11.CheckingComponents
Soyou''veputacircuittogetherandasfarasyouknoweverythingappearstobeok,butit
doesn''tworkasexpected.Evenworse,itrefusestogiveanysignsoflife.Whatdoyoudo?
First,checkthecircuitformechanicalfailures,likenon-connectedwires,brokenviasonthe
board(theseareholesontheprintedcircuitboardthathaveametalcoatingdownthe
lengthoftheholetoconnectonesideoftheboardtotheother),badbatterycontacts
insidethecase,brokenpinsonacomponent,coldsolderjoints,etc.
Ifthisdoesn''tcomeupwitharesult,youshouldcomparevaluesofcomponentswiththe
schematic.
Youmayhaveputacomponentinthewrongplace,orreadvaluesthewrongway.Maybe
youforgotkinfrontofOhms.MaybeyouconnectedthesupplytothewrongpinofanIC.
Thenextstepistotesteachcomponentontheboard.
StarttroubleshootingbymeasuringDCvoltagesatcertainpointsoftheboard,and
comparingthesevaluestotheschematic.So,byknowingtheoperationofthecircuityou
starttheprocessofeliminationtofindthe“suspect”component.
Ifthereareseveral“suspects”,andthisisnotarareoccurrenceincomplexdevices,the
testingisdividedintogroupsofcomponents.Youstartcheckinginreversesolderingorder,
thismeansyoustartwithcomponentslastsoldered,becausethosearethemostsensitive
componentsonthecircuitlikeintegratedcircuits,transistors,diodes,etc.
Thefastestandsimplestmethodtotroubleshootistousean“ohm-meter.”
Inmostcasesyoudon''thaveanohm-meterbyitselfasitisusuallyadedtoanammeter
andvoltmeterinoneinstrument,calledAVOmeterormultimeter.
Thesafestandmostaccuratemethodistodesolderthecomponentfromtheboardwhen
testingit,becauseothercomponentscouldleadtoawrongdiagnosis,soyouhavetobe
verycarefulwhentestingin-circuit.
Ok,youshouldknowsomething
aboutmultimetersnow.Thereare
twokinds:analoganddigital.Analog
onesareitemsofthepast,and
sincetheyuseaneedletotellyou
values,itcanbedifficultdetermining
therightvalue.Digitalmeters,on
theotherhandhaveadisplay.You
shouldgoforthistype,although
bothcomeindifferentsizesand
withdifferentranges.Theirpriceis
fromseveraldollars,toseveral
hundredsofdollarsforreallygood
professionaltypes.
Twoinstrumentsareshownin11.1.
11.1DiodesandTransistors
Whenusingananaloginstrumenttotestadiode,theneedlewillswingalmostfullyacross
thescalewhenthediodeisplacedinonedirectionandhardlymovewhenthediodeis
reversed.
Theneedledoesnotmeasuretheresistanceofthediodebutrathertheflowofcurrentin
onedirectionandno
current-flowinthe
otherdirection.
Ifthevalueisequal
toornearequal,
eitherloworhighin
bothdirections,the
diodeisfaulty,and
shouldbereplaced.
Digitalinstruments
haveapositionon
thedialtomeasure
diodes,asshownin
11.1b.Whenweconnectprobestoeachother,themultimetershouldbuzz,whichsignalsa
shortcircuit,anddisplaytells0.Whenweseparatetheprobesthebuzzingstops,anda
symbolforopencircuitisdisplayed(thiscanbeeither0Lor1).Nowweconnectprobesto
thediode(11.3a).Thenwe
reversethediodeandconnectit
again(11.3b).Ifthemeasured
diodewasok,oneofthetwo
measurementswouldhaveshown
avaluewhichrepresentsa
minimumvoltagethatcouldbe
conductedthroughthediode
(between400mVand800mV),
andtheanodeistheendofthe
diodewhichisconnectedtoprobe
A(redone).Thediodeisfaultyif
youhearabuzz(closedcircuit)or
somevaluewhichrepresents
infinity.
