McIntosh MR 75 User manual

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THEMR75AM/FMTUNER

VARIOUSREGULATORYAGENCIESREQUIRETHATWEBRINGTHEFOLLOWING
INFORMATIONTOYOURATTENTION.PLEASEREADITCAREFULLY.
WARNING:TOPREVENTFIREORSHOCK
HAZARD,DONOTEXPOSETHISUNITTO
RAINORMOISTURE.
TheMclntoshyouhavepurchasedisa ModelMR75.
Ithasa serialnumberlocatedontherearpanelofthe
chassis.Recordthatserialnumberhere:
SerialNumber
Themodel,serialnumberandpurchasedateare
importanttoyouforanyfutureservice.Recordthe
purchasedatehere:
Purchasedate
Uponapplication,MclntoshLaboratoryprovidesa
Three-YearServiceContract.YourMclntoshauthor-
izedServiceAgencycanexpediterepairswhenyou
providetheServiceContractwiththeinstrumentfor
repair.Toassist,recordyourServiceContractnumber
here:
ServiceContractNumber

YourMR75AM/FMTuner
cangiveyoumanyyearsof
satisfactoryperformance.Ifyou
haveanyquestions,pleasecontact:
CUSTOMERSERVICE
MclntoshLaboratoryInc.
2ChambersStreet
Binghamton,NewYork13903
Phone:[phone]
TakeAdvantageof3 years
ofContractService-
FillintheApplicationNOW.
Contents
SERVICECONTRACTINFORMATION1
INSTALLATION2,3
HOWTOCONNECTAND
BACKPANELINFORMATION4-6
FRONTPANELINFORMATION
ANDUSEOFCONTROLS6,7
PERFORMANCELIMITS8
PERFORMANCECHARTS9,10
TECHNICALDESCRIPTION11,12
BLOCKDIAGRAM13
Copyright1981© byMclntoshLaboratoryInc.1
McINTOSHTHREEYEARSERVICECONTRACT
AnapplicationforA THREEYEARSERVICECONTRACTisincludedwiththismanual.
Thetermsofthecontractare:
1.Mclntoshwillprovideallparts,materials
andlaborneededtoreturnthemeasured
performanceoftheinstrumenttothe
originalperformancelimits.TheSER-
VICECONTRACTdoesnotcoverany
shippingcoststoandfromtheauthoriz-
edserviceagencyorthefactory.
2.AnyMclntoshauthorizedserviceagency
willrepairMclntoshinstrumentsatnor-
malservicerates.Toreceiveservice
underthetermsoftheSERVICECON-
TRACT,theSERVICECONTRACTCER-
TIFICATEmustbepresentedwhenthe
instrumentistakentotheserviceagency.
3.Alwayshaveservicedonebya
Mclntoshauthorizedserviceagency.If
theinstrumentismodifiedordamaged
asa resultofunauthorizedrepair,the
SERVICECONTRACTwillbecancelled.
Damagebyimproperuseormishandl-
ingisnotcoveredbytheSERVICE
CONTRACT.
4.TheSERVICECONTRACTisissuedto
youastheoriginalpurchaser.Topro-
tectyoufrommisrepresentation,this
contractcannotbetransferredtoa se-
condowner.
5.ToreceivetheSERVICECONTRACT,
yourpurchasemustbemadefroma
Mclntoshfranchiseddealer.
6.Yourcompletelyfilledinapplicationfor
theSERVICECONTRACTmustbepost-
markedwithin30daysofthedateof
purchaseoftheinstrument.
7.ToreceivetheSERVICECONTRACT,all
informationontheapplicationmustbe
filledin.TheSERVICECONTRACTwill
beissuedwhenthecompletelyfilledin
applicationisreceivedbyMclntosh
LaboratoryIncorporatedinBinghamton,
NewYork.
8.UnitsinoperationoutsidetheUnited
StatesandCanadaarenotcoveredby
theMclntoshFactoryServiceContract,
irrespectiveoftheplaceofpurchase.
Norareunitsacquiredoutsidethe
U.S.A.andCanada,thepurchasersof
whichshouldconsultwiththeirdealer
toascertainwhat,ifany,servicecon-
tractorwarrantymaybeavailablelocally.

