ELEMENTS OF RADIOSERVICING by\V ILL I A MMAR C US,M.S. Coauthorof i'Elementsof Radio" andALE XLEV Y,B.S. Instructorof RadioMechanics, ManhattanTradesCenter forVeterans andChelseaVocationalHighSchool McGRAvV-HILLBOOKCOMPANY,INC. NEWYORKANDLONDON ELEMENTSOFRADIOSERVICING COPYHIGHT,1947,BYTHE MCGRAW-HILLROOKCOMPA:l ;;; s:: b ..... o C/o C":l C APPLICATIONS 45 c('dureforbringingthesecircuitstoresonanceattheiroperating frequenciesiscalled"alignment." Thesignalgeneratorisaninvaluabletoolinreceiveralignment, sinceit isusedto feedthe proper aligning frequencyto each circuit. Theprocedureconsistsessentiallyinconnectinganoutput-measur-ingdeviceacrossthespeaker,whichistheoutputofthereceiver; feeding avoltage at the proper frequency to the circuit being aligned; andadjustingthevariablecomponent,usuallytr:mmercondensers providedforthepurpose,toamaximumdeflectionoftheoutput meter. Alignment isnecessarywhenone ofthe componentsofanytuned circuitbecomesdefective and isreplaced.Alignmentwillalsoperk upareceiverwhere,owing to natural aging ofthe componentswith time and moisture, the tuning-circuit parts change in value. Stage-gainMeasurements.-In asuperheterodynereceiver,each stage,except the diodedetector,amplifiesthe signalbefore it passes itontothenextstage.'Vhentheservicemanhasanideaofthe approximateamplificationor gainthatmaybeexpectedfromeach stageand isequippedto measureit whilemakingasignalcheck of thereceiver,hehasapowerfulservicetoolforquicklydetermining the location of manytroubles. For example, assume an open cathode by-pass condenser in astage of areceiver that isperfect in all other respects.The receiver would produceaweakoutput.Inservicingsuchareceiverbytheold methods,tubeswouldcheck good,voltagemeasurementswouldbe normal,andaroutineohmmetercheckwouldalsoshownothing. The serviceman would then proceed to substitute parts, more or less atrandom,untilhecametothedefectivecondenser. 'Vith the aidofstage-gain lIwasurenwnts,hewouldbeexamining the defectivcstage inamatter ofminutes.Although h('wouldstill be confined to the substitution of parts, hewould be doing so forcomponents ofonly one stage foundto be defective. Accurate stage-gain measurements, as made in engineering labora-tories,wouldrequireaconsiderableoutlayinthematteroftest equipment.However,forservicingpurposes,great accuracyisnot necessarysincetheoffending stagewillusuallybe farbelow normal whenthereceiverisbroughtinasdefective.Adequatestage-gain measurements can be made with the equipment that the serviceman has on hand-a signal generator and an AC voltmeter. Thetheoryunderlyingstage-gainmeasurementsisquitesimple. Thereceiverisheldatalltimesduringthecheckatoneoutput, knownas"standard" output.Asignalfromthe generator isfedin totheinputofastage,andthe voltageofthat signal, necessaryto 46ELEMENTSOFRADIOSERVICING producestandardoutput,isnoted.Thenthe signalisfedintothe outputofthestage.Thevoltagelevelofthesignalisincreased untilstandardoutputisagainobtained.Bydividingthesecond voltage by the first we obtain the gain of the stage.This sequence is illustrated in Fig.7-4. Let ustake an example to illustrate the point.If 1 volt of signal at theinputofastagegivesstandardoutput,andthesignal level must be increased to10volts to maintain the standard output when it is connected to the output ofthe stage being tested,thenthe gain ofthe stage is10/1, or 10. AdjustAttenuator to obtainsmndard output. h - - - - - - - - ~ , II I: II IStaqeI beinq tested InterveningStages 2-AF Output Stage ACVoltmeter ~ Holdat pre-determined voltage for standardoutput FIG.7-4.-Sequence ofmeasurements to obtain the gain ofastage. Thestandardoutputusedinstage-gainmeasurementshasbeen set by the I.R.E. at 50 mw of signal power fed into the speaker.The outputpowermaybemeasuredbyconnectinganACvoltmeter acrossthespeakervoicecoilor,moreconveniently,acrossthe primaryoftheoutputtransformer.Instage-gainmeasurements, thesignalinputlevelisadjustedtokeeptheoutputmeteratthe properfixedvalue.Thisvaluecorrespondstoapproximately 16voltsacrosstheoutputtransformerprimaryformostreceivers. During stage-gain measurements, the A VC system must be inopera-tive, or it will invalidate results.For this reason, the receiver output ismaintained at the lowlevelof50mw sothat input signalsneces-sary to attain that level will be too weak to activate t ~ eA VC system. Themeasurementpointsinthereceiverforstage-gainchecking are usuallytaken fromone gridtothe next.The amount ofsignal necessaryto givestandard output fromanypoint inthe receiveris oftencalledthe"sensitivity"ofthereceiverfromthatpointon. When asignal of 3,500 microvolts is required at an IF amplifier grid togivestandardoutput,thesensitivityofthereceiverattheIF amplifier grid issaid to be 3,500microvolts. For the practicalserviceman,exact sensitivitymeasurementsare not necessary.Comparative sensitivity measurementswillserveas SIGNAL-GENERATORAPPLICATIONS47 well.These may beobtainedby actuallymaking sensitivity meas-urementsfromvariouspointsinreceiversknowntobeinperfect operatingcondition.Ineachcase,theattenuatorreadingofthe signalgeneratornecessarytogivestandardoutputshouldbere-corded.When completed,thc readings foreach point are averaged. Asa result, the serviceman will have comparative data for determin-ingproper sensitivityfromvariouspoints foranyreceiverbrought in.Forexample,iftheattenuatorpositionvariesgreatlyatthe gridoftheIF amplifierofa;nunknownreceiverfromtheaverage setting just obtained,adefect in the IF amplifierstage isindicated, ifalllater stages check perfect. Ontheaverage,thesensitivityofradioreceiversfromvarious pointsmaybesummarizedintheaccompanyingtable.Thediode detectorisomittedbecauseitspurposeisnotamplificationbut rather demodulation. Sensitivity. GeneratorGenerator hot Output from averageinput frequencyleadconnected thereceiver set atto 5-12 microvolts600kcAntennaterminalStandard 50microvolts600kcModulator gridStandard 3,500microvolts455kc(or other IF)IF gridStandard 0.032volt400"'-']:l ::... ......? ::... "Il t-< ......t>:l 0:> :;: c::l ..... (0 ..... 122ELEMENTSOFRADIOSERVICING SUMMARY Test for nonnal operation of thesecond AFstage. Thetipofaplugged-insolderingironappliedtothegridofthetubecausesa growlto beheardinthespeaker. Piagram of atypicalsecond AFstage. The accompanyingfigureshowsthe typicalsecondAF stage. V-5 2-AF 6V6-G From Stage R-12 50QOOO N onnal voltagedata. B+ T-6 Voltageismeasuredfromthechassisorcommonnegativelead.Voltagedata are given in the accompanyingtable. Tube terminal 25L6 andACreceiver,ACjDC receiver, 6V6-GpinNo.voltsvolts Plate .....................323585 Screen ....................425090 Grid ......................500 Cathode ..................812.56 N onnal second AFstage resistancedata. Chassis to cathode ....................................................... 300ohms Chassis to controlgrid ................................................ 500,000 ohms Plate to B plus .. : ................................................... 200-600 ohms The 800 ohms of resistance from chassis to cathode isthe ohmic value of self-bias resistorR-18.When atube other than the 6V6-Gisused,adifferent value willbe found.Refertothediagramofthe receiverbeingtested,orto thetable onpage

The plate toBplusreadingmeasuresthe resistanceofthe primary ofthe output transformer. SECONDORPOWERAUDIO-AMPLIFIERSTAGE123 SERVICEDATACHARTFORTHESECONDAFSTAGE SymptomAbnormal readingLook for NosignalfromthespeakerPlatevoltage=O.ScreenTroubleinthe powersupply. voltage=0SeeChap.8 .Platevoltage=O.ScreenShort-circuitedhighAFby-voltagelowpass condenserC-12 Platevoltage=O.ScreenOpen primary winding of out-voltagenormalorhigh.put transformerT-6 (Screenofaglasssecond AF tube glows) PlatevoltagenormalorWeak secondAF tube.Open high.Screenvoltageself-biasresistor R-IS same asplate ~ . Poor tonequalityPlatevoltagelow.ScreenDefectivesecondAFtube. voltagenormal(largedif- Short-circuited cathodeby-ferencebetweenplateandpasscondenser C-lS.Open screenvoltages)grid-loadresistorR-12. Shortedorleakycoupling condenserC-S2(seeChap. II) VoltagesnormalOpencathodeby-passcon-denserC-lS.Mismatched replacementoutputtrans-former MotorboatingOpenoutputfiltercondenser C-16.Opengrid-loadre-sistorR-12 Squeal or oscillationVoltagesnormalOpenoutputfiltercondenser C-16.OpenhighAFby-passcondenserC-12.De-generativefeedbackcon-nectionfromreplacement outputtransformerincor-rectly phased QUESTIONS 1.Areceiverisbroughtinforrepairs,thecomplaintbeing"noreception." Visual inspection shows ared-hot screen grid in the type 6F6-G power tube.What islikelytobewrong?Indicatetheteststhatshouldbemadetoconfirmyour assumption. 124ELEMENTSOFRADIOSERVICING 2.In adeadreceiver,thepowersupplyisfoundtobeoperatingnormally.A voltagecheckofthesecondAFstageshowsthefollowing: Plate ................................................ 300volts Screen ............................................... 300volts What arethe likely causesofthe trouble?Indicate the tests that should be made to confirmthe actualcauseofthetrouble. 3.AnACreceiver,usinga6V6-GtubeinthesecondAFstage,givesahigh-pitched squeal regardless ofthe setting ofthe volume control or tuning dial.What are the possible causesofthe trouble?How would you check foreach? 4.ThereceiverofFig.10-17hasanopenoutputtransformer.If anoriginal replacementisnotobtainable,usetheuniversaloutput transformerchartofFig. 10-11 for reference and choose(1)the type of transformer that shouldbeused, and (2)thesecondarytapsthat shouldbeused. 5.The receiver of Fig. 10-14 has low volume and sounds tinny.A voltagecheck showsnormal voltagereadings.Substitutionofthebenchtestspeakercausesno improvement.What shouldthe next check be? 6.The receiverofFig.10-14 motorboats.Bridgingtheoutput filtercondenser C-26with a20-mfd/450-volt condenser causes no improvement.What should the next check be? 7.ThereceiverofFig.10-13beginstodistortafterit hasbeenplayingfor15 min.What would you suspect iswrong?How would you confirm your suspicion? 8.Adistortingreceivergivesthe followingvoltage check forthe 6V6-Gtubein the second AF stage: Plate ................................................ 200 volts Screen ............................................... 250volts Grid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..0volts Cathode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..2volts Whatislikelytobethecauseofthedistortion?Howwouldyouconfirmyour assumption? 9.The receiverofFig.10-13 isbrought in as dead and givesthe followingvolt-age readings forthe second AF stage: Plate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..95volts Screen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..95volts Cathode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..30volts Whatislikelytobethecauseofthetrouble?Howwouldyouconfirmyouras-sumption? 10.What precautions should be observed in replacing ashorted high-AF by-pass condenser? CHAPTER11 FIRSTAUDIO-AMPLIFIERSTAGE QuickCheck.-Ifawetfingeroraplugged-insolderingironis appliedtotheinputofthefirstAFstageandaverystronggrowl comes out of the speaker, the stage is probably functioning properly, and the serviceman moves on to the next stage. Function ofFirst AF Stage.