Transistorsaretestedinasimilar
fashion,sincetheyactastwo
connecteddiodes.Accordingto11.4b,thepositiveprobeisconnectedtothebase,andthe
negativeprobeisfirstconnectedtothecollectorandthentheemitter.Inbothcasesthe
resistanceshouldbelow.Afterthat,youdothesamething,onlywithswitchedprobes.The
negativeprobeisconnectedtothebaseandyoutestthecollectorandemitterwitha
positiveprobe.
Bothcasesshouldproduceahighvalue
onthemeter.
WhentestingPNPtransistors,allstepsarethesame,butthemeasurementsshouldbe
opposite:on11.4atheyarehigh,andon11.4ctheyarelow.
Ifyoutesttransistorsusingadigitalinstrument,theprocessremainssimilartotheonewith
diodes.Eachdiodeshouldproduceavaluebetween400mVand800mV.Manymoderndigital
multimetershaveasocketfortestingtransistors.Thereis,asdisplayedon11.5,aspecial
socketwherelowandmediumpowertransistorsfit.Ifyouneedtotesthighpower
transistors,thinwires(0.8mm)shouldbesolderedtotransistor''spinsandthenplugged
intothesocket.Asdisplayedon11.5,atransistorispluggedintothesocketaccordingto
itstype(PNPorNPN)andtheswitchwithahFEmarkingisbroughtintoposition.Ifthe
transistorworks,thedisplayshowsavaluewhichrepresentsthecurrentamplification
coefficient.If,forexample,atransistoris
tested,andthedisplayshows74,this
meansthecollectorcurrentis74times
higherthanthebasecurrent.
11.2Transformersandcoils
Transformersaretestedbymeasuringthe
resistanceofthecopperwireontheprimary
andsecondary.Sincetheprimaryhasmore
turnsthanthesecondary,andiswound
usingathinnerwire,itsresistanceishigher,
anditsvalueisinrangeoftensofohms(in
highpowertransformers)toseveral
hundredsofohms.
Secondaryresistanceislowerandisin
rangebetweenseveralohmstoseveral
tensofohms,wheretheprincipleof
inverserelationsisstillinplace,high
powermeanslowresistance.
Ifthemultimetershowsaninfinitevalue,
itmeansthecoiliseitherpoorly
connectedortheturnsaredisconnected
atsomepoint.
Coilscanbetestedinthesamewayas
transformers–throughtheirresistance.
Allprinciplesremainthesameaswith
transformers.Infiniteresistancemeans
anopenwinding.
11.3Capacitors
Capacitorsshouldproduceaninfinitereadingonamultimeter.Exceptionsareelectrolytics
andveryhighvalueblockcapacitors.Whenthepositiveendofanelectrolyticcapacitoris
connectedtothepositiveprobeofananaloginstrument,andanegativeendtoanegative
probe,theneedlemovesslightlyandgraduallycomesbacktowardsinfinity.Thisisproofthe
capacitorisok,andtheneedle''smovementischargebeingstoredinthecapacitor.(Even
smallcapacitorsgetchargedwhiletesting.)
Variablecapacitorsaretestedbyconnectinganohm-metertothem,andturningtherotor.
Theneedleshouldpointtoinfinityatalltimes,becauseanyothervaluemeanstheplatesof
therotorandstatoraretouchingatsomepoint.
Therearedigitalmetersthathavetheabilitytomeasurecapacitance,whichsimplifiesthe
process.Withthissaid,itisworthmentioningthatcapacitorshaveconsiderablywider
tolerancethanresistors,(about20%).
11.4Potentiometers
Totestapotentiometer,(pot),oravariableresistor,theprocessisrathersimple–you
connectthecomponenttotheprobesofametersettoohmsandturntheshaft.
(A“noisy”potcanberepairedusingaspecialspray.)
11.5Speakersandheadphones
Whentestingspeakers,theirvoice-coilcanbebetween1.5and32Ohms.Thevaluemarked
onthespeakerisanimpedancevalueandtheactualDCresistancewillbelower.When
measuringaspeakerwithananaloguemeter,youshouldhearaclickwhentheprobesare
connected.
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