THEMR75AM/FMTUNER
Installation
ThePANLOCsystemofinstallingequipmentcon-
venientlyandsecurelyisa directresultofMclntosh
research.BydepressingthetwoPANLOCbuttonson
thefrontpanel,theinstrumenteithercanbelocked
firmlyinplaceoritcanbeunlockedsothatthe
chassiscanslideforward,givingyoueasyaccessto
thetopandrearpanels.
Thetrouble-freelifeofanelectronicinstrumentis
greatlyextendedbyprovidingsufficientventilationto
preventthebuild-upofhighinternaltemperatures
thatcausedeterioration.Allowenoughclearanceso
thatcoolaircanenteratthebottomofthecabinet
andbeventedfromthetop.Withadequateventilation
theinstrumentcanbemountedinanyposition.The
recommendedminimumspaceforinstallationis15
inches(38.1cm)deep,17inches(43.2cm)wide,and6
inches(15.2cm)high.
Toinstalltheinstrumentina Mclntoshcabinet,
followtheinstructionsthatareenclosedwiththe
cabinet.Foranyothertypeofinstallation,follow
theseinstructions:
1.OpenthecartonandremovethePANLOCbrack-
ets,hardwarepackage,andmountingtemplate.Re-
movetheMR75fromitsplasticbagandplaceitup-
sidedownontheshippingpallet,thenunscrewthe
fourplasticfeetfromthebottomofthechassis.
2.Markthecabinetpanel
Placethemountingtemplateinthepositiononthe
cabinetpanelwheretheinstrumentistobeinstalled,
andtapeitinplace.Thebrokenlinesthatrepresent
theoutlineoftherectangularcutoutalsorepresent
theoutsidedimensionsofthechassis.Makesure
theselinesclearshelves,partitions,oranyequip-
ment.Withthetemplateinplace,firstmarkthesixA
andB holesandthefoursmallholesthatlocatethe
cornersofthecutout.Then,jointhefourcornermark-
ingswithpencillinesusingtheedgeofthetemplate
asa straightedge.
3.DrillHoles
Usea drillwitha 3/16inchbitheldperpendiculartothe
panelanddrillthesixA andB holes.Then,usinga drill
bitslightlywiderthanthetipofyoursawblade,drill
oneholeateachoftwodiagonallyoppositecorners.
Theholesshouldbarelytouchtheinsideedgeofthe
penciledoutline.Beforetakingthenextstep,make
surethatthesixA andB holeshavebeendrilled.
2

4.SawthePanelCutout
Sawcarefullyontheinsideofthepenciledlines.First
makethetwolongcutsandthenthetwoshortcuts.
Aftertherectangularopeninghasbeencutout,usea
filetosquarethecornersandsmoothanyirregulari-
tiesinthecutedges.
5.InstalltheMountingStrips
Inthehardwarepackageyouwillfindtwomounting
stripsandtwosetsofmachinescrews.Forpanels
thatarelessthan½ inchthick,usethe¾ inch
screws;forpanelsthataremorethan½ inchthick,
usethe1 ¼ inchscrews.
Startingattheright-handsideofthepanel,inserta
screwofproperlengthintothecenterholeinthe
panel,markedB onthetemplate.Onthebackofthe
panel,aligna mountingstripwiththeholesinthe
panelandtightenthescrewuntilthescrewheadis
pulledslightlyintothewood.
Repeatthisproceduretoattachthemountingstrip
totheleftsideofthepanel.
6.AttachthePANLOCBrackets
UsingtwoscrewsofproperlengthintheA holeson
eachside,attachthePANLOCbracketstothecabi-
netpanel;theshortflangeismountedagainstthe
front(face)ofthecabinetpanel.Thescrewspass
throughthePANLOCbracketflange,thecabinetpan-
el,andthenthroughthemountingstripspreviously
mounted.
7.InstalltheInstrument
GuidetheACpowercordthroughthepanelopening
tothebackofthecabinet;then,slidetheinstrument
intotheopeningcarefullysothattherailsonthebot-
tomofeachsideofthechassisengagethetrackson
themountingbrackets.Continuetoslidetheinstru-
mentintothecabinetuntilitisstoppedbytheadjust
positionlatches.Pressthelatchesinward,thisper-
mitstheinstrumenttoslideintothecabinetuntilits
frontpanelisflushwiththecabinetpanel.Depress
thePANLOCbuttonsatthelowerleftandrightcor-
nersoftheinstrumentpaneltolocktheunitfirmlyin
thecabinet.DepressingthePANLOCbuttonsagain
willunlocktheinstrumentsothatitcanslideforward
totheadjustposition;ifyoupressinwardonthead-
justpositionlatchesthenyoucanremovetheinstru-
mentfromthecabinet.
3