-The control gridcircuit isthe stage input and iscoupledtothedetectoroutput circuit.The plate cir-cuit is the stage output, which is in turn coupled to the grid or input circuit of the second AF stage.The detector has an output of rough-ly1 volt ofAF signal.The second AF stage,if it contains a6V6-G beam-power amplifier,requiresan input signal of12.5volts to drive thespeakerto fullvolume.It isthereforethe functionofthe first AF stagetobuild upthedetectoroutput signalvoltage(1volt)to the level necessary to drive the second AF stage (12.5 volts). Theory ofOperation, Functions, and Values of Component Parts. Fromthefunctionofthestage,toamplify1 volt ofsignalto12.5 volts, it would seem that avoltage amplification of12.5 for the stage would be sufficient.However, the detector output may be less than 1volt,inwhichcasetherewouldbe insufficientvolume.The first AF stage,therefore,isusuallydesignedforhighvoltage gain,50or higher,sothatlowinputsignalscanbeamplifiedtotherequired leveltooperatethesecondAFstage.Then,shouldtheinputbe excessive,thedetectorsignallevelfeedingthefirstAFstageis reducedthroughapotentiometer,whichisthemanuallyoperated volumecontrolofthereceiver. The first AF stage is called a"voltage" amplifier, while the second AF stage is called a"power" amplifier.The reason for these descrip-tions lies in their functions.The second AF stage drives the speaker andmustfurnishpowertovibratethespeakerconeandthesur-rounding air.Electric power ismeasuredinwatts,whichincorpo-ratesboth voltageand amperage.ThesecondAFtube,the output transformer, and the speaker are all rated in watts.The second AF stage,therefore,isapoweramplifierdevelopingenoughpowerto drive the speaker.The firstAF stage,on the other hand, furnishes the grid excitation forthe second AF tube.The grid ofthe second AFtubeisalwayskept at anegative potentialbythebiasvoltage 1 ~ 5 126ELEMENTSOFRADIOSERVICING supply,andthesignalvoltagedoesnotnormallyexceedthebias voltage.Asaresult, the grid circuit does not draw current from the previous stage, and the signal grid excitation therefore requires volt-agebutnotcurrent.Forthisreason,thefirstAFstage,which furnishesthegridexcitationforthesecondAFstage,iscalleda "voltageamplifier."If thesignalvoltageatthesecondAFgrid shouldexceedthe biasvoltageand grid current result,thefirstAF stage would also be furnishing power.Likewise, if the first AF stage wereusedtodriveapair ofheadphones,it would be operating as a power amplifier. Thetubeusedasthefirstaudioamplifierisusuallyahigh-mu triode.Mostoften,it isthe triodesectionofadual-purposediode and high-mu triode, like the 6SQ7, which will be used in our standard circuit.Thediodesectionisusedasthedetectorandwillbede-scribed inChap.12. From Detector Output B+ FIG.ll-l.-Typical firstaudio-frequencyamplifierstage. Standard Circuit.-Potentiometer R-27 is the manual volume con-trol forthe receiver.Its usual value is500,000 ohms.The detector signaloutput isconnectedacrossR-27,andthepositionofthepo-tentiometer arm determines how much ofthe detector signal output voltage is fed to the audio amplifier.For example, ifthe arm isnear thegroundedend,littleofthedetectoroutputvoltagedeveloped acrossR-27getsamplified,andthisisthelow-volumeposition. If the arm is nearer the ungrounded end, more of the available signal voltagegetsamplified,andthisisthe high-volumeposition. Condensere-31isthecouplingcondenser.Itfeedstheaudio signal voltage fromthe volume control to the grid or input circuit of the tube and is usually 0.005mfd.It may vary in different receivers from0.001to 0.02mfd. ResistorR-31isthe gridload.It returnsthe grid directlytothe cathode in acircuit known as"contact bias."Aswillbe explained, FIRSTAUDIO-AMPLIFIERSTAGE127 the grid-loadresistorinacontact biascircuitusuallyishigh:2to 15megohms.Theaveragesizeforthestandardcircuitis10meg-ohms. OperationofContactBias.-Whentheschematicdiagramis studied,itwouldseematfirstglancethatthereisnogrid-bias voltageonthetriodesectionofV-4,sincethegridgoesto ground through R-31and the cathode is also at ground potential.To under-standhowabiasvoltageisdevelopedbetweengridandcathode, first assume acondition of no signal input.In the tube,the cathode isemittingelectronswhichareattractedbythepositiveplate,as showninFig.11-2.Someoftheseelectronsimpingeonthegrid locatedbetweencathodeandplate,asshowninFig.11-3.These R-32 B+ FIG.1l-2.-Electrons being attractedfromthecathode tothepositiveplate. R-32 B+ FIG.11-3.-Electrons impinging on the gridofatubeindevelopingcontact bias. willHowthroughthegridloadR-31backto cathode.SinceR-31 usuallyhasahighresistance,itwillnotrequireverymuchgrid current Howto develop avoltage across it.By applying Ohm's law, E=IXR, a current of only 0.1microampere (0.0000001amp)will develop1 volt across10megohms,the usual sizeofR-31.Note the arrow showingdirectionof electron HowthroughR-31inFig.11-3. Since electrons Howfromnegativeto positive,the gridend ofR-31 isnegative,with respect to the ground or cathode end, by this volt-age drop.Therefore, asmall negative bias is established on the grid. This negative bias remains constant foraparticular circuit because, asfastaselectronsleakoffthe gridacrossR-31,newelectronsim-pinge on it, and therefore a condition of equilibrium is set up whereby aslightnegativebiasismaintainedonthegrid.Condensere-31 prevents electrons from leaking.across R-27 to ground. Inamplifiersusedinradioreceivers,gridsaremaintainedat all times at anegative potential.When the signal voltage is placed on 128ELEMENTSOFRADIOSERVICING the grid,it drivesthe grid more negative or lessnegative with each alternation.If thesignalvoltageshouldbe largerthan thesteady negativegrid-biasvoltage,thegridwillbedrivenpositiveonthe positive halfofthe signal cycle,resulting in seriousdistortion.For this reason,thesignalvoltagemustalwaysbe lowerthan the grid-biaspotential.Inthecaseofcontactbias,thegrid-biaspotential islow,and asaresult the signal handling capacity islow.Contact bias, therefore,isusedonlyinthe firstaudiostagewherethe input signalisat alowlevelofpotential. Tubes Used in the First AF Stage.-Vacuum tube V-4isthe volt-age amplifier tube.The one most often used in the first AF stage is the high-mu triode section of the type 6Q7or 6SQ7tube.Receivers equippedwithlocking-basetubesusethesimilar7C6loctaltype. When lower gain forthe stage isdesired or the stage is to be followed by transformer coupling,the type 6R7tube isemployed.Where a separate diodeisused forthe detector stage,the tube employed for the firstAF stage isa6F5or6SF5;these have the same character-isticsasthe triode section ofthe 6SQ7.Even in the latter case,the 6SQ7isoften used with thediodeplates grounded.Older receivers used the 75type oftube in asimilar circuit arrangement. ReceiversoftheAC/DCtypeusethe6Q7and6SQ7incircuits employing 0.3-ampfilamenttubes,and the ortypes in 0.15-amp filamenttubes. CouplingCircuittotheSecondAFStage.-Resistor isthe plate loadofthe firstAF tube.The valuemost often found ismegohm ohms).It may goas highas0.5megohm and as lowas0.1megohm.Highervalueswouldgivesomewhatgreater gain; lowervalueswould result in reduced gain.When the firstAF tube is a low-mu triode like the 6SR7, resistoris lower in value. 50,000 to 100,000 ohms being usual.In all cases,wattage dissipation isrelativelyunimportant.Theresistorsgenerallyinusearethe Y2-wattsize. Condenseristhe audio coupling condenser.This condenser. plate-load resistorand grid-load resistor ofthe following stage make uparesistance couplingcircuit between the two stages. asshowninFig.11-4.Its functionistwofold:It conducts the AF signalfromthe plate circuitofthe firstAFtubetothe gridofthe secondAFtube;atthesametime,itkeepsthepositiveplate potential ofthe firstAF tube fromaffectingthe gridofthe second AF tube. The capacity of coupling condenser varies considerably with differentreceivers.Capacitiesrangingfrom0.01to0.1mfdare FIRSTAUDIO-AMPLIFIERSTAGE common.The standard receiver uses0.05mfd.The larger capaci-tiesgivebetter bass frequencyresponse.Somereceiverspurposely usea condenseratC-32andaregenerallydesigned togiveapoorresponsetolowaudiofrequenciessoastominimize thehumfrequency(120cyclesforafull-waveand60cyclesfora half-wave rectifier). I-AF 2-AF (-32 lIE I--J B+ B+ FIG.11-4.-Resistance coupling between the firstAF and the second AF stages. The insulation ofcondenser C-32must be good,since any leakage wouldputapositivebiasonthesecondAF gridfromthefirstAF plate.Papertubularcondensersareusuallyusedwithavoltage ratingof400or600voltsDC. NORMALTESTDATAFORTHEFIRSTAFSTAGE Signal Check.-In the signal-substitutionmethodofservicepro-cedure,onlythefinalaudiostageismeasuredasasingleunit. Thereafter, as each stage is added, the test is over-all.In the case of the firstAF stage,the test signal isappliedto the first AF stage in-putcircuitwhiletheoutputindicationistakenfromthespeaker. Mostsignalgeneratorsprovideapairofterminals,wherea400-cyclecurrent isavailableforthetestingofAF circuits.Whenthis test signal is applied to the input of an AF amplifier, a 400-cycle note is heard in the speaker. Whentheaudiooutputfromasignalgeneratorisnotreadily available,agoodsubstituteisfoundoneveryservicebench.The tip of the soldering iron is a length of copper rod, partly enclosed in a heatingcoil,whichisenergizedby60-cyclecurrent.Theheating coilinduces asmall voltage inthetip,whichisusableasasource of signal input voltage for AF amplifiers.The test frequency is low, 130ELEMENTSOFRADIOSERVICING 60cycles,whichaccountsforthenoteheardinthespeakerbeing described as agrowl.Also,the human body seemsto pick up some 60-cyclevoltage,andmanypracticalservicemenuseamoistfinger astheir signalsource.Thislast procedureisnot recommendedfor beginners,whomightaccidentallytouchaplateleadat300volts insteadofagridleadat zerovolts. Quick Check for the First AF Stage.-If awet finger or a plugged-in soldering irontip isapplied to the ungrounded(called the "hot") endofthe volumecontrolwiththecontrolinthefullONposition, a very strong growl should be heard in the speaker.If it is not heard or ifit isnot considerablystrongerthan the growlheardwhenthe second AF stage was checked, the trouble is in the first AF stage. The quick signal check can also be used for further narrowing down the location of the trouble.Assume normal response from the second AF grid(alowgrowl)andnoresponsefromtheungrounded(hot) endofthevolumecontrol,asin Fig.11-5. Test signalapplied hereqivesno respon?e .,k' (-31 Test signalapplied heregivesnormal I-AF'!=p-onse2-AF T-6 B+ B+ FIG.11-5.-Trouble shooting an inoperative firstAF stage by asignal check. Then, ifthe test signal isapplied to the plate of the first AF tube, normal response(a low growl in the speaker)indicates that coupling condenserC-32isfunctioningandthe troubleisbeforethe firstAF plate.Noresponseatthispointindicatesanopencouplingcon-denser,orafirstAFplate-to-groundshort. If there isnormal response fromthe first AF plate,the test signal isshiftedtothefirstAFgrid.Normalresponse(astronggrowl) fromthispoint indicatestroubleinthevolumecontrolor coupling condenser C-31.No response means that the trouble is between the first AF grid and the plate.The likelyare 1.AnInoperativeFirstAFTube.