedwithinthestereodecoder.
TheSTEREOFILTERisconnectedintheaudioamp-
lifiertoreducenoisewhenlisteningtoweakstereo
stations.CarefuldesignoftheSTEREOFILTERcircuit
permitsanidealcompromisebetweenchannelsepa-
rationandnoiserejection.
14

McINTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903
607-723-3512
Designsubjecttochangewithoutnotice.
PrintedinU.S.A.
038-580