HowtoConnectand
BackPanelInformation
Refertothedrawingonpage5.
AUDIOOUTPUTS
UsetheFIXEDOUTPUTjacksontherearpanelto
feedprogramtoa stereocontrolpreamplifierorother
equipmentwhichhasitsownvolumecontrol.The
frontpanelVOLUMEcontroldoesnotaffecttheloud-
nessofthetunerattheFIXEDOUTPUTjacks.Theout-
putlevelisa nominal1 voltfor100%FMmodulation.
UsetheVARIABLEOUTPUTjackstoconnectto
equipmentsuchasa poweramplifierora taperecorder
wherecontrolofthevolumeatthetunerisdesired.The
outputattheVARIABLEOUTPUTjacksisa nominal2.5
voltsfor100%FMmodulation.Thereisnodifferencein
thesignalqualityateitherpairofoutputjacks.
BothpairsofOUTPUTjacksmaybeusedsimulta-
neously.Theoutputimpedanceatbothoutputsis
verylowsothatlongaudiocablescanbeusedwith-
outa lossofhighfrequenciesduetocablecapacity.
CONNECTINGANFMANTENNA
Oneofthreeantennasystemscanbeused:(1)an
outdoorFMantenna,or(2)anVHF-TVantenna,or(3)
theindoordipolesupplied.
Anoutdoorantennaisrecommendedforoptimum
performanceinallareas.Infringe(outlying)areas,
bestresultswillbeobtainedwitha highlydirectional
FMantennausedinconjunctionwitha rotator.Ifthe
antennausesa 300ohmlead,connectittothe
ANTENNA300FMpushconnectors.
Thecoaxialcableofanunbalanced75ohmantenna
connectstotherearpanelANTENNA75ftFMtypeF
coaxialconnector.
Anoutsideantennasystemshouldnotbelocated
inthevicinityofoverheadpowerlinesorotherelec-
triclightorpowercircuits,orwhereitcanfallinto
suchpowerlinesorcircuits.Wheninstallinganout-
sideantennasystem,extremecareshouldbetakento
keepfromtouchingsuchpowerlinesorcircuitsas
contactwiththemmightbefatal.
AVHF-TVantennacanbeeffectivewhenitis
designedforbothFMandTVreception.Connectthe
twoleadsfromtheVHF-TVantennatotheANTENNA
300ftFMpushconnectors.
Theflexiblefoldeddipoleantenna(300ohm)sup-
pliedwiththeMR75isforuseinurbanorhigh
strengthsignalareas.Connectthetwoleadsfromthe
dipoletotheANTENNA300FMpushconnector.
Theflexibilityofthetwinflatwireassemblypermits
ittobeplacedundera rug,tackedbehindthestereo
orplacedinanyotherconvenientlocation.Insome
cases,itmaybenecessaryto"position"theantenna
forbestsignalreception.Thisshouldbedonebefore
itispermanentlylocated.
Avoidlocatingtheantennanexttootherwiresor
metalobjects.Anyindoorantennamaybeineffective
inhouseshavingmetalsidingormetalfoilinsulation.
FMPRESELECTORSWITCH
FMreceiverscanbeoverloadedbyverylargeanten-
nainputsignalswhenthereceiverislocatedvery
neartoa FMbroadcastingstationorwhena highgain
directionalantennaisusedina metropolitanarea.An
exampleofoverloadiswherea FMstationcanbetun-
edinattwoormoreplacesonthedial.A preselector
tunedcircuitisbuiltintotheMR75toprevent
overload.
Thepreselectormaybeswitchedintooroutofthe
FMRFcircuitbya slideswitchlocatedontherear
panel.
WerecommendtheFMPRESELECTORswitchbe
usedinthe"out"positionforallFMlisteningcondi-
tionsexceptwherethereisevidenceofoverloadby
strongstations.Inthatcase,the"in"positionwill
protectagainstoverload.Formoststationstherewill
benolisteningdifferencebetweenthetwoswitch
positions.
AMANTENNA
ForlocalandmostmoderatelydistantAMrecep-
tion,thebuilt-inferriteloopstickantennamaybe
used.TheAMloopstickantennaisona swivelbase
andmustbeadjustedawayfromthechassisforbest
reception.Theillustrationatthetopofpagesix
showsexamplesofthemanypositionstheAManten-
namaybeplacedin.Tryseveralpositionsuntilyou
locatethebestperforminglocationforyourlistening
area.
Distantreceptioncanbeimprovedwiththeuseofa
copperantennawire50to150feetinlength.Suspend
thewireina straightlineashighaspossible.Attach
thewireateachendwithsuitableglassorceramicin-
4