-Confirmbysubstitutinga good tube. 2.AGroundedGridLead.-:-Confirmwithanohmmeter.(The ground isprobably causedby defective shielding.) it' FIRSTAUDIO-AMPLIFIERSTAGE131 3.AnOpenPlate-loadResistorR-32.-Confirmbyvoltageand resistancechecks. UseofOutputMeter.-Theear,judging differences in sound in-tensity, can make onlyarough estimate.Except at very low sound levels,the judgment ofthe ear isnot very reliable.Amore quanti-tativecheckforallreceivertestingistomeasuretheactualsignal powerthat isput intothespeaker. Radiomenusuallyworktoadefinitelevelofoutputfromany receiverandthenmakecomparisonsofinputsignalnecessaryto attain that output.This reference level is called "standard output" and is defined as 50 mw (0.05watt)of signal power into the speaker. Notethatthe50mw iswellbelowtheoutputcapabilitiesofany radioreceiverand,therefore,thetestsignallevelatanypointin the receiver,necessaryto attain standard output,willnot overload any tube. Theoutputpowermaybedeterminedbymeasuringthesignal voltageacrossthespeakervoicecoilwithanACvoltmeter.For example,ifwehavea5-ohmvoicecoil,0.5voltwillcorrespondto standard output. W=E2_0.5X0.5_0.25=0.05 watt R- 5- 5 The only trouble with this is that Yzvolt is not easily read on the low ACrangeoftheusualmultimeter. Amore easily read output indication isobtainable at the primary oftheoutputtransformerwhere,owingtotheturnsratioofthe transformer,standardoutputwillcorrespondtoapproximately16 volts.The primary ofthe output transformer,however,is in acir-cuitwheredirect current,the plate current ofthe secondAF tube, is flowing,the signal itself being a pulsation of this current.To keep the direct current ofthe plate circuit from affecting the AC meter, a condenser must be inserted in series, so that the meter will read only theACsignalcomponent.ThisisshowninFig.11-6, whichindi-catestheconnectionsforanoutputmeter.Aconvenientsizefor this seriescondenser is0.1mfd/600volts.Some multimeters have the output condenser built in,in whichcase there willbe test jacks ontheinstrumentlabeledOUTPUTMETER,andtheO.I-mfdcon-denser need not be connected externally.The meter should be used onasuitableACrangewhere16voltswillgiveagoodindication. (About half scale isbest.)It might be advisable forthe serviceman toworkto areading of15or20voltsashisreference level,to take advantage ofaconvenient marker onthe meter scale.Then asfar ashistest bench isconcerned,15or 20 'Volts,as the case may be, is 132ELEMENTSOFRADIOSERVICING standard output, and he willwork at this level except whereservice notesissuedbythemanufacturerofthereceiverconcernedspecify differently.Thevoltagechosentorepresentstandardoutputwill not varytoomuchfrom50mwandissufficientlyaccurate forany servicework. 2-AF 8+ .IMFD. 600V Output Mefer AC Volts FIG.11-6.-Connection of an ACvoltmeter as an output meter. The servicemanwoulddowellto provide himselfwithsomespe-cialtestleadsforconvenienceincheckingtheoutputvoltage.If themultimeter hasabuilt-inoutput condenser,apair oftest leads ClipConnections from Multimeter with built-in OutputCondenser ACVoltmeter connectedup as an OutputMeter PinTip to Voltmeter FIG.11-7.-Test leads forthe output meter. ConvenientMethod forconnectingthe SeriesCondenser terminating inalligatorclipswillbeallthat isneeded.If theout-put condenser isnotbuiltin,onetest lead isprovided with aseries 0.1-mfd/600-volt condenser,asshown in Fig.11-7. FIRSTAUDIO-AMPLIFIERSTAGE133 Stage-gainMeasurernents.-Now,havingestablishedstandard output, let us make some gain checks on areceiver known to be per-fecttodeterminehowthis informationmay be used in later servic-ing.Figure ll-S shows the audio amplifier ofthe standard receiver. The output meter and the AF output of the signal generator are con-nectedtomakegainchecks.The condenserinthehotleadofthe signal generator(which may be connected internally)serves to keep DC plate potentials out ofthe signal generator circuit when the hot SiJ ~ ~ ~ f ~ ~ ~ ~ ~ 'l 0:1 CJi)264ELEMENTSOFRADIOSERVICING presettrimmersacrosstheantennatuningcircuit,eliminatesthe converter signal-grid tuning circuit by converting it into an untuned resistance-coupledcircuit,andthrowsanyone ofarowofpermea-bility-tuned coilsinthe oscillator gridcircuit. Thepredeterminedstationisthentunedinbydepressingthe proper push button.The buttons are sprung sothat they are nor-mally in the OFF position.Then as any button isdepressed, acatch holdsthis button in place while automatically releasingany button Ave Interstage Rf Transformer Shorted tEJbYRtlnge R-3 7H715,000 RF R-6 4700 B+ C-ISwitchIn - Automatic7B8 - PositionConverter ------ StationPush-ButtonSwitch------_--Pre- set Trimmer It Pre-set./C 6 CoilPrd- FIG.15-7.-Simplified diagram of the tuning circuit ofthe receiver ofFig.15-6. previouslydepressed.Eachbuttoncontrolsadouble-poleswitch, one pole of which connects one of the permeability-tuned coils in the oscillator grid circuit,whilethe other pole connects the proper asso-ciated trimmer in the RF grid circuit. Thetrimmersandcoilsintheautomatictunerhavealimited range(approximately400kc),sothateachbuttoncannottune many desiredstations in the broadcastband.However,the values ofcoilsandcondensersarestaggered,sothatanystationcanbe tuned inonsomeonebutton.The tuningrangeofeachbutton is usuallymarkedneartheadjustmentscrews. Figure 15-7 shows asimplified drawing of the tuning circuit of the ZenithreceiverofFig.15-6,whenthe rangeswitchisintheauto-maticposition.Onepushbuttonisdepressed,showingonepreset trimmer connected across the RF tuning circuit and one preadjusted permeabilitycoilinthe oscillatortuningcircuit.The coupling be-FURTHERNOTESONTHEOONVERTER-VARIATIONS tween the RF and converter tubes is of the resistance-capacity type. Thiscouplingalsoremainsthesamefortheshort-wavepositionof the range switch.The circuit is tuned only in the manually operated broadcastpositionoftherangeswitch. The system just described is typical for push-button tuners of the switching type.Thesesystemsdiffermainly in thenumberofpre-setstationsavailable.Insomecases,switchingfrommanualto automatic tuning istaken careofbyanextra similarpushbutton, rather than aposition on the range switch.Often the radio-phono-graph switch is also an extra similar push button.In addition, some typesprovidetwosetsoftrimmercondensers,insteadofoneset of trimmersandonesetofpermeability-tunedcoils.In thesetypes, theregularbroadcastoscillatorcoilisused,theoscillatortuning condenser isswitched out ofthe circuit,and one ofthe preset trim-mer condensers issubstituted forit. ServicingPush-buttonTunersoftheType.-Push-buttonsystemsoftheswitchedtuning-circuittypegiveverylittle service Occasionally, the switches do not make good con-tact.When this happens,the followingcleaning procedure iseffec-tive:Dusttheentireswitchassemblywithasoftbrush.Depress the firstswitch,andapplycarbontetrachloridetoitscontactsand also the arm and contacts ofthe next switch.Then depress the two switchesalternately:firstthesecond,thenthefirst.Repeatthe procedure forthe firstand second switches,thistime depressing the second button before applying the carbon tetrachloride to it.Repeat onthe next pair,makingsure that eachswitch has beenwashed in boththeopenandtheclosedposition. Another service problem isresetting the adjustment screws, which may change their position with time.When doing this,the receiver shouldbeallowedawarm-upperiodofabout15min,toallowall components to reach normal operating temperature.The oscillator control isadjusted first,followed by the antenna adjustment.If the adjusting screwsare not marked,the servicemancan identify them bycheckingthewiringdiagramorbytheoperationoftheadjust-ments.Theoscillatoradjustmentiscritical-a fractionofaturn will bring the station in or out.The antenna adjustment is broad in comparison.If the receiver isequipped with amagic eye,it should beusedtoindicateexactresonance.Anoutputmetercannotbe usedforthispurpose,sincethereadingwillvary withthe modula-tionoftheprogram.Avacuum-tubevoltmeter,ifavailable,con. nectedto the Ave bus,can also be used asthe resonance mdicator. If neitherthemagiceyenoravacuum-tube voltmeter is available, theadjustmentsaresetforbestvolumeandtonebyear.Agood 266ELEMENTSOFRADIOSERVICING checkforcorrectsettingsistotunetothesamestationwiththe swit c:hsetformanualoperation,andthenswitchfrommanualto push button and note any difference.Operation should be the same, exceptinthecaseofareceiverlikethatofFig.15-6,wherethe manual switch throws in an extra tuning circuit. When push buttons are set up or when the adjustment screwsare farfromtheircorrectalignmentpositions,itwouldbetimesaving to usethe signal generator for finding the desired stations. Figure15-8 showsthe method ofconnecting the signal generator to the receiver.Adjust the signal generator for amodulated output at the frequencyofthe firstdesiredstation.Depress the firstpush button,andadjustthe associated oscillator control until the signal-q .. J r7\ ~ - - - - i L Be~.00025 Ant.00000a-Buttons Mod Gnd000000 Ant ././ .,."-!-Att 000000 Osc SignalGenerator Receiver FIG.I5-S.-Using asignalgenerator as an aid in quickly presetting push buttons .. generator note isheard.It willbe accompanied by asqueal, caused bythebeatingactionbetweenthegeneratorsignalandthesignal fromthedesiredstation.Disconnectthesignal-generator"hot" lead.If thesquealdoesnotstopbecauseofleakage,detunethe signal generator.Then readjust the oscillator control formaximum responsefromthestation.Finally,adjusttheantennatrimmer. Repeattheprocedurefortheotherbuttons. Mechanically Operated Push-button Tuners.-Figure 15-9 shows twoviewsofatypicalmechanicallyoperatedpush-buttontuning system.Thistypeisknownasa"rocker-bar"mechanismandis probablythe most popular ofallpush-buttontuners.Each button depressesapresetpawl,whichturnstherockerbarasfarasthe pawl setting willallow.A gear connected to therockerbarrotates thegang tuningcondenser.Thetuningknobanddialpointerro-tate with the condenser gang.The return spring maintains the push button in its normalout positionand,at the sametime,keepsthe pawlawayfromtherockerbar. FURTHERNOTESONTHECONVERTER-VARIATIONSfl67 When abutton isset up,the locking screwisloosened.Ascrew driveriskeptpressedagainsttheloosenedlockingscrew,thereby depressingthepushbarandpushingthepawlagainsttherocker bar.Thedesiredstationforeachbuttonistunedinmanually, therebypushing thepawlto its propersetting.The lockingscrew ReturnSpring Button Button Rocker. Bar Return