ALTERNATE
AMANTENNA
TO
EXTERNAL
GROUND
FMANTENNA
OPTIONAL
75ANTENNA
LEAD-IN
TOAC
POWER
SOURCE
MAXIMUMPERFORMANCE
INDICATOR
OR
OSCILLOSCOPE
LEFTSPEAKER
PREAMPLIFIER
RIGHTSPEAKER
5
POWERAMPLIFIER

sulators.Connecta lead-inwireatanyconvenient
pointontheantenna.Itisrecommendedthata
lightingarresterbeusedwithanoutdoorAMantenna.
Thearrestershouldbewellgroundedtoa groundrod
orcoldwaterpipe.
CONNECTINGA MAXIMUM
PERFORMANCEINDICATOR
Thescopeverticalandhorizontaljacksontherear
panelareusedtoconnecta Mclntoshmaximumper-
formanceindicatororanoscilloscope.Followdirec-
tionsoutlinedinthemaximumperformanceindicator
owner'smanual.Themaximumperformancein-
dicatorgivesa displayofsignalstrengthversusfre-
quencyandallowsadjustinga directionalantennafor
thebestmultipathfreeFMreceptioncondition.
UNSWITCHEDACPOWEROUTLET
Thisoutletmaybeusedtopowerauxiliaryequip-
mentsuchasanantennarotatorora maximumper-
formanceindicator.Powerisavailablewheneverthe
MR75isconnectedtoa powersource.
FUSE
A0.5AMPfuseprotectstheMR75circuits.The
fusedoesnotprotectadditionalequipmentcon-
nectedtothebackpanelACoutlet.
FrontPanelInformationand
UseofControls
TheMR75Tunerhasa flexiblecontrolsystem.Pro-
peruseofthesecontrolswillyielda highlevelofper-
formance.
INPUTSELECTOR
Thetwopositioninputselector
functionsasfollows:
AM:ConnectstheAMtunercir-
cuitsforAMreception.
FM:ConnectstheFMtunercircuits
forFMstereoormonoreception.
MODE:
Themodeswitchhastwoposi-
tions:STEREO- Inthisposition
thetunerautomaticallyswitches
fromFMmonotoFMstereode-
pendingonthetypeofbroadcast
beingreceived.MONO- Inthis
positionofthemodeswitch,all
broadcastsarereceivedmonophonically.
MUTE:
Themuteswitchhastwoposi-
tions:ON-backgroundnoiseand
hissheardbetweenFMstations
issuppressed.OFF-inter-station
mutingisdefeated.
VOLUMECONTROLAND
ACPOWERSWITCH
TheVOLUMEcontrolhasbeen
precisiontrackedthroughoutthe
listeningrange(0to-65dB)for
accuratestereobalance.Itsets
theoutputleveloftheVARIABLE
OUTPUTjacksandthefrontpanel
HEADPHONEjack.TheFIXEDOUT-
PUTjacksarenotaffectedbythepositionofthe
VOLUMEcontrol.TheACpowerswitchispartofthe
VOLUMEcontrol.TurningtheVOLUMEcontrolfully
counterclockwiseturnstheACpoweroff.
6