LockingScrew Gearto Tuning Condenser Gang SimplifiedSideViewof aPushButton FIG.I5-9.-Mechanically operated push-button tuner. is then tightened, fixingthe pawl firmlybetween the shoeandpush rod.Subsequently,whenthe button isdepressed,the pawlpushes therockerbartoitssetposition,therebybringinginthedesired station. From the servicing point of view,loosened adjustments are about the only difficulty experienced with mechanical buttons of this type. A complete adjustment procedure follows. Adjustment of PushButtonsforMechanicalAutomaticTuners. -Rotatetherangeswitchtothebroadcastposition.Selectthe stations desired forautomatic tuning.Choose one of "thesestations Q68ELEMENTSOFRADIOSERVICING and any button to be adjusted for it.Follow the procedure outlined below: Seatscrew driver in screwslot.Pushscrew driverallthewayin. Loosen ScrewI to!y,turns. c e FrG.15-10.-Adjustmentof pushbuttonformechanical p,utomatictuners. 1.Grasp the button firmlyand remove itfromitsshaftbypullingstraightout (seeFig.15-lOA). .2.Insert ascrew driver into the slot of thelockingscrew.Pressinandloosen the screw 1 to 172 turns (see Fig.15-10B). 3.With thescrewdriverseatedinthe screw slot, press the screw in as far as pos-sible.Holditinfirmlywithonehand, andtuneinthedesiredstationwiththe otherhandbypressinginandrotating the selector knob(see Fig.15-10C). 4.Release the selector knob and tighten the screw firmly. 5.Check the adjustment by tuning well pastthestation,usingtheselectorknob and then pushing in the button shaft.The station shouldcome back in again clearly andwithmaximumvolume.Afterthe adjustment is tested, check to see that the lockingscrewistightenedfirmly.Re-place the button on itsshaft. 6.Adjust the remainder of the buttons in the same manner asoutlined above. Figure 15-10D shows a common method ofinserting station tabs. MechanicallyOperatedPush-button TunersoftheMotor-drivenType.-Motor-drivenpush-button tunersare too varied in their operation, adjustment, and serviceproblemsforanygeneralized treatment in abook ofthis nature.The servicemanisreferredtothemanufac-turer's servicenoteswhenheexperiences difficulty with any ofthese devices.For teaching purposes, as an example, the dia-gramandstation-settinginstructionsof theStromberg-CarlsonNo.440receiver are included inthe text. InstructionsforSettingUpPush Buttons.-Beforereadingtheinstruc-II,--=- I'. OFFt.C'eA8$I" ':t.t.1\.:7A . ""'-IF C! "OJ OO,oooJ\. "3TOPAT

3'J1SGT RECT. "'NE CONTROL FIG.diagram ofthe RCA VictorModel16X-l recei"er. I':oXr-l AP?Rc(.GAII'I (400 "')I \. OATA 3!:>L6GT OUTPUT RCAAIDeR CHANALVST T'II'G /1 L9.1I " ',j :.. !2 Q 1/.) ;;:; '" ...-____""Tr..,immer ~ = = = * = = = l l Mixer DTTme+r+-+ Ant Tri mme-tr --+-+@ WaveTrap' L.::======-____ --.::::::::::::.. ____ ---l Adjustment FIG.22-5.-Layout diagram ofasix-tubesuperheterodyne receiver. and which issecondary isnot easily determined.However, it does notmatter toomuchwhichweadjust first,sincethealignmentof them isrepeated. Sometimes,the IF cans looklikethose in Fig.22-5.Here,only oneadjustmentscrewisvisiblefromthetopofeachcan.This arrangementiscommonforpermeability-tunedIFtransformers. ALIGNMENTOFSUPERHETERODYNERECEIVER419 The top adjustment screw will be for either the primary or secondary ofthe IF transformer.The adjustment screwforthe other halfof thetransformerextendsfromthe bottomendofthe IF canand is adjusted from the underside of the chassis. Location ofOscillator Trimmers and Padders.-The two controls forthe oscillator-tuned circuit are the 600or low-frequency padder, andthe 1,500or high-frequencytrimmer.If the tuning gang con-denser is of the variety with cut plates for the oscillator section, as in Fig.22-4,the oscillator trimmer may be easilylocated.Receivers usingthistypeofcondenserusuallyarenotequippedwitha600 padder. Whenthe condensersofthe tuning gang all lookalike,asin Fig. 22-5, the oscillator section can easily be foundas follows:Tune in a station and touch the stator plates of each cOBdenser section.When theantennaandRFsectionsaretouched,receptionwillbelittle affected.Buttouchingthoseoftheoscillatorsectionwilldetune the receiver.The trimmer mounted on this oscillator section isthe high-frequencyoscillatoradjustment.The600padderwillbe located closeto thisoscillator section. Some receivers do not use a padding condenser, but a permeability-tuned oscillator coil instead.The alignment procedure isthe same ineither case.Theoscillatorcoilis"usuallyclosetotheoscillator section of the gang tuning condenser.The permeability adjustment isusuallylocatedinthetopofthe coil,whichissomountedasto make the screwaccessible fromthe top,side,or rearofthe chassis. LocationofRF,Antenna,andWave-trapAdjustments.-Trace the antenna wireto the antenna coil.The wave trap willbe close to the antenna coil.The antenna coil willalso lead to the antenna sectionofthecondensertuninggang.Thislocatestheantenna trimmer.The RF trimmer isthe onlyone to be located.It may be found by tracing the stator lead to the mixer coil. When the receiver isofthe multiband type,the IF trimmersare identifiedasbefore.TheRFtrimmers,however,areusually mounted on the coilassembliesratherthan onthe "gangcondenser. Thismakesthelocationofthetrimmerssomewhatmoredifficult, but it can be done. Figure22-6showsthelayoutofatwo-bandsuperheterodyne receiver.Allthe coilcans,RF andIF, lookalike.However,the secondIF can isbetween the detector andthe IF tubes.The first IF can is between the IF and the converter tubes.The oscillator coil can isin one line with the rear gang condenser section and the con-verter tube.The 600 padder is between the gang condenser and the oscillator coil.The RF coil can linesupwiththe center sectionof 420ELEMENTSOFRADIOSERVICING the 3-gang condenser,andtheantenna coillinesupwiththe front sectionofthegangcondenserandtheRFtube.Somewherebe-tween theantenna leadandthe RF coilcan,the wavetrapwillbe found.In Fig. 22-6, the wave-trap adjustment is the trimmer near theantennapost.Itcanbeconfirmedbytracingtheantenna WIrmg.The only thing left isto determine which trimmer on each coil isforthe broadcast band and which isforshort wave.This is nottoodifficult.Startthealignmentprocedurewiththeadjust-o @ Adjustment o0 1stIF @F 6SK7 FIG.22-6.-Layout diagram of asix-tube two-band receiver. mentoftheIFamplifierintheusualway.Whenthefirstcoil trimmer adjustment isreached, set the receiver for broadcast recep-tion and give either trimmer ahalf turn.If it has no effect,return it toitsoriginalposition.The other trimmer isthebroadcast coil trimmer.To verify,give it ahalf turn and note the effect. AdjustingaTrimmertoaPeakResponse.-Whenmakinga trimmeradjustment,turn the trimmer screw back and forthslowly oneachsideofthepeakoutput position.Notethepeakposition reading.The final adjustment is always one of tightening the screw, stopping at the peak response. Sometimes,when the alignment toolisremoved fromthe adjust-ing screw,the response fallsbelowthe peakobtained whilethe tool wasinposition.Thiseffectoccursbecause theweightor capacity of the tool affectsthe trimmer capacity.When this happens, make thefinaltighteningadjustmenttothepeakposition,andtighten the screw an extra fraction of a turn.When this is done, the output meter reading willfallbelow the peak reading,but it shouldreturn to peak when the tool isremoved. ALIGNMENTOFSUPERHETERODYNERECEIVER 421 Insomereceivers,peakresponsepositionoccursatthe . low-capacitysettingofthetrimmercondenser(whenitiswideopen), as shown in Fig. 22-8.Atrimmer should not be left with this type ofadjustment since,inthis position,thespringtension ofthetopplateisatits weakest.Any jar or vibra-tion,likethatfromthe speaker,willcausethe upperplateofthetrimmer to vibrate, with accompany-ing noiseand microphonics. Whenpeakresponseposi-tion is at minimum capacity, removetheadjustment screwandbendthetop plateback,asshownin Fig.22-9.Microphonics willtherebybeeliminated. Determinationofthe IntermediateFrequencyof aReceiver.-Beforeat-temptingtoaligntheIF trimmers,theserviceman mustknowtheinterme-diate frequencyforthe par-ticularreceivertobe aligned.Thisinformation isalwaysfoundinthe manufacturer'sservice alignmentnotes,oronthe schematic diagram. Wheretheusualinfor-mation isnot available,the serviceman can assume that theintermediatefrequency isprobably455kc,ifthe receiverisofmodernvin-Alignment Tool FIG.atrimmer condenser. FIG:trimmer condenser at its low-capacity setting. FIG.toavoidmicrophonics whenthetrimmercondenserpeaksat thelow-capacity setting. tage.The IF amplifiersofthe past fewyearshavebeendesigned topeakatsomevaluebetween440and480kc.If,becauseof lackofinformation,areceiverdesignedtopeakitsIFamplifier at465kcisalignedat455kcinstead,nogreatharmwillhave beendone.Thereceiverwilloperatenormallyan9- satisfactorily, ELEMENTSOFRADIOSERVICING althoughthetrackingofthetuningdialmaybeslightlyoff. If theservicemanisentirelyuncertainoftheintermediatefre-quency, he can determineit approximately by connecting the"hot" lead of the signal generator to the mixer grid.Then, by rotating the signal-generator frequencycontrol from500to 150 kc,he can seeat what frequency aresponse is heard.Usually,amisalignedreceiver isnottoofarfromitscorrectsettingandabroadresponse,with possiblytwopeaks,willbeobtained.Forexample,ifareceiver showsabroadresponsecenteringat approximately270kc,it may safelybeassumedthatthereceiverwasoriginallydesignedto operateat260kc,thenearestcommonlyusedintermediatefre-quency.Themostcommonlyusedintermediatefrequenciesare 175,260,455,and465kc. When the procedure just described is used, it is important to make surethat theoscillatorofthe receiverismadeinoperative.If the receiveroscillatorisoperating,therewillbemanysquealsandre-sponsesheard,dependingonthepositionofthereceivertuning control.Theoscillatorismadeinoperativebyplacingashort acrossthe oscillatortuning condenser. It isalsoimportantintheproceduretomakesurethatthere-ceiver is not responding to aharmonic of the signal-generator output. Thisunwantedresponsecanbeavoidedbystartingthesearchfor theintermediatefrequencyatthehigh-frequencyendoftheIF spectrum, that is, at 500 kc.Then, when a receiver shows a response at460kc,theservicemanshouldexpectanotherweakerresponse at230kc,thesecondharmonicofwhichis460kc.Stillanother veryweakresponseshouldoccurat153kc,whosethirdharmonic isabout460kc.Werethe servicemantomake the checkat ran-domfrequencies,hemightstopat the230-kcresponse,assumean intermediatefrequencyof260kc,andtrytoalignthereceiverat that frequency,withverypoor results. AligningtheIFAmplifier.-Thereceiver,signalgenerator,and outputmeterarehookedupasshowninFig.22-10.Theoutput meterisadjustedforahigh-voltageACrange,andconnected through a0.1-mfd/600-volt condenser to the plate pin of the second AF tube.The signalgeneratorisadjustedforamodulatedoutput at the intermediate frequency of the receiver.The signal-generator outputisconnectedthrougha0.1-mfd/600-voltcondensertothe mixergridofthereceiver.Thestatorterminaloftheoscillator condenser isshorted to the condenser frame.The receivercontrols are set formaximumgain.Both the receiverand the signalgener-ator are allowed awarm-up period of 15min. The signal-generatorattenuator isadjustedtogiveanoutputas ALIGNMENTOFSUPERHETERODYNERECEIVER423 lowaspossible,withthemodulationnoteheardfaintlyinthe speaker of the receiver.Either trimmer in the output IF can is then adjusted fortheloudestnotef.comthespeaker.Theattenuatoris then reduced until the note can just be heard again.Then the other trimmerinthesecondoroutputIF can isadjustedforthe loudest notefromthespeaker.Theprocedureisrepeatedforthetwo trimmers in the input or first IF shield can-first reducing the output toafaintnoteandthenadjustingthetrimmerforamaximum note. Thealignmentisthenrepeatedforthefourtrimmersinturn, startingwitheithertrimmerinthesecondIFcan.Thistimethe outputmetersettingisreducedtothe50-voltACrange.The attenuatorisreducedtogiveareadingofabout5volts,andeach trimmer isadjusted forpeak voltage reading.Iftheoutput meter ()'\55 KMODIF // SignalGeneratorReceiver FIG.22-10.-Connections to areceiver foraligningthe IF amplifier. shows areading over 30voltswhileanyone trimmer isaligned,the attenuator setting ofthe signalgeneratorisreducedagain,andthe adjustment continued. Whenallfourtrimmershavebeenreadjustedtogivepeakre-sponse on the output meter,the IF alignment iscomplete. SettingtheReceiverTuning-dialScaleAdjustment.-Thenext step in the alignment procedure isto adjust the receiver tuning-dial pointer.The mostcommonadjustment istorotatethegangcon-denser to maximum capacity(plates fullyengaged)position.Then set the dial pointer to the last calibration mark on the low-frequency end ofthe dialscale,asshown in Fig.Aligning the Oscillator Circuits.-After the IF trimmers have been adjusted,thereremaintheoscillatorandRForantennaadjust-ments.Normally,theoscillatorwillhavetwoadjustments:the high-frequencytrimmerandthe600padder.However,whena receiverusescut platesintheoscillatorsectionofthetuninggang condenser,andno600padder,theadjustment oftheoscillatorcir-cuitisrelativelysimple.Theremainderofthissectionwillgive 424ELEMENTSOFRADIOSERVICING the alignment procedure forsuchan oscillator circuit, andit willbe followedbythe RF andantennacircuitadjustments.The section afterthatwillpresentthealignmentprocedureforanoscillator stagewitha600padder. The short(used in the IF alignment)isremoved fromthe oscilla-torsectionofthegangtuningcondenser.Thesignal-generator outputleadisshiftedtotheantennapostofthereceiver,anda 0.00025-mfd/600-voltmicacondenserisaddedasthedummy antenna in series with the generator output lead.The output meter Dialpointer onlast calibration pointattheLow-Frequency end Pulley Idler/' Pulley FIG.22-11.-Setting the dialpointer. is switched to ahigh-voltage AC range.The receiver istunedto a quiet point onthe tuning range,between1,500and1,700kc.The receivercontrolsarekeptat the maximumgainpositions. Now, the signal-generator frequency control is adjusted todeliver thesamefrequencyasthatshownonthereceivertuningdial,let ussay 1,500 kc.The oscillator trimmer isthen loosened all the way andtightenedcarefullyuntilthesignalisheard.Thisadjustment isvery critical.When it isreached,the signal-generatoroutputis reduced,and the output meter isswitched tothe 50-volt AC range. Then thetrimmeradjustmentisrepeatedforpeak voltage output. If the reading goes down after removing the aligning tool, followthe procedure suggested in the section on adjusting a trimmer (page 420) . AdjustingtheRFandAntennaTrimmers.-Thereremainnow only the RF and antenna trimmer adjustments.The signal-genera'" tor dialisturned to1,400 kc.Then the receiveristuned formaxi-mum or peak responseat or near1,400kc.The generator attenua-torisreducedtogivealowreadingontheoutputmeter,andthe ALIGNMENTOFSUPERHETERODYNERECEIVER425 mixertrimmerisadjustedforpeakresponseonthemeter.If an antennatrimmerispresent,itisthenadjustedforpeakresponse. Alignmentofthereceiverisnowcomplete,exceptfortheadjust-ment of awave trap, if present. AdjustingtheOscillatorandRFCircuitsWhena600Padder Is Present.-The alignmentprocedureissomewhat differentwhen a600padderorpermeabilityadjustmentispresentforalignment at 600 kc.The signal-generator output isfedto the antenna ofthe receiverthroughamica condenser, acting as adummy antenna.The generator isadjusted foramodulated output at 600 kc.Thenthereceiveristunedto600kc,andthe600padderis adjustedforpeakresponse. Then,thereceiveristunedtoaquietpointnearthehigh-fre-quencyendofthedial.Thesignalgeneratorfrequencycontrolis adjustedforthesamefrequencyasthatshownonthereceiver tuningdial.Thehigh-frequencytrimmerontheoscillatorsection ofthe gangcondenseristhencarefullyadjustedforpeak response. Thislastadjustmentiscritical.Inmakingit,thetrimmerscrew shouldfirstbeloosenedandthen slowlytighteneduntilthe noteis heard.The attenuator is then reduced for alow reading on the out-putmeter,andtheadjustmentrepeatedforpeakvoltageonthe output meter. ThenextstepcomprisestheRFandantennatrimmeradjust-ments.The oscillator isset to1,400 kc.The receiver tuning knob istunedbackandforthnear1,400kcandleftatthepositionof greatest response.The RF and antenna trimmers are now adjusted forpeak response,asdescribedpreviously. Alignmentoftheoscillatorcircuit isnot yet complete.The gang tuningcondensermust be rockedat600kconthe tuningdial,and the 600padder adjusted forpeak response.This rocking procedure isdescribedin the nextsection. Rocking the Gang TuningCondenser at 600Kc.-In the rocking procedure,performed step by step, the receiver and signal generator are both tuned to 600kc,and the 600padder isreadjusted forpeak response.The attenuator isthen set to give an output meter read-ing of16volts.Thesignal,ofcourse,isbeingfedto theantenna. Thereceiveristhentunedslightlyhigherthan600kc,suchas 605kc.Thesignalgenerator,however,isleftat600kc,andthe 600padderisreadjustedforpeakresponse.If theoutputmeter readingincreasesabovethepreviousreadingof16volts,thema-neuver isrepeated until amaximum voltage isobtained. If the out-put meterreadingdoesnotincrease,the receivertuningcondenser 426ELEMENTSOFRADIOSERVICING is rocked in the other direction; that is,the receiver istuned slightly lowerthan600kc,suchas595kc,andthe600padder isadjusted. The output is noted.If there is an increase, the condenser is rocked stilllower,thepadderisadjusted'again,andthepeakoutputis noted.Therockingandpaddingadjustmentsaremadeandre-peateduntilmaximumoutputisreached. Theadjustmentofthehigh-frequencyoscillatortrimmer isthen checkedat 1,500kc forpeak responseat the high-frequencyendof the broadcast band. AdjustingtheWaveTrap.-Thelaststepinreceiveralignment isadjustment ofthe wavetrap.The signalgeneratorisconnected to the anteI1lla and ground of the receiver, the "hot" lead fed through a0.1-mfd/600-voltcondenser.Thegeneratoristhenadjustedto give astrong response at the intermediate frequencyofthe receiver, say455kc.Thereceiveristunedto1,000kc,approximatelythe center ofthe tuning range.Then the wave trap isadjusted to give minimumresponseintheoutputmeter.The receiverisnowcom-pletelyaligned. ALIGNMENTOFSUPERHETERODYNERECEIVER427 SUMMARYOFALIGNMENTPROCEDURE 1.Set receiver controls formaximum gain. Q.Connect ground lead fromsignalgenerator to receiver chassis(orBminusin the caseofan AC/DC receiver). 3.Connect output meter to plate pin ofsecond AF tube and chassis. 4.Allow signal generator and receiver to operate for 15min as awarm-up period before aligning. SignalgeneratorReceiverNotes GeneratorTuning FrequencyDummy"hot"leadcondenserAdjustment setting, kc.antenna,to receiversetting, mfdterminalkc 455 or IF0.1Mixer gridAnyIF trimmers1,2 MaximumTuning dial capacitypointer 6000.00025Antenna600600padder 1,5000.00025Antenna1,500High -frequency oscillator trimmer 1,4000.00025AntennaTune formaxi- RF and antenna mum outputtrimmers 6000.00025Antenna600600padder3 1,5000.00025Antenna1,500High-frequency4 oscillator trimmer 4550.1Antenna1,000Wavetrap5 NOTES: 1.Shortoscillatorsectionofgang tuningcondenser. Q.Align IF trimmers in the followingorder: a.Detector input trimmer(secondaryofsecond IF transformer). b.IF plate trimmer(primary ofsecond IF transformer). c.IF grid trimmer(secondary offirstIF transformer). d.Converter plate trimmer(primary offirst IF transformer). e.Repeat the adjustment of the trimmers inthe sameorder. s.Rock the tuning condenser during this adjustment as follows:Turn the rotor ofthegangcondenserbackandforthandadjustthe600padderuntilapeakis obtained. 4.This step isacheck to see whether the previous adjustment of the 600 padder hasaffectedthe setting at the high-frequencyend.If the high-frequency trimmer requiresconsiderablereadjustment,the600paddermustalsobereadjustedby repeatingthepreviousstep.. 5.Adjust forminimum output. CHAPTER28 SURVEYOFTHESERVICINGPROCEDURE The precedingchapters ofthisbook have analyzedeach stage of the receiveranddiscussedtroublesthat mightarisefromdefective componentswithineachstage.Theemphasishasbeenprimarily on defects that produce no reception or weak reception. However,whenadefectivereceiverisbroughtinforservicing, the defedive stage isnot usually self-evident.It istherefore neces-sarytopresentanover-allservicingprocedurefortrackingdown troubles.Inaddition,otherdefects,likehum,distortion,motor-boating,modulationhum,noise,andintermittent operation,which havebeentreatedincidentally,requireanover-allapproach.It is thepurposeofthischaptertopresentsuchaninclusiveprocedure forallthe defectslisted. SERVICINGPROCEDUREFORNORECEPTION When the complaint is"no reception," the trouble may be caused bybreakdownofalmostanycomponentthroughoutthereceiver signalchain.Forthebeginnerorservicingapprentice,aroutine checkoftubes,followedbyaroutinevoltagecheck,isagoodap-proach.Butitistootime-consumingforthemoreexperienced serviceman,whowillbeginwitharoutinesignalcheck.Thefol-lowingsteps representthe moreexperiencedapproach: 1.CheckthePowerSupply.-Theservicemanwillaskhimself variousquestionswithrespecttotheinoperativereceiver.Doall thetubesinthereceiverlightorwarmup?Isthereanysignof unusual overheating?Is the hum excessive?DoesBplus measure itsnormal200to300volts?If theanswerstothesequestionsare those applicable to anormal receiver, he proceedsto the next sfage. If not,there istrouble inthe power supply and it must be found. The causes forlack ofreceiver reception originating inthe power supply are listedbelow: Open line fuse. Defective line switch. Defective linecord. Open power transformer primary. 428 SURVEYOFSERVICINGPROCEDURE Dead rectifiertube. Open filterchoke(speaker field). Filter choke winding that shorts to chassis. Shorted filtercondensers. Short in the Bplus line. Open voltage divider resistor. 429 2.ChecktheSpeaker.-If thepowersupplychecksperfect,the speakercomesupforinspection.Tocheckitsnormaloperation, momentarily unseat the second AF tube.If aloud click isheard in thespeaker,thelatterisnotthecauseofinoperation,andthe serviceman goeson to the next check. If the click isnot heard, the speaker may be defective in some re-spect.Possiblecausesforinoperationoriginatinginthespeaker or associated circuits are Open speaker voicecoil. Open speaker voice-coilleads. Open output transformer primary. Dead secondAF tube. 3.ChecktheSecondAFStage.