FRONTPANELINDICATORS
TheMR75hasthreedialscales:1.AM- marked550
to1600kHz;2.FM- marked88to108MHz;3.Logging
scale0 to100.
Asthetuningknobisturned,thepointermoves
acrossthedial.TherearetwoLEDsonthepointer.
WhenyouhaveselectedFM,oneLEDglowsbehind
theFMscale,onAMtheotherglowsbehindtheAM
scale.
TheLoggingScaleisusefulforretuningtoyour
favoritestationwithgreataccuracy.
TUNINGKNOB
TheTuningKnobselectsAMorFMstations.AnAM
stationistunedincorrectlywhentheverticalcolumn
signalstrengthindicatorshowsa maximum.AnFM
stationiscorrectlytunedwhenthecenterbarin-
dicatoratthebottomofthesignalstrengthcolumnil-
luminatesandthesignalstrengthdisplayisata max-
imum.Thereareindicatorsmarked+ and- oneach
sideoftheFMcenterbar.Theselightasyouap-
proachFMstationsandindicateifyouareabove( + )
orbelow(-)thestation.Whenthecentertuningbar
lights,theFMAFL(automaticfrequencylock)circuit
isactiveandthetunerislockedtotheFMstation.
ThislockedconditionwillgivethebestFMreception
withlowestbackgroundnoise,lowestdistortionand
beststereoseparation.
BecauseofthelockingactionoftheAFLcircuit,
theTuningKnobcanbetunedmechanicallyslightly
awayfromthefrequencyoftheFMstationwithout
detuningtheMR75.ThismakestuningtheMR75very
easybutcancausetheMR75tonotreturntotheFM
stationwhenpoweristurnedoffandonagain.This
conditioncanbeavoidedif,whentuningina station,
youstopturningtheTuningKnobassoonasthe
centerbarindicatorlights.ThentheMR75willreturn
tothesamestationwhenpoweristurnedonagain.
INPUTINDICATORS
Atthebottomrightofthetuningdialareaarethree
LEDlamps;twoindicatethestatusoftheSELECTOR
switch.TheilluminatedLEDindicateswhetherAMor
FMisselected.
TheremainingLEDmarked"mute"indicateswhen
themuteswitchisinthe"on"position.
SOLIDSTATETUNINGINDICATOR
TheMR75utilizesanallsolidstatetuningin-
dicator.FourteenLEDsmakeupthedisplay,which
consistsoffourbarsand10smalldots.
Thebaratthetopindicatesreceptionofa FM
stereomultiplex(MPX)station.The10littledotsar-
rangedina verticalcolumnindicaterelativestation
signalstrengthforAMorFM.Thethreebarsatthe
bottomofthedisplayindicate ifyouaretunedabove
(+),below(-),orrighton-stationwhenreceivingFM.
Whentuninga FMstation,theproperadjustmentis
whenonlythecenteron-stationverticalbarindicator
lights.
HEADPHONEJACK
AHEADPHONEjackisprovidedtodrivelowim-
pedancedynamicheadphonesfromaninternalpower
amplifier.
7