-Ifthespeakerisperfect,the servicemanproceedstocheckthesecondAFstage.Aplugged-in soldering iron isapplied to the signal grid pin of the second AF tube. If alow growlisheard in the speaker, the stage isallright,and the next stage ischecked. If thegrowlisnotheard,thetroubleisinthesecondAFstage, which isthen subjected to avoltage and resistance check to localize the cause ofthe trouble.Causesoflackofreceiverreception orig-inating in the second AF stage are as follows: Dead semnd AF tube. Openoutput transformer primary. Shorted plate by-pass condenser. Open cathode self-bias resister. 4.Check theFirst AFStage.-With allpreviouschecksshowing normalconditions,theservicemanproceedstocheckthefirstAF stage.Whenaplugged-insolderingironistouchedtotheun-groundedendofthevolumecontrol,averystronggrowlshould normally be heard in the speaker.If it is heard, the serviceman may go on to check the next stage in the signal chain. If the growlisnot heard,the causeofnoreceptionisin the first AF stage and itsassociated parts.Such possible causes are Dead firstAF tube. Opencouplingcondenserinthe gridorplatecircuit. Open volumecontrol. 480ELEMENTSOFRADIOSERVICING V olume-controll ug. shorting to chassis. Short in gridwiring(shielding). Open plate loadresistor. 5.ChecktheDetectorStage.-Thedetectorstageisthenext checkwhenallpreviouscheckresultsarenormal.Amodulated signal at the intermediate frequencyofthe receiver is fedto the grid of the IF tube.If the signal-generator modulation note isheard in th(>speakerasthegeneratorfrequencycontroliswobbledaround theintermediate frequency,thedetectorisallright.The service-man then goeson to check the IF amplifier stage. If the modulation note isnot heard, the trouble isin the detector stage or the IF tube.Possible causes of receiver in operation here are Dead IF amplifiertube. Shorted grid circuit inthe IF tube. Open or shorted plate, screen,or cathode in the IF tube circuit. Defective output IF transformer: a.Open windings. b.Shorted trimmers. c.Leads shorting to the shieldcan. Defective detector tube. Openvolume control. Misalignmentofthe IF transformer. 6.ChecktheIFStage.-Whenthemodulatedsignal-generator output is fedto the control gridofthe IF tube and its note is heard. indicating normal detector operation, the "hot" lead is shifted to the mixergridoftheconverter.If thenoteisnowheardatgreatly increased volume,the mixer and IF amplifier are functioning.The serviceman then proceeds to check the osciHatorofthe converter. If thesignal-generatornoteisnotheardwhenthe"hot"leadis applied to the mixer grid,the followingfactorsmay be defective: Dead mixer(converter)tube. Shorted mixer grid circuit(tuning condenser). Shortedoropenplate,screen,orcathodecircuitsinthemixer circuit. Defective input IF transformer: a.Open windings. b.Shorted trimmers. c.Leads shorting to the shieldcan. Misalignment. 7.ChecktheOscillatorCircuitoftheConverter.-After thesig-nal-generator output isfedat the intermediate frequencyofthe re-ceivertothemixergridanditsnoteisheard,thereceiverdialis adjusted to 600 kc.The signal-generator frequencycontrol isthen SURVEYOFSERVICINGPROCEDURE481 wobbled back and fortharound 600kc.If the note isnow heard at aboutthesamevolumeasthe formermodulatedIF signal,the os-cillatorcircuitisfunctioning,andtheservicemanproceedstothe next check. If the modulation note fromthe generator whichisset to600kc isnotheard,theoscillatorcircuitisinoperative.Possiblecauses are Defective oscillator(converter)tube. Open oscillator coil(either winding). Open or shorted oscillator anode by-passcondenser. Open oscillatoranode dropping resistor. Shortorresistanceintheoscillatorsectionofthegangtuning condenser. Defective oscillator padder condenser. Defectiveoscillator grid condenser. Defective oscillator grid resistor. S.Check theMixerCircuit of theConverter.-If the oscillator is functioning normally, the "hot" lead of the signal generator is shifted tothe controlgridofthe RF tube,ortothe antenna ifthereisno RF stage.The receiver is tuned to 1,400 kc, and the signal generator is wobbled back and forth around 1,400 kc.If the modulation note isheardat increasedvolume,the mixer circuit isfunctioning. If the note isnot heard,the trouble liesinacomponent between the RF grid(orantenna)andthe mixer grid,andthese might be Dead RF tube. Shorted RF control grid circuit(tuning condenser). Openorshortedplate,screen,orcathodecircuitsIIItheRF stage. 9.Check theRFInputCircuit.-If thesignal-generatormodula-tionnoteisheardwhenthehotleadisconnectedtoRFgridbut thereisnoreceptionfromtheantenna,thetroublemustbe inthe antenna coilor leads.Possible causes in this regardare Antenna lead shorting to chassis. Open connection betweenantenna andantenna coil. Open or burned antenna coilprimary. A2-point Check of aSuperheterodyne Receiver.-As the service-man gainsinexperience,hedevelopsshorter methodsofprocedure whichreducethetimeconsumed.Suchashortcut isthe2-point servicingprocedureforcheckingan ,inoperativesuperheterodyne receIver. Ifvisualinspectiondoesnotdisclosethesourceofthetrouble, thetipofaplugged-insolderingironisappliedtotheungrounded endofthe volume control.Thisisthe beginning .ofthe AF signal 432ELEMENTSOFRADIOSERVICING chain.Normally, astrong growl from the s p ~ a k e rshould be heard. Ifitisnotheard,thetroubleisintheaudioamplifier(firstAF, second AF, and speaker)or the power supply,and they are checked stagebystageforthespecificdefect.Ifthestronggrowlis heard,thisonecheckclearsthefir.stAFstage,thesecondAF stage,the speaker,andthe powersupplyofblame forthereceiver inoperation. The serviceman then moves on to the second check point.This is themixergridoftheconverter.Amodulatedtestsignalatthe intermediate frequency of the receiver is fed to this mixer grid.The normal response is the modulation note of the signal generator in the speaker.If this note is not heard, the trouble isin the IF amplifier or the detector stage.The signal-generator output isincreasedand the frequency control iswobbled around the intermediate frequency to see if the receiver is misaligned.If the response is still not heard, the test signal is fedto the IF grid to localize the defect further. If the normal response is heard when the modulated IF test signal is fedto the mixer grid,the IF amplifier and detector stages may be presumed to be functioning.The signal-generator frequency control andreceiverdialsaresetto600kcinorderto checktheoscillator circuit ofthe converter. Thenormalresponseinthislattercheckisthesignal-generator modulation note fromthe speaker.If it isnot heard, the oscillator circuit is not functioning.Avoltage check of the converter stage is then made. If the normal response is heard, the defect must be before the con-verter.A check of the RF amplifier stage and the antenna circuits is now in order. Bythisshort2-pointcheck,thesignalchannelmaybequickly analyzed into three blocks,which are checked over all before resort-ing, if necessary, to stage-by-stage checking. SERVICINGPROCEDUREFORWEAKSIGNALS Thedefectsthatcauseweakreceptionaredifferentfromthose whichresultinnoreception.However,theservicingprocedure that localizesthestageinwhichthedefectliesisthesamesignal checkjustoutlinedforthecomplaintofnoreception.Themain difference in the two checks is the receiver response to the generator signal. For adeadreceiver,allsignalchecksresultinanormalspeaker responseuntilthedefectivestageisreached.Atthatpoint,the receiver will give no response.For aweak receiver, all signal checks give anormal response until the defective stage is reached.At that SURVEY OFSERVICINGPROCEDURE433 point, the receiver will give a weak response, as shown by a loss or no gain over the last normal check before this check. Manyfactorswithinthereceivermayresultinweakresponse. These are tabulated below: 1.Weak tube in any stage. fl.Short in the power transformer winding. 3.Short in the filamentwiring. 4.Jammed voicecoilin the speaker. 5.Weak excitation circuit forthe speaker field. 6.Shorted turns in the output transformer. 7.Opencathode by-passcondenserinthesecondAF,IF,con-verter,and RF stages. 8.Open AVe by-pass condenser. 9.Receiver misalignment. 10.Openplateby-passcondenserintheIF,converter,andRF stages. 11.Openantenna coil. Ifl.Resistance in the gang tuning condenser. 13.Poor wiper contact in the gang tuning condenser. SERVICINGPROCEDUREFORHUM Acommonreceiverdefectisahumlevelthat issohighthatit mars normal receiver reception.This sectionwilldescribethe type of hum that appears all over the receiver dial. CheckingthePowerSupply.-Whenareceiverisbeingserviced fQrthe complaintofanabnormallyhighhum level,themost com-mon defect that causes this condition is the breakdown of the power-supply filtercondensers.Thisisusuallyduetotheagingofthese condensers. The first step, therefore, in trouble shooting for hum is to connect asubstitutecondenseracrosseachofthepower-supplyfiltercon-densers in turn.If the hum level is reduced as a result, the defective filtercondenserisreplaced.The filterchokeofthepowersupply must also be checked forashort that results in inadequate filtering. Tubes as aSource of Hum.-If the filter condensers check perfect, agoodsecondstepistoreplacethetubeswithnewones,oneat a time.Tubesoften introduce hum,especiallythe AFtubes.Such humresultsfromheater-cathodeleakagethroughtheirinsulation, capacitivecouplingbetweentheheaterandotherelectrodes,and emission from the heater to other electrodes or vice versa.Although elimination of hum from these sources is primarily adesign problem, replacement of tubes with new ones may reduce the hum level. 434ELEMENTSOFRADIOSERVICING GridCircuitsasaSourceofHum.-Another possibility forhum isanopengridcircuitinanystageofthereceiver.Thistypeof hum results from abuild-up and discharge of signal at the open grid at arate that may be close to the power-supply hum, and is mistaken forit. Thenextcheckinhumelimination,therefore,isacontinuity check made withan ohmmeter ofallgrid circuits. Previous Service Work as aSource of Hum.-If the cause of hum still proves elusive,the next check isto see if previous service work may not have introduced the trouble.For instance, replacement of part of aspeaker may have resulted in the reversal of polarity of the hum-buckingcoil.Orthewiringmayhavebeendisturbedwith resultingpoor leaddress,particularlyintheregionofthedetector and first AF tube.The diode-plate leads, volume-control leads, and firstAF grid leads must allbe short.They should be dressed close tothechassisandawayfromthefilamentorotherwiringthat carries60-cycle current. TrackingDownElusiveHum.-Thesuggestionsmadeabove should locate most of the common causes of hum.Occasionally,an elusivecausewillescapethenormalprocedurethathasbeensug-gested.Insuchacase,thereceivermustbeexaminedstageby stage. Todothis,removeallthereceivertubes,excepttherectifier. Sinceit isdangeroustooperateapowersupplywithoutanyload, aheavy-duty 5,000- to to 50-watt resistor should be connected as aload from Bplus to ground.Then turn the receiver onand listenforhum.If humispresent,it isduetosomefactor that wasoverlookedinthepower supply,andit must be carefully sought for. If the hum level isnormal, insert the second AF tube and remove the power-supplyresistor load.If the hum now isheard,it isdue to some defect in the secondAF stage.If the hum levelisnormal, insert the firstAF tube and listen forhum.In this way, the tubes arereinserted,onestageatatime,untiltheoffendingstageis reached.Then the components ofonly one stage need be carefully checkedto findthe defect. InthecaseofanACjDCreceiver,wheretubeheatersarein series,tubesmaynot beremoved,asabove.Thestagesmustbe made inoperative in another manner.Ashort fromthe secondAF gridtogroundmakeseverythingbeforethispointinoperative,so farastheireffectonthespeakerisconcerned.Anyhumpresent limitsthedefecttothepowersupplyorsecondAFplatecircuit. If the hum levelisnormal,the short isshiftedtothe firstAF grid SURVEYOFSERVICINGPROCEDURE435 and ground.This adds the first AF plate circuit and the second AF gridcircuittothepartofthereceiverbeingchecked.Thus,in shifting the short to groundfromgridtogridofthe varioustubes, moreandmorepartsofthereceiverthatwillaffectthespeaker are brought in forcheck until the point of hum islocated. Oncethetube-removalprocedureorgrid-groundingprocedure haslocalizedthestageinwhichhumoriginates,nothinginthis stage should be overlooked in the careful recheck.On some remote occasions,anewtubethat replacesabadonemayhaveasimilar defectthatstillresultsinhum.If allelseinthehum-producing stage has been foundto be goodit may be necessaryto replace the oldtube with severalnewonesbefore the hum disappears. Anotherpossibilityinthecarefulrecheckof astage isthepossi-bility of leakage between sections of aby-pass condenser block.For example, aline filtercOlldenser or acondenser connected to aheater lead willcarry alternating current.If they are part of ablock, leak-age to other condensers in the same block may introduce hum. Normally,condensersarechecked foropens,shorts,leakage,and intermitt8'lltopens.Noneofthesechecksrequiresthe removalof the condenserfromthecircuit.However,inmakingacarefulre-check of astage, the condenser lead must be opened, and asubstitute condenserconnectedinitsplace.Thisprocedurewilltakecareof leakage between sections of ablock. Summary of the Causes of Hum in Receivers.-The causes of hum inareceiver may nowbe summarized forquicker use. 1.Open power-supplyfiltercondensers. 2.Defective tubes(cathode-heater leakage). 3.Open gridcircuit. 4.Reversedspeaker hum-bucking coil. 5.Closenessofaudiogridleadsto 'wiringcarrying60-cyclecur-rent. 6.Leakage between sections of aby-pass or filter condenser block. 7.Shorted filterchoke. SERVICINGPROCEDUREFORNOISYOPERATION When areceiver isbrought in with the complaint that it isnoisy, the condition isone of hissing and crackling sounds that are extrane-ousto the desired station signal.Noise may result fromanyone of agreatnumberofcauses,includingnoisepickupbytheantenna, anoisy power line,and noise produced by defective llnits within the. receiver itself.The first two are installation problems.A procedure forhandling them isgiven inChap.17. DeterminationoftheReceiverastheSourceofN oise.-The 436ELEMENTSOFRADIOSERVICING servicemanmustbe abletodeterminebycheckifthenoiseresults from some defect within the receiver.This check isbest handled on the service bench,wherenoise fromantenna andpower line pickup iseither absent or,at least, isafactorwhosenormal level isknown. Todetermineifthereceiveritselfisthesourceofnoise,thean-tennaandgroundconnectionstothereceivershouldberemoved, andtheantennaandgroundterminalsofthereceiverconnected togetherbymeansofashortlink.Then,ifthereceiveristurned on and noises are heard in the speaker, especially when the receiver is jarred, the noise is due to adefective component within the receiver. CausesofNoisewithinaReceiver.-Thecomponentsinare-ceiver that usuallycausenoisy operation areasfollows: 1.Noisy tubes(looseelements). ~ .Corrosionincoilwindings: a.RF transformers. b.IF transformers. c.Audio transformers. d.Speaker fields. 3.Speaker defects: a.Rubbing voicecoil. b.Torn paper cone. c.Loose rim. 4.Poor connections. 5.Noisyvolume control. 6.Swinging shorts in IF transformers. 7.Conductive dirt in vital spots(likesockets). 8.Tuning condensers(shorts and poor wiper contacts). Locating the Source of Noise in aReceiver.-Several of the causes of noisewithin the receiver have already been presented in previous chapters.Chapter9givestliechecksforthespeakerdefectsthat cause noise,and these checksmay be usedin noise analysis.Prob-ably,replacementwiththebenchtestspeakerwilldisclosethis source.InChap.14,thenoisytuningcondenserisdescribed.In Chap.11, defective volume controls are described.Both of them are commonsourcesofnoiseandareeasilyidentifiedasthesources, sincethenoisecomesonwhenthecontrolsareadjusted.Apro-cedureforcleaningtuningcondensersisgiveninChap.14.A noisyvolumecontrolshouldbereplacedasdescribedinChap.11. Agoodprocedureto followwhenhuntingforthesourceofnoise issimilar tothat usedinchecking forhum.Remove allthe tubes, except the second AF tube and rectifier tube.In the case of the ACj DC receiver, connect a short between the second AF grid and ground. Tap the tube and other components in the second AF stage and listen SURVEYOFSERVICINGPROCEDURE437 fornoise.If the noiseisheard,allconnectionsandcomponentsin thesecondAFstageandpower.-supplystagearecheckeduntila poorjointordefectivecomponentisfound.If thenoiseisnot heard, the second AF and power-supply stages are probably in good condition.The firstAF tube isthenadded,or the groundconnec-tionismovedtothe firstAF grid.Then componentsinthisstage arecheckedasbefore.If theyprovesatisfactory,theprocedure isrepeatedforeachstageinthereceiveruntilthetroublesome stageisfound.Thesearchwithinthedefectivestagemustbe thoroughandnotoverlookanyoddandunusualconditionofa component. Aseachtube isreplacedor madeoperative,it shouldbe slightly jarred by tapping the radio sharply.When this is done, anoisy tube willbecomemorenoisy.Thenreplacementwithanewtubefol-lowedby the jarring test willtell ifthe tube wasat fault.In some cases, simply replacing one tube at atime with new ones and tapping the receiversharplywilllocate anoisy tube. The stage-by-stageanalysismayshowanoisystage.If the de-fectisduetocorrosionincoilwindings,jarringtheradiowillnot affect the noise.In this case, the defective winding can be found by anohmmetercheck.GoodwindingsinRFand IFtransformers normallymeasurelessthan100ohms;acorrodedwindingusually measuresseveralhundredohms.Theresistancemeasurementof AFtransformers andchokesalso increaseswhenthey are corroded. Often, unsoldered or poorly soldered connections,or bits of solder or other conducting dirt, may be the cause of noise in astage.They may be difficult to locate because they may be in an out-of-the-way place.Suchdefectsmaycauseintermittentnoiseorintermittent operation.Jarringthereceiverusuallyincreasesthenoisewhen thosedefectsarethecause.Anextremelycarefulsearchmustbe made forthem. If the procedurelocalizesthe IF amplifierasthe sourceofnoise, removethe IF transformersfromtheirshieldcansforacarefulin-spection.Eventhoughanohmmetercheckshowsfreedomfrom corrosion,the leadsfromthecoilstothetrimmers,whichliealong theshieldcan,mayvibrateintocontactwiththecan,producing noise.Inspect the leadsand route them sothat they cannot possi-blytouchtheshieldcan. SERVICINGPROCEDUREFORINTERMITTENTOPERATION Intermittent receptioncanbe dividedintotwomain groups.In one,theradiosuddenlyclicksoffandremainsinoperativefora while;then,just asmysteriously,it resumesnormal operation.In 438ELEMENTSOFRADIOSERVICING theother type,the volumesuddenlydecreasesandthenreturnsto normalalittlelater.Sometimes,thesechangesaregradualrather than sudden.Thisconditionisoften called"fading." Causes of Intermittent Operation.-Intermittent operation(often accompaniedby noise)maybedueto intermittent breaksorother defectsintheantenna-groundsystem,whichshouldbecarefully checked forbreaks,asdescribed inChap.17. In the receiver,manycomponentsmaybethecause.Totackle thecomplaint,theservicemanmightreplaceallcomponentslikely tocausethetrouble,hopingbyeliminationtoremovethecause. Or,he might track downand repair the causative factor.The first procedureisexpensive;the secondistime-consuming. Thewholesalereplacementofsuspectedreceivercomponents includes 1.Alltubes. 2.Allby-pass andcoupling condensers. 3.AnyresistorsthatdissipateheatandmaychangeIIIohmic value as aresult, like voltage-divider resistors. 4.The volume control. In addition,the condensergangshouldbe cleanedandoverhauled. Finally,athoroughsearch should be made forpoorlysolderedcon-nections. TrackingDownIntermittentReception.-Intrackingdownthe cause of intermittent reception, the receiver is allowed to play on the servicebenchuntilthefadingoutoccurs.Thisconditionmaybe hastenedbyjarringthereceiver;or,thereceivermaybemadeto operate inside apacking box to cut off ventilation and produce over-heating;or,it mightbeoperatedthroughanautotransformercon-nected to the power lines,sothat it operates under the condition of an abnormally high line voltage. Thenwhenthereceiverfadesout,anyaccompanyingsymptom, !ikenoiseorsqueal,willbeofaidinlocatingthetrouble.If the receiver stays out, it is serviced as though it were adead or weak one. If receptionisresumedbeforeanyconclusiveevidencehasbeen reached, the serviceman waits for the next fade-out, or attempts once again to induce it. SERVICINGPROCEDUREFORMODULATIONHUM "Modulation" hum isthe name applied to a hum that is heard to-gether with the stationvoiceor musiconly whenastation istuned in.The hum level isnormal at an off-station position on the tuning range.This type ofdefect in receivers is also called "tunable" hum. SURVEY OFSERVICINGPROCEDURE439 CausesofModulationHum.-Themostcommoncauseforthis condition isan open line filtercondenser,or inefficient grounding of the receiver.Whenareceiverischecked at the service bench fora complaint of tunable hum, the first step is to operate the receiver on astation where the hum isvery noticeable.No ground lead should be connected to the receiver, since the home installation may not use one.Thenconnectacondenserofsimilarcapacityacrosstheline filtercondenserandlistenforareductioninthe hum.If thisstep isnot effective,try connecting the condenser fromthe other side of the line to the chassis,asshown in Fig.23-1. I -L ~ - r -"I AddItionalLine'- ~ FoiterCondenser connectedhereto reduceModulcdlon Hum I.... "Oroqlnar -=-'LIne-Filter Condenser FIG.23-1,-Procedure forreducing modulation hum. If the modulation humstillpersists,the next likely cause isleak-age or capacity effectsfromthe heater to other elementsin the RF or'converter tubes.The next step,therefore,isthesubstitutionof tubes, knownto be good,forthe RF and converter tubes. SERVICINGPROCEDUREFORSIGNALDISTORTION Distortioninareceiverresultsinpoortonequalityfromthe loudspeaker.It isusuallyduetotheoverloadingofsomestagein the receiver by asignal that is too large forthe stage to handle. Causes ofReceiver Distortion.-It isunusual forthe signalto be toolarge.Thecommondifficultyisthatthestageoperationhas deterioratedtoapointwhereitcannothandleasignalofnormal strength. Theusualdifficultyistroubleinthegrid-biascircuits,whichis foundbyvoltageanalysis.Thespeaker,ofcourse,isanother possible cause of poor tone.This condition ischecked by substitut-ingthebenchtest speaker forthe receiverspeaker. 