PerformanceLimits
Performancelimitsarethemaximumdeviationfrom
perfectionpermittedfora Mclntoshinstrument.We
promiseyouthatwhenyoupurchasea newMR75
froma MclntoshFranchiseddealeritwillbecapable
ofperformanceatorexceedingtheselimitsoryou
canreturntheunitandgetyourmoneyback.
Mclntoshistheonlymanufacturerthatmakesthis
statement.
FMSECTION
Tuning
88to108MHz
AntennaInputs
One300ftbalancedandone75unbalanced.
IntermediateFrequency
10.7MHz
UsableSensitivity
2µM(11.2dBf)IHF
QuietingSensitivity
5µV(19.1dBf)IHF-50dB(Mono)
50µV(39.5dBf)IHF-50dB(Stereo)
SignalToNoiseRatio
70dBIHFminimumbothMonoandStereo
HarmonicDistortion
0.18%(Mono)IHFmaximum
0.38%(Stereo)IHFmaximum
AlternateChannelSelectivity
75dBIHFminimum
ImageRejection
100dBIHFminimum
StereoSeparation
45dBminimumat1 kHz
AudioFrequencyResponse
20Hzto15kHz+0,- 1 dB
CaptureRatio
1.8dB
SpuriousRejection
100dBIHFminimum
SCARejection
60dBminimum
AMSECTION
Sensitivity
75µVIHFwithexternalantenna
SignalToNoiseRatio
45dBminimumIHFor55dBat100%modulation
FrequencyResponse
+0 - 6 dBfrom20Hzto3500Hz
HarmonicDistortion
0.8%maximumat30%modulation
AdjacentChannelSelectivity
30dBminimumIHF
ImageRejection
65dBminimumfrom550kHzto1600kH
GENERALINFORMATION
AudioOutput
Variable:2.5Vinto5k
Fixed:1Vinto5k
AudioHum
75dBdownfrom100% modulation
PowerRequirement
120Volts50/60Hz,20Watts
SemiConductorComplement
22BipolarTransistors
4FieldEffectTransistors
23SiliconDiodes
15IntegratedCircuits
19LED's
2BridgeRectifiers
MECHANICALINFORMATION
SIZE:
Frontpanelmeasures16incheswide(40.6cm)by5
7/16incheshigh(13.8cm).Chassismeasures143/4
incheswide(37.5cm)by4 13/16incheshigh(12.2cm)
by13inchesdeep(33cm),includingconnectors.
Knobclearancerequiredis1 1/4inches(3.2cm)in
frontofmountingpanel.
FINISH:
Frontpanelisanodizedgoldandblackwithspecial
gold/tea!nomenclatureillumination.Chassisisblack.
MOUNTING:
ExclusiveMclntoshdevelopedprofessionalPANLOC.
WEIGHT:
23pounds(10.43kg)net,35pounds(15.88kg)in
shippingcarton.
8

PerformanceCharts
FMSIGNALPERFORMANCE
0
-10
-20
-30
-40
-50
-60
-70
RELATIVEOUTPUTINdB
0.11 101001k10k
SIGNALINPUTINMICROVOLTS
F.M.HARMONICDISTORTION
100%MODULATION
0.5
0.4
0.3
0.2
0.1
0
201001k10k20k
FREQUENCYINHERTZ
HARMONICDISTORTIONINPERCENT
RESPONSEANDSEPARATIONINdB
+10
0
-10
-20
-30
-40
-50
F.M.FREQUENCYRESPONSEAND
STEREOSEPARATION
201001k10k20k
9
FREQUENCYINHERTZ

A.M.FREQUENCYRESPONSE
80%Modulation
201001k10k20k
RFINPUTUSINGDUMMYANTENNAINMICROVOLTS
AMMINIMUMUSABLE
SENSITIVITY(IHF)
AMSELECTIVITY
AT1000kHz
9709809901000101010201030
10
FREQUENCYINHERTZ
FREQUENCYINkHz
50060070080090010001100120013001400150016001700
0
120
100
80
60
40
20
RELATIVEOUTPUTINdB
0
-10
-20
-30
-40
-20
-40
-60
-80
-100
RELATIVEOUTPUTINdB
FREQUENCYINkHz

TechnicalDescription
FMSECTION
Theradiofrequency(RF)assemblyhousesthecom
pleteFM-RFfrontendandpartoftheAM-RFcircuitry.
Aneightsectionvariablecapacitoristheheartofthe
RFassembly.Fivesections,1,2,4,6 and8 areused
forFMandthreesections,3,5 and7 areforAM.Sec-
tions1 and2 ofthecapacitorarefortheFMantenna
RFinputcircuits.Section2 isalwaysinuse,however,
ifadditionalRFselectivityisdesiredtoprotect
againststrongsignaloverload,additionalpreselector
filteringisaccomplishedbyaddingsection1.Thisis
accomplishedbyoperationofthepreselectorswitch
ontherearpanel.Thisswitchdirectsa DCvoltage
thatcontrolsPINsemiconductordiodestoperform
theRFswitchingfunctionselectronically.
AMOS-FETRFamplifierfollowssection2 topro-
videexceptionalsensitivitywhilemaintainingsuper-
iorcross-modulationrejectionovera widedynamic
range.Capacitorsections4 and6 followtheRFampli-
fiertoincreaseselectivityandtoprovidecorrectim-
pedancematchingforthefollowingfullybalanced
MOS-FETmixer.
Thelocaloscillatoristunedbycapacitorsection8.
AMOS-FETbufferamplifierisemployedbetweenthe
oscillatorandthemixer.Thehighinputimpedanceof
thisamplifierinsuresthatoscillatorloadingis
minimal,therefore,oscillatorstabilityisgreatly
enhanced.Theoscillatorisfinetunedbya varactor
diodeoperatedbya correctionvoltagegeneratedby
theAutomaticFrequencyLock(AFL)circuit.AFLisa
Mclntoshpatentedcircuitthatturnsona lockvoltage
whenmanualtuningreachesthecenteroftheFMsta-
tioncarrier.ThisAFLtuningaidinsuresthetuningis
accurateforminimumdistortionandbestperformance.
TheFM-IFsystemthatfollowsthemixerconsistsof
fiveintegratedcircuitamplifiersandfourpiezoelectric
filters.Theycombinetogivea totalamplificationof
140dB.TheIF responsecharacteristicis veryselec-
tive,being170kHzwideat-3dBandonly500kHz
wideat-50dB.Theresponseissymmetricalabout
thecenterfrequencyof10.7MHz.TheIFfiltersareper-
manentlysealedandcannotdriftnorvibrateoutofad-
justment.TheexceptionallyhighgainoftheIFsystem
assuresproperlimitingatallinputsignallevels.A
Foster-SeelydiscriminatorcompletestheIFsystem.
Thedetectedoutputofthisdiscriminatorhasa high
signaltonoiseratioandextremelylowdistortion.This
outputfeedstotheFMstereomultiplexsection.
FMSTEREOMULTIPLEX
Theheartofthemultiplexsectionisa newthird
generationphaselockedloop(PLL)stereodecoderin-
tegratedcircuit(IC).ThisPLLICincorporatestwo
specialsystems,anautomaticvariableseparation
controlcircuittoreducebackgroundnoisewhen
receivingweakstereostations,andtri-leveldigital
waveformgenerationwhicheliminatesinterference
fromSCAsignalsandfromthesidebandsofadjacent
channelFMsignals.
Thevariableseparationcontrolisoperatedfrom
theIFamplifier'ssignalstrengthdetector.A smooth
transitionisprovidedfrommonotostereoorfrom
stereotomonoatweaksignallevelstoprovidethe
optimumsignaltonoiseratioandbeststereosepara-
tionfortheprevailingsignalconditions.Thecircuit
operatesonlyduringstereoreception.Itswitches
automaticallytomonophonicifthe19kHzpilottone
isabsent.
InthePLLtheinternaloscillatoroperatesat228
kHz,lockedontothe19kHzpilottone.The228kHz
feedsa 3 stageJohnsoncounterviaa binarydivider
togeneratea seriesofsquarewaves.Suitablycon-
nectedNANDgatesandexclusiveORgatesproduce
thetri-leveldrivewaveformforthevarious
demodulatorsinthecircuit.Theusualsquare
waveformshavebeenreplacedinthePLLand
decodersectionsbytri-levelwaveforms.Thesetri-
levelformscontainnoharmonicswhicharemultiples
of2 or3.Thiseliminatesfrequencytranslationand
detectionofinterferencefromtheside-bandsofadja-
centstationssincethethirdharmonicofthesub-
carrier(114kHz)isexcluded.Italsoeliminatesin-
terferencefromSCAbroadcastssincethethirdhar-
monicofthepilottone(57kHz)isexcluded.Un-
wantedspuriousaudiblecomponentsandphasejitter
inthePLLareinherentlyeliminatedbythistechnique.
Additionaladvantagesofthephaselockedloop
stereodemodulationaretheeliminationofinductors
tominimizedrift,integrallampdrivingcapabilityto
indicatethepresenceofthe19kHzpilotcarrier,ex-
cellentchannelseparationovertheentireaudiofre-
quencyrange,extremelylowdistortion,lowoutput
impedance,andtransient-freemono/stereoswitching.
Aftermultiplexdetection,19kHzpilotand38kHz
carriersuppressioncircuitsareusedtopreventtape
recorderinterference.
TheFMmutingcircuitisunusual.Itoperatesboth
bydetectingultrasonicnoiseandbysensingcorrect
tuningofthedetectorcircuit.To'un-mute'itis
necessaryforthesignaltohaveanadequatesignal-
to-noiseratioandtobetunedtothecenteroftheFM
carrier.TheMUTINGcircuitcanbeactivatedorde-
featedbythefrontpane!mutingswitch.Themuting
oftheaudiosignalisdonewithFETanalogswitches.
11

AMSECTION
TheAM-RFamplifiercircuitincludesa threesec-
tionvariabletuningcapacitorina metalenclosure.
Thisthreesectionvariablecapacitorisusedfor
greaterspuriousrejection.TheMclntoshAM-RFamp-
lifiercircuitisunique.Ithasconstantsensitivity,con-
stantselectivityandhighimagerejectionacrossthe
completeAMband.OrdinaryAM-RFcircuitscannot
doallofthissimultaneously.Thisdesignachieves
equalsensitivityevendowntothelowendofthe
band.Spurious,imageandintermediatefrequencyre-
jectionareallsuperior.Thesamecircuitdelivers
equalselectivityacrosstheentireband.Thus,thereis
nolossofaudiofrequencyresponseatthelowendof
theband.AnotheradvantageoftheMclntoshcircuit
isfreedomfromcrossmodulationandoverloadingby
stronglocalstations.
Aloopstickantennaisprovidedwhichcanbe
rotatedformaximumsensitivityover180degrees.
EachMR75loopstickisindividuallytuned.This
custommatchingoftheloopsticktotheAM-RFfront
endmaximizestheperformanceoftheantenna.A
rearpanelantennaconnectorisprovidedforconnec-
tinganexternalantennaifdesired.
Tomaintaintheexcellentimagerejectionandspur-
iouscrossmodulationrejectionoftheAM-RFampli-
fier,anautodynecircuitisusedfortheAMmixer.The
AM-IFusestwodoubletunedIFtransformersdesign-
edtoobtaina highdegreeofselectivityyetallowing
goodaudiofidelity.A 10kHzactivefiltereliminates
thewhistleandirritating"monkeychatter"causedby
anadjacentstation.Theautomaticvolumecontrol
systemwasdesignedtopreventburstingorthumps
whentheAMistunedthrougha strongsignal.Distor-
tionatlowaudiofrequenciesisminimizedbyusinga
two-sectionAVCfilterinsteadoftheconventional
singlesection.
OUTPUT/HEADPHONEAMPLIFIER
Theoutputaudioamplifierisa push-pullcom-
plementaryclassABamplifierusinga signalinver-
tingdifferentialstageatitsinput.Thisamplifier
drivesthevariableoutputandtheheadphonejack.
TURNONDELAY
Aturn-ondelaycircuitislocatedaheadoftheout-
putamplifier.Thiscircuitusesa lightemitting
diode/lightdependentresistornetworkthatfunctions
totransmitnosignaluntiltwosecondsafterthe
powerswitchisturnedonandtoremovetheoutput
signalalmostinstantlyuponpowerturn-off.Thusthe
turn-onandturn-offistransientandnoisefree.
POWERSUPPLY
Tominimizehum,radiation,andthusimprove
signaltonoiseratiotheMR75powertransformeris
tripleshielded.Shieldingincludesa copperstrap,a
siliconsteelstrap,andfinallya steeloutershell.The
transformersecondaryoutputvoltageisfedtoa full
wavebridgerectifierand2,200microfaradfilter
capacitorstoprovidethe+24voltsforpoweringthe
plusregulatorand- 24voltsfortheminusregulator.
The± 18voltsneededforlowlevelamplifierstagesis
suppliedbyintegratedcircuitvoltageregulators.
12

McINTOSHMR75AM/FMTUNER
Block
Diagram
13

McINTOSHLABORATORYINC.
2CHAMBERSST.,BINGHAMTON,N.Y.13903
607-723-3512
Thecontinuousimprovementofitsproductsisthepolicyof
MclntoshLaboratoryIncorporatedwhoreservetherightto
improvedesignwithoutnotice.
PrintedinU.S.A.
039281