440ELEMENTSOFRADIOSERVICING Amorecompletelistofpossiblecausesofreceiverdistortionis given below. 1.Rubbing speaker voicecoildue to a.Off-centervoice coil. b.Warped speaker cone. c.Off-center speaker fieldgap. 2.Shorted cathode by-pass condenser in the secondAF stage. 3.Changedvalue ofsecondAF bias resistor. 4.Open grid leak in the firstor second AF stage. 5.Open volume control. 6.Defective tubes. 7.Shorted or leaking audio coupling condensers. 8.Shortedor leaking A VCby-pass condensers. Less frequent causes ofpoor tone quality are those resulting from previous replacement of defective parts.These include amismatch resulting fromthe replacement of aspeaker or output transformer; a replacement plate circuit by-pass condenser inthe secondAF stage thatistoohighorlowincapacity,resultingintoohighorIowa response;rarely,side-bandcutting resultingfromthe useofanIF replacementtransformerwithextremeselectivecharacteristics. In the latter case,the side-band cutting may be reduced by slightly mistuningeachIF trimmer,broadeningitsresponse characteristic. SERVICINGPROCEDUREFORMOTORBOATING Motorboatingisadefectinareceiverthatresultsinaput-put noisesimilartotheexhaustofamotorboat.Themostcommon causeformotorboatingisanopenoutputfiltercondenserinthe power supply.The only other common cause is an open grid circuit inanyofthe stagesofthe receiver . . RemovingMotorboatinginaReceiver.-Aservicingprocedure for this defect in areceiver isto bridge the output filter condenser in the power supply with atest condenser of similar capacity and to see ifthetroubleiseliminated.If thisprovesineffective,theservice-man proceeds to make an ohmmeter check of all grid circuits, look-ing for an open.In this regard,it should be remembered that AVC decoupling filter resistors are part of their grid circuits.They must not beoverlooked,eventhoughtheyrarelyopen.The mostcom-monopensoccurinthegrid-loadresistorsofthefirstandsecond AF stages. SERVICINGPROCEDUREFORSQUEALSANDOSCILLATIONS Therearemanytypesofsquealsandhowlsinareceiver,allof whichare classifiedunder the generaltermof"oscillation."Their SURVEYOFSERVICINGPROCEDURE441 causesaremanyandvariedandcallfordifferentservicingproce-dures. ChirpsorBirdies.-First,thereisthetypeofsquealorbirdie that appears to spoil reception fromonly one or two stations.This isprobablyimage-frequencyinterference.Aprocedureforhand-ling these isgiveninChap.16. MicrophonicNoise.-Thereisanothertypeofhowlknownas "microphonic"noise.It usuallystartsonaloudsignal,orwhen the radioisjarred,and builds upto astrong howlthat drownsout allreception.It canbecausedbylooseelementsinatubeorby vibratingtuningcondenserplates.Thehowlisstartedbyeither the jarring of the receiver or the vibration resulting fromaloud sig-nal fromthe speaker.The looseelementsbeginto vibraterapidly and introduce sustained high-pitchedAF notes into the tube. Whenareceiverwithmicrophonichowlisserviced,thereceiver isoperatedatlowvolume.Eachtubeinturnisgreatlytapped. Whentheoffenderisreached,a"bong"isstartedwhichsoondies out,sincethespeakervolumeistoolowtosustainthevibration. Anytubeinthereceivermaybethecauseofthemicrophonic howl.However,thedetectorfirstAFtubeisthemostcommon offender. If acheckofthetubesdisclosesnodefect,thetuningcondenser should be investigated.Microphonics due to the tuning condenser areusuallyfoundinsmallreceivers,wherethespeakerandtuning gang assemblyare incloseproximity,or in large receiversdesigned forandoperatedathigh-volumelevels.Inbothcases,original design takes care of the condition by mounting the tuning condensers orthechassis,orboth,onarubbersuspension.Sometimes,even the speaker ismounted on rubber to dampen vibrations.It isonly necessarythereaftertocheckthemountingprovisionstoseethat therubberhasnot becomeoldandcracked,or that thesuspended mounts are still floatingfreely. SquealsovertheMajorPartoftheReceiverTuningDial.-Another type ofsqueal is the one that occursover the entire tuning range ofthe receiveror alarge part ofit.If this squeal isaffected somewhat by tuning,the defectivecomponentisusuallyintheRF or IF portions of the receiver.If the squeal is unaffected by tuning butisaffectedbytheoperationofthetonecontrol,thedefective component isprobably in the audio endofthe receiver.However, theseconsiderationsarenotoftoogreatconsequence,sincethe servicing procedure isthe same forboth conditions. Squeals of either type are usually caused by regenerative coupling. ,The latter is usually caused by poor contact between.a shield and the 442ELEMENTSOFRADIOSERVICING chassisor by the opening ofaby-pass condenser.The service pro-cedureissuggestedbythecause.Shieldsarecheckedfortheir contacttothechassis.Ohmmetercheckingisinadequate,since evenasmallresistancecontact(toosmalltobereadontheohm-meter)maystillcauseinadequateshielding.Thebestprocedure istocleanandtightenallshield-groundcontacts.Whereatube shield has been inadvertently discarded, it should be replaced by the serviceman.This shielding isespecially importantinthe caseofa high-gaintube like the IF amplifier. Openby-passcondensersarecheckedbybridgingatestcon-denser across each by-pass condenser in the receiver.It is important whenmakingthesecheckstoshorttheterminalsofthetestcon-denser after each condenser ischecked. Atest condenser ofabout 0.1mfd can be used forall RF by-pass condensers,even though the condenser being tested differsconsider-ablyfromthat capacity.The substitutionbox describedinChap. 23isvery convenient forrapid testing of thistype.The audio by-passand the power-supply filtercondensers should not be neglected in this test. Sometimes, the broad squeal is due to an error that crept in during previousservicework.Disarrangedorpoorlydressedleadsmay comeaboutinthereplacementofanIForRFtransformer.Or, aninversefeedbackwindingfromthesecondaryofanoutput transformer mayhavebeenreversedduringthereplacementofthe transformer.In theformercase,the leadsmaycouplewithother parts of the receiver and deliver regenerative feedback.In the latter case,areversedinverse feedbackwindingmaydeliverregenerative feedback,rather than degenerative feedback.As aresult, an audio oscillation isset up. Poorlydressedwiringmaybecheckedbymovingthesuspected wires with abakelite rod, while ;the receiver is oscillating.Achange in the squeal indicates that the wire isat fault.Generally, the grid and plate leads are the "hot" leads and should be routed close to the chassisanddirecttotheirconnectionpointswithoutcrossingeach otherorcomingclosetoother wiring. Thereversedinversefeedbackwindingmaybecheckedforby reversing the primary or secondary wiresofthe output transformer and by observing ifthere isany improvement. Asummary listingoffactorsthat might cause broad squealsand oscillationsfollows: 1.Open power-supply output filtercondenser. 2.Opensecond AF plate by-pass condenser. SURVEYOFSERVICINGPROCEDURE443 3.Reversed feedbackwinding(after output transformer has been replaced). 4.Open shielding. 5.Incorrect lead dress. 6.Open A VCby-passcondenser. 7.Openscreenby-passcondenserIIItheRF,IF,orconverter stage. s.Openplatedecouplingby-passcondenserintheRF,IF,or converter stage. AIRCHECKOFARECEIVER Thefinalstepinservicingareceiverisfirsttocheckthatthe original complaint has been removed,and then to check the receiver inaUrespectsfornormaloperation.Thisfinalcheckisknownas the"air check." To make the air check, the receiver is connected to an antenna and turned on.The tuning dialisrotated to anonstation position,and the hum level is noted for normal operation. At the same dial position, the volume control is rotated from mini-mum to maximum, in order to determine if it isnoisy.The same is thendoneforthetonecontrol,ifpresent. Thenthedialisrotatedtothelow-frequency(550-kc)end,the volumecontrolissetforamoderatevolumelevel,andthedialis rotated toward the high-frequency(1,500-kc)end.The stations are checked offasthey appear.This procedure checksthe dialcalibra-tion and the sensitivity of the receiver.All stations that the service-man knowsare normallypickedupinhislocalityshouldbepicked upbythereceiverbeingchecked.Failuretoreceiveanyofthem indicatesaweakreceiver.Goodjudgmentshouldbeusedbythe servicemaninthistest.Obviously,asensitivesuperheterodyne receiverwithanRFstageandtwoIFstagesshouldpickupmore stations than areceiverwith no RF stage and only one IF stage. Asthe stationsarepickedup,the selectivityofthe receiver may be determined by the dialspacethat eachstation covers,especially the strong localstations.If astrong localstation stretches over 30 kc of the dial,the receiver selectivity ispoor and should be checked. Misalignment isindicated. Tonequality ismost easilychecked by listening to speech rather than to music.Clear, crisp,intelligible speech isasign of good tone quality,especiallyforthehighaudiofrequencies.Thenturnto somesymphonicmusicprogram,andlistenfortheresponsetothe lowfrequencies. Thenextcheckisforthepowerhandlingofthereceiver-from 444ELEMENTSOFRADIOSERVICING whisperstothelimitthatthespeakerwilltakewithoutrattling. This willnot be much forasmall speaker.But alarge speaker in a high-fidelityreceiverought to be capable ofroaringwith goodtone quality. The course over the tuning dialwilldisclosewhistles,birdies,and squeals.The serviceman must be on thealert fortheseeffects.A sharp slapon the receiver willshowupany noisyconditions. The finalstep isthe check ofthe operationofanyother controls onthereceiver,likefidelitycontrols,phonographswitches,push buttons, and short-wave operation. CASEHISTORIESOFDEFECTIVERECEIVERS Atthispoint,it isadvisabletoexaminetheworkofapractical serviceman making repairs on the bench.The followingtabulations are the actual case-history records made by aserviceman with wide experIence. Case1.Complaint: Receiver isdead. 1.AppliedAF signalto firstAF grid ........................... '".Normal. fl.Applied IF signal to IF grid .................................... Normal. S.Applied IF signalto mixergrid. . . . . . . . . . . . . . . . . . . . ......... No response. 4.Replaced mixer tube ........................... Receiveroperates normal. 5.Air check ........................................................ O.K. Case 2.Complaint: No reception. 1.Signalto firstAF grid .......................................... Normal. fl.IF signal to second IF grid ................................. No response. 8.Replaced secondIF tube ........................ Receiveroperates normal. 4.Checked: Sensitivity ................................................... O.K. Selectivity ..................................................... O.K. Quality ........................................................ O.K. Dial mechanism ............................................... \Vorn. 5.Replaced dial belt. Case 3.Complaint: Noisyand fading. 1.Jarred receiverwhileplaying ................................. Very noisy. 2.Lightly tapped tubes and components ....... , ........ Noiseallover chassis. S.Replaced tubes oneat atime and t: