AVO 160A Instructions for use

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AVOCT160AOperating Instructionsand ServiceManual
ThisAVOCT160Amanualistheresult ofcombining(and editing)twodifferentsetsofphotocopiesof
AVOCT160AmanualsandalsofrominformationfromtheoriginalAVOCT160manuals.ThisAVO
CT160Amanualhasbeen kept asoriginalaspossible,eventhoughsomephotocopiesof the AVO
CT160A pageshavebeen replacedwithAVOCT160 pageswherethetextdidnot differinanypoint, but
wherethe qualityoftheCT160 scannedpageswasmuchbetterthan theCT160A scannedpages. All
figuresandphotoshavebeenkept intheiroriginalshape althoughmost havehadtobeeditedduetothe
poorqualityofthe originalphotocopies.
The schematicsat the endofthe manualhavebeenextendedwiththreenewsetswherethefirst isan
unalteredoriginalredrawnschematic,thesecond schematicisaredrawnand corrected schematicand
thethirdisaredrawn,correctedandmodifiedschematic.Thesecondschematichasbeencorrectedin
thefollowingareas:
D2: Changedfrom66V RMS winding to99VRMS winding,calculatingthecurrentinthe circuit
showsthatthe 99V RMS winding isnecessaryforthecircuittoworkcorrectly.
R4& R41:Changedplacesinschematic,the componentliston page17 ofthe ServiceManual
andalsothecomponentliston thepagebeforetheoriginalschematicshowsthisasthe correct
order,aswell asan internalcomponent placementcomparisonbetween aCT160A andaCT160.
R37&R37 shownasR37A and R37B aspercomponentliston page18 ofthe ServiceManual,
consistingofone 13 "selected"resistoreach.
SH6: Ground connection fortags2-5, wasmissingfromoriginalCT160A schematicbut canbe
foundinlaterCT160schematicsand alsointheoriginalCT160A & CT160 testers.
WIRES & COMPONENTS:Movedforclarity.
The thirdschematichasbeenmodifiedinthefollowing areas:
All ofthecorrectionsabovefromthesecond schematicplus:-
D5, R42& R38: Anodevoltagerectification added,componentsusedD5=D1, R42=R38=100
k±10%1W–thismodificationhasbeentested andproved toworkinanoriginalAVOCT160
andisalsousedbyAVOinthe AVOMkIV.
D6, D7& C4: Meterprotectioncircuitadded,D6=D7=D1,C4=8µF/ 63VDCNonpolarized
(preferablyPolypropylene)–thismodification hasbeentestedand proved toworkinanoriginal
AVOCT160andisalsousedbyAVOintheAVOMkIV.
Ifyou wanttoread moreabout the innerworkingsoftheAVOValveTestersincluding theAVOCT160
and AVOCT160AI recommendthatyouvisitthe“UKVintageRadioRepairandRestoration
Discussion Forum“here http://www.vintage-radio.net/forum/index.php and readthethreads:
“Everythingyou wantedtoknowaboutthe innerworkingsofan AVOMkIV Valvetester!”
“Somemorethat youwanted toknowabouttheinnerworkingsofanAVOValvetester!”
“Allthatyou wantedtoknowabout the innerworkingsofan AVOCT160Valvetester!”
“AVOCT160-anodecurrentmeasurements, calibrationresistorsand themeterworkings!”
“AVOCT160A –Specialeditionof theAVOCT160 -comparison!”
I haveincludedadaptedversionsoftheCT160A andCT160textsaboveinthisdocument.
The workofeditingandtestingthecorrectionsandmodificationsinthe AVOCT160Amanual, andwriting
of thearticlesabove, hasbeen performedbyMartinForsberg,Sweden, and Euan MacKenzie, Australia.
Sweden2010-11-18

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The followingtextsareadaptationsdone tothetextspublishedbyme,MartinForsbergandEuan
MacKenzie,at the “UKVintageRadioRepairandRestoration Discussion Forum“and
containimportantinformationabouttheAVOCT160A ValveTesteraswellasimportant informationonthe
AVOCT160 ValveTesterwhichtheAVOCT160Aisbased on.
AVOCT160A–Special edition oftheAVOCT160-comparison!
Thistextwillendeavourtoexplainhowthe AVOCT160A worksincertainareasincomparison tothewell
knownAVOCT160.Someofthe informationcomesfromMr.YutakaMatsuzakawhokindlysentmea
copyoftheAVOCT160AOperating InstructionsandcalibrationInstructions.Mr.YutakaMatsuzakahave
publishedsomeinformationon hiswebsite, http://www6.wind.ne.jp/yutak/avo_ct160/index.htm,about
calibrationoftheCT160AandalsoacomparisonoftheCT160 and CT160A. Ihavealsobeen given
photocopiesofanotherversionoftheCT160AmanualbyFrankPhilipsewhichhereceived fromWimDe
Grotte,thesecopieshaverecentlybeenpublishedon FrankPhilipse’swebsite:
http://frank.pocnet.net/instruments/AVO/index.html .
Someofthemodificationsthat AVOdidtotheCT160A havealreadybeenmentionedinthe article:”AVO
CT160-anodecurrentmeasurements, calibrationresistorsandthe meterworkings!”on the“UK
VintageRadioRepairandRestorationDiscussionForum“,whichalsofollowsthistext.But
thedifferenceswillbe describedinmoredetailinthisarticle.
The AVOCT160AhasbeenreferencedtoastheDutchMilitaryversionsinafewplacesandIhavefound
no othermentionsofit.
PleasealsounderstandthatI havenot had access toanAVOCT160Amyselfforthisarticle,onlyto
secondhandpicturesandinformationplusphotocopiesofAVOmanuals.Howeverthemodificationsare
notcomplicatedonceyouunderstandhowanAVOCT160 and anAVOMkIVworksunderthehood.
Anyerrorsaremyown,but Ihavemademybest tocheckeverything beforepublishingithere.
Part1:ModificationsdonebyAVO
AVOhaveupdatedtheAVOCT160A toworkmorelikethe laterAVOMkIVinmanyareas,butnotall.
Apartfromthe looksonthefrontpanelsandtheextravalveholderpanelconnected viathenewcontact
on theoriginalvalveholderpanelmostmodificationsareeasy todescribe andunderstand if you
understandhowthe originalAVOCT160 works.ModificationsthatAVOhavedonetothe AVOCT160A
are:
A. All valveshavebeenreplaced withSilicon diodes.(D1–D4),making it necessaryforfurther
modificationsdescribedbelowtocompensatefordifferent voltage dropthan valves, and alsofordifferent
behaviourcompared tovalves.
B. Therangeforcurrentcapabilitiesforrectifiervalveshavebeenextendedupto180mA,thissimple
modificationwillbedescribedbelowinthe partdescribing theover-currentrelaymodificationastheyare
closelyconnected. (R40 +modificationsforovercurrentrelayRL3)
C. Sincethevalveshavebeen disposedofthe 6.3V(markedas5.8V inthe AVOCT160 schematic)
winding fortheheatershavebeen removedfromtheschematicfortheHeatervoltage transformer.
D. The500 resistorintheD/Rcircuithavebeenreplaced withtwo1K resistortohandlethe higher
currentrange.(R12& R13)

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E. Theovercurrent relaywindingRL3havebeenshunted witharesistorforthe 120mAand180mA
ranges,anextraswitchwaferfortheSETIa& DRswitchhavebeenadded forthismodification. (R37&
SJ1, unfortunately,inmyopinion,re-numbered byAVOtoSJ1asit wouldlogicallyhavebeen SJ5asSJ1
–SJ4arealreadypresentinthe AVOCT160)
F. The gridvoltscontrolhavebeen modifiedwithaswitchforthe capabilityof doubled voltage upto80V,
thecircuit havealsobeenmodified,moredetailsofthisinthetextbelow.(R4, R39,RV5andSL1&2have
beenintroduced plusanewwinding on theAnode&Screenvoltstransformer)
G. Thecalibration resistorcircuit havebeenmodifieddue tothemodificationdone tothegridVoltscontrol,
thismodificationisalsodescribedbelow. (D2,RV6, R41andR3plus99VRMS windingfrom
Anode/Screen voltstransformer–oldR4removedfromcircuit)
H. TheAnodeCurrentcontrolshavehad theseriesresistorchangedtocompensateforthecurrent
throughthecircuit whenaSilicondiodehavebeeninserted.(R6)
I.The SilicondiodeintheScreenvoltscircuithavemadeitnecessarytointroduceanextraresistorto
ground.(R38)
Inall otheraspectsthemodificationstatusof the AVOCT160Acorrespondstothelastmodificationstatus
of theAVOCT160 uptoresistorR36 fortheGridtoCathodeconnection.
ModificationA&C:
Replacing the valveswithSilicon diodesmeant that theextraheaterwindingisnotnecessaryanymore
and ithasbeenremovedfromtheschematics,butsinceno actualAVOCT160Ahavebeen availablefor
check itisnotknownbymeifthewindingisstill thereornot.TheSilicondiodesalsomeansthat some
circuitswill havetobecompensatedsincetheyhavealowervoltagedrop thanthevalves.
ModificationB, D&E:Currentmeasuringrangeforrectifiervalves increasedup to180mA.
Tomeasurerectifiervalvesonthe180mArangeAVOhavemodifiedtheCT160A inthe samewaythat the
AVOMkIVisdesigned,withoneextraresistorontheSETIa& DR switchforthemeasuringrangeand
alsoaresistor, R37,whichisconnectedinparallell withtheovercurrent relaycoilRL3.Thisextraresistor,
R37, isworkingasashuntmaking lesscurrent runthroughtherelaycoilsoit willnotactuateasbeforeon
theold120mA rangeandon thenew180mA range.Thisisnecessarysorectifierswithahighercurrent
capabilitycanbe measured,the relaywouldotherwiseactuatetoearly.The 500 resistorR12onthe
D/Rcircuit intheAVOCT160havebeen replacedwithtworesistors,R12 & R13at1K inparallell
making500 , tobeabletohandlethecurrentflowingthroughthematthehighercurrentrangesfor
rectifiers.

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ModificationF:
The GridVoltscontrolhavehaditsrange extended byaswitchwhichmakesitpossibletodoublethe
voltageon thescale, intheX1positionyouhave0-40Vand intheX2positionyouinstead have0-80V.
Thismodificationhasbeendone byintroducinganewwindingon theAnode/Screenvoltstransformer.
Thiswindingnowhavethree taps,ground,55V RMS and 120V RMS. TheswitchSL1&2forthe X1and
X2rangeswitchesbetweenthe55VRMSandthe120VRMS windingsplusitalsoinsertsorshorts
resistorR39whichcompensatesthe gmmeasuringvoltagecircuit.Whenthe120VRMSwindingis
selected resistorR4and potentiometerRV5isconnected forcalibrationpurposesofthe0-80V rangein
thesamemannerasRV3isusedforthe 0-40V range. Thebiggestchangetothe circuit isthat the
negativevoltage whichearlierwasusedfromanoutof phasewinding of66V RMShasbeenremoved all
together!ThisisthencontradictingAVOstext inthepatentthatthisvoltageisnecessaryforthevalvenot
toformadiodebetween thegrid(orscreen)tothe cathodewhen thevalveundertestisnotconducting.
Maybethisisatrade-off forsomereason,maybe it wasmoreexpensivetohaveitthereand haveit
changed instep withthegridvoltsswitch,ormaybeAVOfound that it wasnot necessarytohavethat
voltagethere.
ModificationG:
HereAVOhavechangedthecalibrationresistorcircuitintroducingapotentiometer,RV6,forcalibration
purposesandalsochangedresistorR3to1.22 M .AVOalsochanged thecircuit byintroducingaresistor
(R4, wronglynamed R41intheoriginalschematics)toground,thishasearlierbeen mentionedas
necessaryforshortingthechargebuildupinsidetheSilicon diode when itisnotconducting.Atthesame
timethe windingwhichsuppliesthiscircuit hasbeenchanged tothe99VRMSwinding,thisisnecessary
toensurethatthe samecurrentasintheAVOCT160 isdriven throughthecircuitforthemeasuring
purposes.
The calculationforthe99VRMS windingbecomes:
99V RMS equals89.13V Mean,theconversion factorbeing1.1107.
99V RMS / 1.1107 =89.13V Mean.
Thisvoltageishalf-waverectifiedwhichresultsinhalfthe voltage aftertherectification, minusthe voltage
drop ofthediode,whichcan beassumedtobe 0.7V.
89.13VMean/ 2–0.7V =43.87V
When thisvoltageisdividedbythecurrentnecessaryinthemeasuring circuit, whichhasearlierbeen
showntobe35.775µA theresultingcalibration resistancewillbe:
43.87V/ 35.775µA=1.226M ,witha+/-1%1.22 M resistorwhichrangesfrom1.207M upto1.232
Myou mightend upjustoutsidetherangepossibletoadjustwiththe20 K potentiometerRV6soitis
best tousearesistorcloseto1.22M .

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ModificationH:
Thismodification,of seriesresistorR6, hasalreadybeen describedintheearlierarticleonAVOCT160
modificationsandAnode Current control“inthe UKVintageRadioRepairandRestoration
Discussion Forum“andonlyconfirmsourcalculationsthere.Thecurrentthrough theAnode Current
controlsneedstobe exactly12.5mAandwhen thevalveisreplacedwithaSilicon diodewithamuch
lowervoltagedrop thedifferenceinvoltage dropwill havetobecompensated forwithanincreasein
resistorR6.
ModificationI:
AVOalsochanged thecircuitbyintroducingaresistorR38inthe ScreenVoltscontroltoground, thishas
earlierbeenmentionedasnecessaryforshortingthecharge buildupinside theSilicon diode when itisnot
conducting.
Ashasearlierbeenmentioned itispossibletoinsertaSilicon diode intheAnodeVoltscircuit justafterthe
Anode VoltsswitchbeforetheElectrodeSelectorswitch,plusan extraresistorof100K +/-10%at1
Watt –thisresistorneedstobe thereforthesamereasonasR38 above.
NewValveSockets:
The extraValveHolderpanelthatisconnected viaacabletotheValveHolderpanelon theCT160A box
havebeenequippedwiththefollowingvalvesockets:
PencilTubes,5WiresinLine,FlyingLead, 7WiresinLine, B7A,B9D,SpecialBasefor2C39, Acorn,
Nuvistor,F8andB8D
Thisfinishesthe currentcomparisonbetweentheAVOCT160andtheAVOCT160A.
Part2:ModificationsdonebyAVOtotheover-currentrelayprotection
HereissomemorenewinformationasI havestudied the,soon finished,AVOCT160Amanualinsome
moredepth!Byfinished Imeanthat Ihavesoonfinishedcleaningupallthepages.
WhileAVOwaschangingthecurrent rangeforDiodesand Rectifiers, byaddingthe 180mA range,they
alsochangedwhichvoltagesthatareused fortestingDiodesand Rectifiers. IntheAVOCT160theyused
thewindingsfor40V, 75V and125V tocoverthe rangesfrom1mA up to120mA but intheAVOCT160A
thesewindingshavebeenchangedto40V, 75V,150Vand200V. Thetwohighervoltagesareused in
conjunctionwiththe added SwitchSJ1whichconnectsresistorR37inparallelwithrelaycoilRL3forover
currentprotection. Fromreading the componentlistIcouldalsosee that AVOusedtworesistorstomake
up theresistanceofR37,whichisstatedas13 each. Thiswill thenmakeR37 tohavearesistance
value of 6.5 .
Withacoilresistancecloseto5 , whichEuan MacKenziehashelpedmetomeasure,forRL3this
meansthatyouwill haveroughlya57%decreaseinsensitivityinRL3asthecurrent isdividedbetween
thetworesistorsmaking upR37and the relaycoil.Themathsforthisissimply:
13 inparallelwith13 givesyou6.5 .Thisthengivesthecurrent through therelaycoilRL3 tobe:
6.5 / (6.5 +5 )=6.5/ 11.5=56.5%
Thisfigureof57%correspondsfantasticallywell withwhatAVOdidintheirAVOMkIVtesteronthe
currentrangesforDiode and Rectifiertestingontheranges60mA,120mAand180mA.HereAVOused
two3.9 resistorsinparallelwiththerelaycoilRL3,whichhasaresistancecloseto1.5 inanAVOMk
IV, thiswill alsomeana57% decreaseincurrent sensitivity.Themathsforthisissimply:

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3.9 inparallelwith3.9 givesyou1.95 . Thisthen givesthecurrent throughtherelaycoilRL3tobe:
1.95 /(1.95 +1.5 )=1.95/ 3.45 =56.5%
Ifyou check thecalculationsaboveandtakealldecimalsintoaccount youwillactuallyseethatthesetwo
calculationsgiveexactlythe sameresult, thefactorbetweenthenumbersisexactlythreeand one third
(3.333.....).ThatwaswhatI meantwiththefantasticallywellcorrespondancebetween the AVOMkIV and
theAVOCT160Ahereinthiscircuit!
Isthisjustacoincidenceorsomething thatAVOplanned?Myguessisthatitisplanned butthe resistance
forthe resistorsandRL3 willofcoursevarybetweeneachtester,butinthe AVOCT160A servicemanual
AVOsays that theseresistorswill be "selected",whichprobablymeansthat theyareselectedtoworkin
eachAVOCT160Atesterbeingbuilt.AVOstatedthe tolerancesfortheseresistorstobe+/-10% inthe
AVOMkIVand+/-20%intheAVOCT160A.

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AVOCT160 -anodecurrentmeasurements,calibration resistorsandthemeterworkings!
Thistextwillendeavourtoexplainhowthe AVOCT160worksincertainareas,especiallytheAnode
Currentcontrols, thecalibrationresistorsand the metercircuit.It will explainhowtoreplacethe rectifier
valveswithSilicon diodes,asAVOdidintheirlatermodel,theCT160A; andalsoexplainwhythereisa
slightmodification tothecircuitasaconsequence.It will alsoexplainhowyoucanimprovethecalibration,
bymakingafurtherslightmodification tothe calibrationcircuit.It will alsoexplainhowyou can placea
SilicondiodeintheAnodevoltage circuit and asmallmodificationthat thisentails. Allofthese
modificationshaveactuallybeen triedouton anAVOCT160(SerialNo. 4087YF)andthemodificationdo
notchangetheresultsinanyway;you getexactlythesameresultswhen comparisonstoastandardised
valve(aCV455)aremade, bothbeforeandafterthe modificationswereperformed.Thereisalsoashort
descriptionofthefunction of R14 andSW3,whichareused intheSETZEROpositionon themA/Vdial.
The protectionofthe meterisalsodiscussed and acomparison withAVO’sotherValveCharacteristic
Metersismade.
Thistext isacollaboration betweenme,MartinForsberg, and myfriend EuanMacKenzie, whereI have
writtenmost ofthe textandEuan havemade mostofthemeasurementsandallofthe modificationsinhis
AVOCT160. Euan hashelped mewithproof readingof thetextandalsosuppliedindepthinformation
aboutcomponentsandmathematics.
Itisquitelong, but I reallyhopethatyou find itworthwhiletoread through totheend!
Thereismoretocomeinthefuturewithalookat theDutchMilitaryspecialeditionoftheAVOCT160
namedAVOCT160A.The AVOCT160Amight havebeenused byotherstoobutithasbeenreferenced
toastheDutchMilitaryversionsinafewplacesandI havefound noothermentionsofit.
Part1:AnodeCurrentControls;and replacing theCV140 valveswithSilicondiodes:-
Inthe AVOCT160 theanodecurrent ismeasuredbymeansofaPotentiometer, whichisalaboratory
instrument fortheprecisionmeasurementof anunknownvoltage.If youdidPhysicsinthesixthformat
school,youwouldhaveencountered the Potentiometer; initssimplestformitiscomprisedofa1mlength
ofNichromewire,alongside a1mwoodenruler;driven bya2V leadacidcellandcalibrated using a
galvanometerinserieswithWestonStandardcell,todetect thenull. It isacommon misconceptionthat
theCT160 isaBridge; itisnot, becauseaBridge hasfourarms,whereasthe Potentiometeronlyhas
three.IntheCT160,the anode currentflowsthrougha200 sensing resistor,thevoltagedropproduced
acrossthisresistoristhen compared toaknownvoltage; whichisdevelopedinaseparatecircuit,
comprised ofaconstantcurrentflowingthroughnine switchedresistors, R15to23,andone variable
resistor,RV4;whicharelabelled the ‘ANODE CURRENT’controls.
The anodecurrent controlsperformthe sameactionasthe backing-off controlsdointheAVOMkIII and
MkIV,theyeffectivelybalanceoutthe largestandinganodecurrent, whichisflowingthroughthe200
resistorinthe anode circuit.Thismakesitpossibleforthe metertoshowthesmallcurrent change
producedbythemA/V control, whenthemA/V measurementisbeingperformed.Inthe CT160,theanode
currentcontrolsaredesignedinsuchawaythatyou havetoread theanodecurrent, whichisflowing
throughthevalve,onthescalesprovidedon thecontrols, not onthemeter. Whereasinthe AVOMkIII
and MkIV,thebackingoffcontrolsarenotprovidedwithanyscales,andtheanode currentisindicatedon
themeter.
The anodecurrent controls(orbacking-offcircuit)act asapowersupply,whichisdesigned toproduce
exactlyonevoltforeachstepfrom0mA(0V)to90mA(9V)onthe rotaryswitch, orcoarseanodecurrent
control;plusthevariable0mA (0V)-10mA (1V)fromthefine anode current control; (infact,the maximum
on thefineanodecurrentcontrolisactually11.25mA, or1.125V). Thusmakesit possibleto‘balanceout’
amaximumof10.125V;whichcorrespondstoan anodecurrent of101.25mA. Thesmallextravoltage on
thefine anode currentcontrolisintended toproduceasmalloverlapon eachrange; sinceitispossibleto

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turnthefine controlpastthe 10onthe scale. Thisisprovidedsoyoucansee if thecurrentisslightly
higherthanthemaximumon that current range,orifyoushouldswitchtoahigherrange.When the
maximum(indicated)anodecurrentof100mAisbeing measured,inrealityonly50mA isflowingthrough
the200 anode current resistor; becausethe valveworksasahalfwaverectifierfortheappliedAC
voltagetoit. Now50mA flowingthrough200 producesavoltagedrop of10Vacrossthe anodecurrent
senseresistor; whichisthen compared,usingthePotentiometer,tothevoltageproducedbytheanode
currentcontrolcircuit.
Inthe coarseanodecurrentcontrol,thevoltagedropineachstep ofthe rotaryswitchmustequalexactly
1V;sinceeachstep hasaresistanceof80 , then theremust be aconstant current of12.5mAflowing
throughthecircuit. Nowthisvalueexceedsthe manufacturer’smaximumvalueof 9mAforthe
CV140/EB91/6AL5valves, whichAVOusedforrectificationintheCT160, andislikelytolead toa
shortened servicelife. Theseparticularvalveswereactuallydesigned asFMdetectordiodes,notas
currentrectifiers!
Asyouwill havenodoubtnoticed,the maximumavailablescreen voltage intheCT160is300V; thisis
due tothe valvemanufacturer’sPIVlimitof330V, (nowreferred toasRepetitiveReverseMaximum,
RRM). FittingsuitableSilicon diodeswillremovethislimitation;soif youdowishtoutiliseahigherscreen
voltage,youcoulddisconnectthe125V wirefromthe screenvoltageselectorswitch;asthisisthe‘odd
one out’,inthesensethat125Visnotavailableonthe anode voltage selectorswitch. Then movethe
highervoltagesback onetagon thescreen switchwafer, and thenconnectthe lasttagtothe400Vtapon
theHTtransformer.AVOmarkedthetapson theirHTtransformerwiththe‘mean voltage’, soitislabelled
as‘H400’(insteadofusingthe customaryrmsvalue,ie440V).Thescreenselectoroptionswill thenbe
exactlythesameasontheanodevoltageselector.
AsforsuitableSilicondiodes;don’t fitanyolddiodesthat just happen tobelyinginyourscrap box; itis
wellworthfitting soft recoverydiodes,whicharedesignedtominimisecircuitswitchingoscillations;for
examplePhilips’BYW96E, whichisrated at3A and1kVVrrm.Admittedly, wedon’trequireaforward
currentof 3A,howeverthelowercurrentsoftrecoverydiodesdon’tseemtohaveasufficientlyhigh
enoughVrrm;800Vwouldbesufficient.
Ifyou aregoingtoreplacethevalvesintheCT160withSilicondiodes,you will havetoalterthe series
resistanceR6(730 accordingtothelegend,butinreality750 wasused)whichisusedtoobtainthe
correctcurrent flowingthrough theanodecurrent controls,sothat thevoltage acrosseach80 resistor
will still beexactly1V.Ifyou placeastandardSilicondiode inplaceofeachvalvediode (rememberthe
CV140isadoublediode,soone Silicondiodewill be needed foreachdiode)thenyouwill haveto
increasetheseriesresistor,R6,toapproximately935 ,inthe anodecontrolcircuit.Thecalculationfor
thisisasfollows:-theanode currentcontrolcircuit issupplied fromatransformerwindingof50V rms;
whichisequivalent toameanvoltageof45.02V;(theratiobetween rmsandmeanvoltagesbeing 1.1107
forasinusoidalwaveform;whichgivesus50V RMS÷1.1107=45.02VMean).Nowtheapproximate
forwardvoltagedrop foraSilicon diodeis0.7V; howeverforaCV140 itis2.2V(asmeasured atthe
manufacturer’smaximumcurrent of9mA);sothevalueofR6mustbeincreased.
The anodecurrent controlcircuitrequiresanexactcurrentof 12.5mA (derivedfromthe1V volt drop
acrosseach80 resistor,1V ÷80 =0.0125A or12.5mA). Afterrectification bytheSilicondiode, you
then haveavoltage of 45.02V÷2-0.7V =21.81V DCmean. (The45.02Vhastobedividedby2,
becauseitisahalfwaverectifier; themeanvoltage following half-waverectification isobviouslyhalved,
and thefigureof0.7Volt comesfromtheforwardvoltage dropof theSilicon diode).
The totalresistanceinthe circuitwillthen havetobe 21.81V÷12.5mA=1744.8 ;fromthiswemust
subtract the existing resistorsintheanodecontrolcircuit=9x80 +90 =810 ;thusthenewvalueof
R6=1744.8-810 =934.8 . Youcould,ifyouprefer, useasmallvariableresistorinserieswithalarger
resistor,sothatyou canadjust the mean currenttoprecisely12.5mA. Alternatively, you might chooseto

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useacombinationofresistors, eitherinparallel,eg 1.1k// 6.2k; orinseries, eg 910 +24 , toadjustthe
totalcurrent.
Ifyou wantedtomeasurethecurrent, youcouldinsertacurrentmeterinserieswiththecircuit,but you
wouldthenhavetoadjustthecircuitfortheadditionalresistanceintroduced byyourcurrent meter, asit
will alsoproduceavoltagedrop.Itisthereforeeasiertomeasurethevoltagedrop across eachresistor
withadigitalmultimeter,asthatwill haveahighinput resistance,typically10M ,whichwillnotaffectthe
circuitasmuchasthe currentmeasurementwoulddo–quiteapartfromthat,you donot need breakthe
circuitforvoltagemeasurements!
Thisalsomeansthat if youneed toreplacethe90 anode currentpotentiometer, withanothervalue,say
100 (since90 willbe hardtofindnowadays)youcando that, but thenyouwillneedtoreducethe
seriesresistor, R6, bytheadditionalresistanceinthenewpotentiometer,inordertokeepthe currentat
12.5mA through the backing-offcircuit; inthisexample,by10 . Howeveryouwillneedtomakeanew
scalefortheanodecurrent potentiometer,and youwillgetalargeroverlapon eachrange, butitwillstill
givethe correctmeasurementsforanodecurrent. Thepracticalminimumvaluewill be closeto90 , asit
wasoriginally.Ifforanyreason,you needtogolowerthan 80 ,thenyouwillhavetolowereach80
resistor,and increasethecurrent, tomaintainexactly1V. A typicalreason forreplacingitwouldbethat
theoldoneisopencircuit, orperhapshasbecomenon-linear,duetowear,orisotherwisedamaged.
Makinganewscaleisquiteeasy if youusea360°protractor, togetherwithamultimetertomeasureout
eachstep ofeither0.1V, or8 ,andmarkthemontheprotractor,thentransferringthemtoapaperscale.
Or,alternatively, youcandrill aholethroughthecentreofafairlylargeprotractor, orapieceofPCB, then
fasten the potentiometerintheholeandusingalarge knobonthepotentiometer,and asyouturnit,mark
eachpoint on theprotractor/pcb, whichyoucanthentransfertoapaperscale.
NB:-Thereisonecasethathasnotbeencheckedthoroughlysofar,andthat iswhetherthe gm
measurementswill beaffected, if thepotentiometerischangedtoanyvalue otherthan90 !Thereisonly
aslightriskof that, sinceeachofthethree240 resistors,R24 toR26,areusedtocompensateforthe
anode current controlresistances-thisshouldbeinvestigatedfurther, beforeI canrecommendchanging
thepotentiometerforanothervalue;butmyguessisthatthe changedoesn'tmatter, asitisthevoltage
deliveredbetween the twopointsthat formthebacking-offcircuit, iswhatthemeasurementiscompared
with, andsincethatisunchanged,sothenisthe currentandresistanceinthat circuitpath.
However,therealsothe possibilitytoputapotentiometerwithahighervaluein, andthenshortingoutthe
lastpart ofthe trackabove90 ,andthenmakeanewscaletofitthe newpotentiometer;thenthiswill
workjustaswellastheoldone,except thatthenewscalewill be morecramped!
Fortheanode currentcontrolmeasurementstobe accurate,you mustensurethat theremainderof the
componentsintheAVOCT160arewithintolerance,andalsothat thetesteriscalibrated;howeverforthe
anode current controlsinthemselvestobe accurate,you mustensurethatavoltageof1V MeanDCis
developed across eachof the80 resistors.
The otherdiodesintheCV140 valves,V1&V2,canalsobereplaced withSilicon diodes,and fortunately
thevalueused forthe SETVgpotentiometer, RV3,islargeenough sothatnothingneedstobechanged
oraddedthere.EachrectifierintheCV140 hasaforwardvoltage dropof approximately2.2-2.9V,atthe
currentsinvolved(whichrange fromcloseto11mA, toup to14mA,through eachdiode), whereasa
Silicondiodehascloseto0.7V atthesecurrents. The differencebetween thesevoltagedropswillbe
takenupbyRV3asithasenoughresistanceforthisadjustment.
A quickcalculationofthisfollows:-55VRMSisequivalentto55÷1.1107 =49.52VMean, whichhalfwave
rectified, becomes49.52 ÷2=24.76V MeanDC.
ForSETVgcalibrationpurposes,AVOstateintheircalibration procedurethat avoltageof20.8VMean
DCshouldbepresentacrosstheGridVoltscontrol, RV2; thisthenmeansthatthediodeintheCV140

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plusthe SETVgpotentiometerhasatotalvoltagedropgivenby24.76V -20.8V =3.96V MeanDC. Ifwe
replacetheCV140 diodewithaSilicon diode,withaforwardvoltagedrop of0.7V, that leaves3.96V -
0.7V =3.26VmeanDCtobe accommodated bytheRV3potentiometer. Thecurrent throughthiscircuitis
20.8V÷1937.9 =10.733mA (the totalresistanceof the gridvoltagecontrolcircuitis1937.9 -I leave
thatasacalculation exerciseforyoutoperformbyyourself!(Oryoucancheat,and lookattheappendix
atthe end ofthismessage!). Thisvoltagedropand currentthroughthecircuitthennecessitatea
resistanceforRV3ofapproximately304 ,whichiswell insidethe500 valueof RV3.
Part2:Gmmeasurements, resistor R14and switchSW3onthemA/V control:-
When themA/V controlisinitsresting position, themA/V potentiometer,RV1, isturnedtoaposition
wherethe wiper(whichisconnectedtocommonground)isconnecting thepositiveside ofthenegative
gridvoltagesupplytothecommon ground(and atthesametimeshortingR5).Thisisdone sothatthe
extravoltagefromthemA/V control,whichisaddedduring thegmmeasurements,will be zerowhen the
largestandingAnodecurrentisbalanced out; whichinitself, isanecessaryprerequisiteforthegm
measurementstobe performed.
When thelargestandingcurrenthasbeenbalancedout bythe Anode Current controls,(orbacked-off)the
gmmeasurementcancommence. When the mA/V dialisinitsrestingposition,ieturnedfullyanti-
clockwisebythespringmechanism,theswitchSW3isclosedandR14 isshorted.WhenthemA/V dialis
movedtothe‘SETZERO’region,theswitchSW3isopened,and theresistorR14,22 k , isinsertedin
theAnode Current circuit;thisresultsinaverysmallvoltagedrop fromthecurrent then flowingthrough
thecircuit, usuallybelow1µA.Forexample,acurrent of1µA will result inamaximumvoltagedrop of
22mV, whichcorrespondstoastandingAnode currentof0.22mA. Ifthebalancingout oftheAnode
Currentcontrolshasbeenexact, thennocurrentwillflow. However, if thisisnot thecase, anyresidual
currentflowinginthecircuitcannowbebalanced out byfine adjustmentof theAnode Current control,
thusenablingthebalancingoutproceduretobemoreprecise.Thiswillensurethatwhenthegm
measurementisperformed,whenthemA/Vdialisturned intothegmmeasurement zone, the
measurementwillbe moreexact. Youwill alsobeabletogetamoreprecisemeasurementfromthe
Anode Current controls,afterthisfineadjustment havebeenperformed.
Thisfineadjustment procedureinnowayeffectsthelargestanding currentflowingthroughthevalve
duringthe testing,asthelarge Anodecurrentisstill flowing, aslong asthetesterisinthe ‘TEST’position;
asonlyinthisposition the Anode voltage isappliedtothe valveundertest.Soreleasingthe dialand
lettingitgoback totheresting position stillleavesthelarge standingAnodecurrentflowingthroughthe
valve;the onlydifferenceinthepositionof themA/Vdial,ishowmuchthegridvoltageischanged, from
zeroto0.260mV -thisisthe extravoltagenecessaryforthegmmeasurement tobe performed.
Part3:Calibrationresistors:
Tomakesurethatitispossibletocalibratethe CT160(andthisalsoappliestotheAVOMkIII andMkIV)
you will needtoensurethatallthe componentsarewithinthetolerancelevelsthat AVOspecified;notonly
thecalibration resistors,butit isobviouslyworthlesstohavethecorrect value forthecalibration resistors,
iftherestof thetesterisnotwithintolerance(especiallythemeter,anodecurrentcontrolsand 200
resistorinthe anode circuit).
The calibrationresistorsinalloftheAVOCT160, MkIIIand MkIVfamilyareequallyimportant asthey
providethesamefunction inallofthesetesters-lettingapredeterminedcurrentflowthroughthemeter;
sothe meterneedleindicateseitheron,orverycloseto,thered‘SET~’line on the meterscale.Thiswill
ensurethatthesettingforthetransformerprimary(and thereforethesecondary)voltageiscorrect, sothat
allthe subsequentmeasurement willalsobecorrect.Inaddition,theleakageandinsulation testsalsorely
on thiscurrent through the calibrationresistors.

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Inthe AVOCT160,andalsothe MkIIIandMkIV, thesamecurrent issupposedtoflowthroughthe meter
atthe centreofthe SET~calibrationregiononthemeterscale.Thecentreofthisarea isusuallymarked
byaredline,whichisalignedwiththe 90mA line, or90% ofFSD.
Howeveron somemetersthisredcolourmayhavefaded completely, soyou havetouseeitherthecentre
of theblackarea asareference,orusethe 90mAline.Alternatively, youcouldtest yourmetertocheck
thatitisgivingthe correctdeflection, compared tothescale;thenyou can markyourownpointonthe
scalewherethe needleshows90%of FSD.
Whatisitwereallywewishtoaccomplishwiththe calibrationresistors Theimportantthingto
understandisthatthe calibrationresistorsarenot partof theactualmeasurement done onthevalveinthe
TESTorGAS positions,onlyintheotherleakage andinsulationtests. Thecalibrationresistorsarenot
onlytheretogiveyouareadingthattellsyouthat yourtesterisstillinworkingorder, comparedtothe last
timeit wascalibrated,and that youhavechosenthecorrectmainsvoltagesetting(forthe measurement
thatyouintendtoperform)butalsotodeliveracurrentthatisusedforothermeasurement purposes!
Measurementsmaybeperformedwhentheneedleiswithintheblack areaof the SET~region,butit
shouldbeclosetotheredline inthe centreforthe‘best’correlationtothecalibrationperformed
previously. Howeverthe currentthroughthecalibration resistorsisalsousedtoperformtheinsulation and
leakage tests;soinorderforthesetogivethecorrect results,thecalibrationcurrentmustalsobecorrect.
The valve, V1(B), issupplied bythevoltagefromthe 55Vrmswinding; asthatvoltageisinphasewiththe
Anode andScreen voltage,andthevalveundertest itselfactsasarectifierinthe AVOCT160,andinthe
MkIII.(However, intheAVOMkIV,theAnode voltageisalreadyrectifiedbyasilicondiode).Thevoltage
fromthe 66Vrmswindingisonlyusedtokeepthegridnegative,during the halfcyclewheretheanodeof
thevalveisdriven negativebythe ACAnode voltage, andisnotused foranymeasurementpurposesin
theTESTandGASpositions.According tothe AVOPatent No. 606707,thisistoensurethatno current is
drawnthrough thediode formedbythegridandcathode, when noanodecurrentisflowing,whichcould
damage thevalve’semission.
Thismeansthatitisveryimportant tomakesurethat the AnodeandScreenvoltagesarecorrect,by
meansoftheSET~, andthen setthegridvoltagecorrectlyviaRV3.Theadditionalvoltagefromthe 66V
RMSwindingmeansthattherearetwovoltagesthatneedtobecorrect onthetransformer,fortheneedle
toreadthecalibration region.Thisthenworks asanextrameasuretoensurethat the transformer
voltagesarecorrect. Sincethe66Vrmswindingispartof theAnode/Screen voltagewindings,you have
thusensuredthattheyarealsocorrect (apart fromapossibilityofopencircuit somewhereelseinthe
transformer). IntheAVOMkIII andMkIV,theadditionalvoltageisalsotaken fromtheAnode/Screen
voltswindings, soyou havethesamefunctionthere.
Inthe AVOMkIII andMkIV, youhaveanextrapotentiometerintheSET~calibrationcircuit, which
meansthatyoucan adjust the deflection oftheneedlewhenyouhavemadequitesurethatthe grid
voltagecontrolandAnode & Screenvoltagesarecorrect. Howeverinthe AVOCT160 you donothave
suchapotentiometer;soitiseven moreimportantherethat youhavecheckedthattheAnode,Screen
and Gridcontrolvoltagesarecorrect, (apartfromalltheothercomponentsof course), otherwiseyouwill
notgetthecorrect reading onthemeter.
Allthismeansthat youneed the calibrationresistorsfortwopurposes,oneistoensurethatthe voltage
settingsarecorrect,sothattheAnode, Screen and Gridvoltagecontrolswill deliverthe correctvoltagesin
theTESTandGASpositions,andalsotoensurethat the currentat the othermeasurementpositionsis
correct.
Thisdualpurposemeansthatyou needthesecalibration resistorstohavethe correctresistanceforthe
measurementstobe correct.

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Thereisreallyonlyonevalueof thesecalibrationresistorsthatwill work,asyouneed the correctcurrent
throughthemeter. Byperformingthecalculation backwards,andcomparingthe result withtheAVO
CT160ServiceManual,itwill hopefullygiveyouan understandingof thenecessityforthe required
resistanceinthecalibrationresistors.
Letusfirstcheckthe calculations,withthe valuesgivenbyAVOfortheircalibration resistors.Themeter
resistanceis3,250 (nominally)and themetershunt, R9, isaresistorof10k ;theseareconnected in
parallelandhaveatotalresistanceof2452.8 . Thecurrent throughthe meter,whentheneedleisaligned
withthe calibrationmarkat 90% ofFSD,correspondsto27µA. Thiscurrentof27µA passing through the
resistanceofthemetercausesavoltage dropof 87.75mV(3,250 x27µA=87.75mV).Thismeansthat
thecurrent flowing through themetershunt, R9,is8.775µA (87.75mV ÷10k =8.775µA).Thetotal
currentflowingthroughthe metercircuit istherefore35.775µA(27µA +8.755µA)attheredline.This
currentwill requireavoltage 47.223voltsacrossthecalibrationresistorsR3& R4(35.775µAx1.32M =
47.223V);add thistothevoltage dropacrossthemeter,87.75mV andyouthereforerequireatotalvoltage
of47.3108V. Inthecalibration part oftheAVOCT160Servicemanual,AVOsimplystatethat,withthe
Gridvoltagelinkconnected,you shouldhave47VDCacrossthegridvoltspotentiometer;whichisexactly
thesameastakingthe measurement acrossthecalibrationresistorsandmeter, sincetheyareconnected
inparallel. Sowedidthecalculation correctly,onlybackwardsandarrivedatthecorrect voltagedrop;
although AVOdidnotstipulateanydecimalplaceshere,but47.3V isthecorrect voltagefromthe
calculations. NB: itisanothercommon misconceptionwiththeCT160,thatR3& R4compriseamatched
pairof1.32M resistors; whatAVOmeantwasthat thevaluesof R3& R4shouldbematched,soasto
giveanoverall resistanceof 1.32M .
NOTE: Thereisnoreason whyaSilicondiodecouldnotalsobeplaced intheAnodevoltagecircuit,
somewhereaftertheAnodevoltageselectorswitch.ThisisexactlywhatwasdonebyAVOintheirMkIV;
thecircuitsaremoreorless identicalineveryotheraspect.Twominorpointsthough,firstlyit isdifficultto
accesstheanode voltageselectorswitch,(withouthaving toremovealotofthe mechanicalstructure)so
theeasiestplacetofit thediodeisat one ofthetagsfortheanodecurrent relaycoil, RL3.It isthe middle
rowofthe sixtags, andtheanodecoilhasamuchlowerresistancethanthetwoothercoils, between5
and 6 .Secondly, asAVOdiscovered, itisnecessarytofit a100k resistortoearth,afterthediode; in
ordertoavoidanyspuriousvoltagereadings; presumablyduetochargestoredinthe reversecapacitance
of thediode.
Inthe AVOMkIII andMkIV thereisanextrapotentiometerforadjustmentof theSET~current, soitis
possibletoput theneedleexactlyontheredcentrelineoftheSET~region.WhileI didthe calculations
forthe AVOCT160,Irealisedthatitwouldbebeneficial,ifyou weretoputsuchapotentiometerinthe
CT160’s66Vrmscircuit too;asitwouldthen giveyouthe samemeansofadjustmentthat isavailablein
theAVOMkIII andMkIV(the MkIIIismoreorlessthesametesterastheCT160,apartfromsomeminor
differencesinbothelectricaland mechanicaldesign). Without thispotentiometerinthe66Vrmscircuit,
thereisinevitablyacompromisebetweenthenegativeGridvoltagecalibrationandthecalibration ofthe
SET~,andtheircombined current throughthemeteratthatpoint. Thiswillbecomemoreevident,ifyou
useSilicondiodes, instead ofCV140s; sincethedifferenceintheirforwardvoltagedropsthenbecomes
moreapparent. IntheCV140/6AL5/EB91valves, the forwardvoltageismoredependent on thecurrent
flowingthroughthevalve(particularlyasAVOwereusing thevalvesbeyondtheirrecommendedcurrent
range!)thantheforwardvoltage dropofaSilicon diode, whichisrelativelyconstantinthese
circumstances.Theforwardvoltagedropcurveisflatterforthe valvethan fortheSilicon diode,soitvaries
morewhenthecurrent through the diodesisvaried. IhaveseentwoAVOCT160swhichhavebeen
modified withaSilicon diode intheAnodecircuit,and the ownershavenotreported anyadverseeffect;
although theyhadnotdonethemodificationsthemselvesand hadno information astobywhom,orwhen,
themodification had beendone.Ihaveneverseen anymodificationforthe 66Vrmswinding, butmy
calculationsand thefactthat AVOusedit on theirotherValveCharacteristicMetersshowthatitisa
modificationworthdoing, andcalculationsshowthat itwill giveabettercalibrationresult, anditwill also
makethecalibrationeasiertoperform, withouthavingtogo back and forthbetween thedifferent steps.

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Butremember, themostimportantthingisthatthe voltagesarecorrectonthe secondarysideofthe
transformers;onlyafterthatshouldanymodificationstothecalibrationcircuit beperformed!
Part4:theMeter:
Forthemetertogivecorrect readings,the movementmusthavethecorrect resistanceof 3,250 , which
includesthe internalswampresistor.Inaddition,italsoneedstohavethe correctdeflection at two
particularpoints, namely22.2µA and 27µA. Obviouslyit needstogivethecorrectreading acrossthe
wholescale, but thosepointsareused duringcalibration of the meterand tester.The22.2µApoint
correspondstothe1mA/V markon the green scale,andthe27µA pointisthered centrelineoftheSET~
region.
Ifyou can, Irecommendthat youremovethemeterandtest it, tocheck that theFSDiscorrect,andthat it
givescorrect readingsright across the scale. Quiteoften, thesemetersareknowntohavelost someof
thefluxinthemagnetsovertheyears,sothattheynolongergivethecorrectreadings.Theonlycompany
thatI knowofthatcan re-magnetisethemeterisHertsMeterCoLtd., Unit 10, BerryRoad,Hatfield,
England AL10 8BJ,butofcourse,theremaybeothercompaniesaround.
Alternatively, if youarecompetentand confidentenoughtotackleit yourself,you canfitapairofthe
modernNeodymium(oneof therareearthseriesofelements)discmagnets,(the onesEuanused were
10Øx3mmthick,butsmallerdiameterswouldbejustassuitable, it’sthe thickness thatisthe morecritical
parameter) ontheoutside of the existing ferromagneticpoles. Fortunately, itiseasytodeterminethe
correctpolarity,set up amidscaledeflectiononthemeterwithasuitableexternalcircuit, ega1M
resistorinserieswithavariabletransistorpowersupply; then withaveryfirmgriponthediscmagnet,
slowlyapproachthepolepiece.Iftheindicated currentrises,it’sthecorrect pole;ifitfalls, thenit’sthe
wrong pole.Finaltrimming, togivethe exactFSDvalueof30µA,isdoneviathemagneticshunt, whichis
locatedundertheRHS needlestop. It isahinged platedsteelarm,whichismountedon aneoprene bush;
it ‘shortsout’someofthemagneticfluxacross the polegap. Itis,orit shouldbe,cementedtothe existing
ferromagnet; youwill needtobreakthe cement, inordertoadjust the shunt, andthenre-cement it when
theadjustment iscorrect.
Ifyou needtochangeyourmeter,forexample,if the movingcoilisburntout,you willhavetoensurethat
you find a30µAmeterwithan internalresistancelowerthanthestipulated3,250 ,sothat youhavesome
leewaytocorrect itupwards; AVOstatebelow1,600 intheirServicemanual,but Ihaveneverfoundone
below2,400 inofall of thosethat Ihavetested.Ifyouhavetoreplacethe meterwithonewhichhas
someothervalueofcurrentandinternalresistance, thenyou willneedtorecalculateall ofthe resistorsin
themeasuringcircuitsinordertogetit toworkcorrectly; thatistoo muchworktodo here. However,I
havebeengivensuchaschematicofanAVOMkIII, whichI will comeback tointhefuture,afterIhave
checked itthoroughly.Anothermethod istoinstallan OP-AMPmeteramplifierandanewmeter
movement. Ifallresistorsneed toberecalculated inthemeasuring circuitthe totalresistanceinthe
measuringcircuitwill bealteredandthatwill makedifferencestothemeasurementsasitthen
necessitatesachangeofthe Anodemeasuring resistorresistanceand thewholeValvetesterwillnow
havean alteredinternalresistancefromtheviewofthevalve.
Thereisalsoasuperbwebpagewhichshowsyouhowtoput asmall magnetonthemetermagnettohelp
it togetbacktothecorrect fluxsoitwill havethe correctdeflection again, onMr.YutakaMatsuzaka's
websitehere: http://www6.wind.ne.jp/yutak/avo_ct160/mk4-2.htm , Mr. Yutaka'smainsitecan befound
here: http://www6.wind.ne.jp/yutak/avo_ct160/index.htm . Thereisalotofotherinterestinginformationon
calibrationoftheAVOCT160theretoo.TheelusiveAVOCT160A(whichdoesincorporateSilicon diodes
and alsohasanadditionalswitch,whichdoublestheGridvoltage range)isshowninsomesmalldetail
there!Ifyou useGoogleTranslate, youcanreadmostof theinformationon Mr.Yutaka'swebpage
translatedintoEnglish, forinstance.

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I haveincludedaPDF-fileformaking metermovementmeasurementsof theCT160 30µA FSDmoving
coilmeter, theschematicshowsasimplemeansofmeasuringtheFSDand theinternalresistanceofthe
meter. Beforeyou makeanymeasurementsinthiscircuit,youshouldshortthe twoconnectionswherethe
CT160meterwill betested,soyou can set the maximumcurrentinthe circuitwiththeDMM/AVO
connected inserieswiththe circuit.Thiswill protect theCT160 meterfromoverload ifyou havesetthe
potentiometerstoo low,whichcouldresultinatoohighcurrentflowingthroughthecircuit,possibly
destroyingtheCT160meter.
The meterFSDcanbe checkedwiththe switchSW1intheopenposition and theinternalresistanceof
themetercanbecheckedwithswitchSW1closed. Thisisdonebyfirst havingswitchSW1openand
settingtheFSDof the meterand thenbyclosing SW1and adjustingthe potentiometeracrossthemeter
untilthe metershows50%ofFSDwiththeneedlepointing straightup,thenopenswitchSW1againand
measuretheresistanceof the potentiometer, whichwill correspond exactlytothe internalresistanceofthe
meter. Thiswill giveyou the exactfigurefortheinternalresistance,sincethemeterand potentiometer
workasacurrentsplitter,letting exactly50% ofthe currentflowthrough eachbranch,whichOhm’slaw
dictatesthat the resistancesmustbe the same,asthevoltagedrop acrosstheresistanceandmeteristhe
samewhentheyareconnectedinparallel.
When youaregoingtomakethe measurementson theAVOCT160meter,Irecommendthatyouusea
small1.2V NiMHbattery, afew1% resistorsandthreepotentiometers(preferably10turnmodelsfor
easieradjustment),oneswitch, plusaDMM(oranAVO8)andsomecableswithcrocodileclipsfora
quickhook-up ofallcomponentstocheckthe metermovement.
The meterintheAVOCT160hasatotalinternalresistanceofapproximately3250 .Sotoprotect the
meterfromoverload, byatoohigh current,youwillneedtohaveatotalresistanceinthemeasuring circuit
thatwillonlyallowamaximumcurrentcloseto30µA tobedrawn.Withastandard1.2V NiMHthatmeans
atotalresistanceof40k ;whenthemeterresistanceissubtractedthat leaves36,750 . Withthe
followingresistorsinseries,R1at18k ,plusR2at 15k , plusatrim-potentiometer, RV1, witha
resistanceof10k youcanvarythe resistancefrom33k up to43k ,whichmeansthatthe currentcan
be variedfrom26µA to33µA. Thenyouput another10 k 10 turnpotentiometerinserieswiththis,sothat
you can makefine adjustmentstothecurrent through the metertochecktheFSDvalue. It isbesttousea
trimpotentiometerforRV1soyoudon’t accidentallychangethe current toatoohighavalue,anduse
regular10-turnpotentiometersforRV2& RV3.Ifyou wish,you couldusealockingnutonthese, adjusted
sothat thereisjust sufficient frictiontopreventanyaccidentalrotation,whenyou let goof the shaft.
Nowyoushouldshortthepointswherethe CT160meterwill be connectedlater,andset bothofthe 10k
10 turnpotentiometerstotheirminimumvaluesandconnecttheDMM inthepositionshowninthe
schematic. Set theDMM toitsminimumcurrent range(whichisusually200µA)thenadjustRV1, thefirst
10 k 10turnpotentiometer, sothatthecurrentis32µA.NowtryRV2,thesecond10 k 10 turn
potentiometer, toseethatyou canvarythecurrentbelow30µA, andthen leaveitinapositionbelow
30µA.Ifthisworksokyou canremovetheshortandinstall theCT160 meter,notewhichconnection
shouldgotothe electrodeclosesttothebattery,thepositiveone, andwhichshouldgototheDMMside of
theconnection,thenegativeone.
Byadjustingthe second10k 10turnpotentiometerand looking at themeteryoucanadjusttheFSD
pointandthen checkthe DMM/AVOforthe currentdrawn.
Ifyou can’tobtainFSDwithRV1setat 32µA, thenyou willneedtoadjust yourmeter;eitherbyhavingit
sent toaprofessionalcompanytobere-magnetised,orbytrying toadd extramagnetsasisdescribed
earlier.

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Part5:Meterprotection:
Sincethe circuit oftheAVOCT160 ismoreorless thesameastheAVOMkIII and notverydifferentfrom
thelaterAVOMkIV,it ispossibletoprotectthemeterbythe samemeansused intheseotherAVO
testers.Youcan fittheexactlythe samecircuit, asusedbyAVOintheMkIV, toprotect the meter
somewhat, without affecting thereadingsinanyway.You willhavetouseSilicondiodesacrossthe
meter, not Germaniumdiodes, asthosehaveatoolowvoltagedrop,whichwillaffectthemeterreading,
makingitnon-linear. AsintheAVOMkIV, you fittwoSilicondiodes,oneineachdirection,acrossthe
meterandalsoanon-polarisedcapacitor.WithAVO’smicroammeterremoved,ameasured currentof
‘100mA’wasset upinthe anode circuit (a4k load on‘400V’)andbothANODECURRENTCONTROLS
weredeliberatelyleft onzero.Withthe protectioncircuitinstalled, usingaDMM,themaximummeasured
voltageacrossthe meterterminalswas0.27V;withthe DMM set tocurrentinstead,the corresponding
currentthroughthemeterwas86µA. However, withoutthe protectiondiodes,the maximumvoltagewas
3.03V, andthe current was0.30mA;atentimesoverload.
Youcan usean 8µFcapacitor, preferablyapolypropylenetype, orfailing that, anon-polarisedelectrolytic;
justbesuretouseagoodlong lifemodelwithverylowleakage.AVOoriginallyusedasmall8µF12V
electrolytic; thisisnotrecommended, asthereisinsufficient polarisingvoltage,sotheyrapidlybecome
leaky.Note:thereisnothingmagicabout thevalueof8µF,justrememberitwasastandardvalue inthe
industryat thetime.The timeconstantis3,250 //10k x8µF=20ms;sousing amodern10µFwillnot
makeanydiscernibledifference. The diodescanbethesameSilicondiodesasusedelsewhereinthis
document, suchasthe BYW96E, asthesewillworkjust aswellhere.
TherehavebeensomesuggestionsthatSilicondiodesdonotworkintheCT160metercircuit;
additionallyithasalsobeenallegedthatusing thesediodesupsetsthe measurements. I canassureyou
thatthereisnoprobleminmaking thesemodificationstoeitheranAVOMkIII ortheAVOCT160,which
aremoreorlessthesameconstruction.BothI, andmanyotherpeople, havetriedthismodificationand it
hasbeensuccessfullyusedformanyyears,withoutcausinganydiscerniblereductioninthemeter
reading. AVOthemselvesmadethesemodificationstotheirlatermodelsof theMkIV and alsothe
VCM163.Admittedlytheearlymodelsof the MkIV didnothavethismeterprotection,the circuit thenonly
showsoneSilicondiode inone direction and nocapacitoracrossthemetereither. Howeverthey
introducedthe capacitormodificationinApril1960,andthe twodiodesacrossthe meterinOctober1960
(see theAVOMkIVServiceManual, p38).

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Part6:WhattodowhenFSDcurrentisnot30µAortheneedle“sticks”on theAVOmeter:
Ifyou areunluckyand haveameterthatdoesnothavean FSDcurrent thatis30µA, eitherlowerorhigher
oryourneedle“sticks”at certainpointswhilemoving overthescaleyouneedtohavethatmeterserviced
beforeyoudoanycalibrationoruseyourtester.Itisverycommonthatthemeterneedsahighercurrent
atthe FSDpoint and across thewholescaleand thisisdue tothatthemagnethaslostsomemagnetic
fluxovertheyears. If yourmeterneedsahighercurrent or“sticks”youcansendit totheonlycompany
thatI knowofthatcan re-magnetisethemeterwhichisHertsMeterCoLtd., Unit10, BerryRoad,Hatfield,
England AL10 8BJ,butofcourse,theremaybeothercompaniesaround.HertsMeterCoLtd. Willalso
clean yourmeter,fixthe balanceand theneedle/jewelbearingsandtreatthe glasswithantistaticspray
and alsore-glue theglass ifnecessary. A meterthathasbeenrefurbishedbyHertsMeterCoLtd.islike
newwhenyou haveitbackfromthem.
Thereissomeinformation onthe Internet whichsays that therearetwotypesofmetersintheAVOValve
Testers, oneat30µAFSDandoneat33µAandwithdifferent internalresistancethan thestated 3,250 .
Tothe bestofmyknowledgeandfromwhatI havebeentoldbypeoplewho haveeitherbeenemployed at
companieswhoserviced AVOValveTestersorbypeoplewho wereemployedbyAVOthemselvesthere
wasonlyoneFSDand that wasat30µA, all othermetersarebrokenordamagedmetersandthey
probablysufferfromaweakened magnet. One morepieceof informationthatspeaksagainsttheideathat
therearetwodifferent metersisthattherehavenot been anymodificationsdonetothe AVOValve
Testerstoaccommodatesuchameterinthetest circuit, itwouldhavebeennecessarytocorrectthe
shuntresistorandthe restofthe circuittoget acorrectreading. A meterthatshowsthesymptomsofa
higherFSDcurrent ismost probablysufferingfromaweakened magnet andneedstobere-magnetised.
IfyourmeterforsomereasonneedsalowerFSDcurrentitismostprobablysuffering fromawrong
adjustmentof themagneticshuntarminsidethe meter, oritcouldhavebeenstored inaplacewherethe
meterhasbeenexposedtoapowerfulmagneticfieldwhichhaveaffected themagnet although thisisnot
suchaprobablecause. Inthiscaseit isalsobettertosend themetertoHertsMeterCoLtdthantofiddle
aroundwithit yourself astheyaresoeasilydamaged.Themagneticshuntarmissituated onthemagnet
and looks likean Lletterand isusuallyglued tothemagnet andscrewedtight on theleft pillarholding the
screwwiththescale. Youneedtoopenthe meter,removethescale(whilenotbendingtheneedlewhich
mightbreakit offfromthemovingcoilif youarenot verycarefulwhileremovingit). You will thenhaveto
loosenthe magneticshunt fromtheglueand at thesametimemakesurethat youdonotgetanyof the
glue residue insidethemeterasitwill disturbthe movementof themovingcoil.Movingthemagnetic
shuntupanddownaminisculeamount willadjustthemagneticfluxinthemagnet circuit andthatwill
resultinahigherorlowerFSDcurrentnecessaryforthemovingcoiltoreachthe FSDpoint. Butyou
mighthaveameterthathasatoostrong magnet anywayand thismight not beenoughtogetthecorrect
reading soIstillrecommend you tohavethemetersenttoHertsMeterCoLtd.foraproperadjustment
and refurbishing.
The internalresistanceoftheAVOmetersalsodifferbetweendifferent meters,Imyself haveseen the
internalresistanceofthemeterrangingfrom3,180 upto3,304 but when thesemeterswhereshunted
withthe internalshuntofthe correspondingAVOValveTesterwheretheycamefromthetotalresistance
wasveryclosetothenecessaryresistanceof2,453 (3,250 inparallelwiththe shuntat 10,000 is
2,452.83 ).Itthen lookslikeAVOselected theshunt resistancetomatcheachmeterinsteadofcorrecting
theinternalresistanceofthe meteraslong asthemeterhadthecorrect FSDcurrentat30µA. Thisisonly
an observationIhavemadeandnotsomething thatIcouldverifywiththesourcesIhavebeenincontact
with.
Soaslong asyourmeterhasan internalresistancesomewherebetween3,000 and 3,500 andanFSD
currentat30µAanddoesnot stickwhilemovingalongthescaleyoushouldbeabletoadjusttheshunt
resistoraccordinglytohavethemeterworkasdesignedinyourtester.Adjustingtheshunt resistoris
simple,onlyusetheruleforresistorsinparallelconnection tocalculatethenewshunt resistor. Usethe

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ThisdocumentisacollaborationbetweenMartinforsberg,Sweden, andEuanMacKenzie,Australia. CopyrightMartinForsberg&
EuanMacKenzie2010-11-18
wantedinternalresistanceof2,452.83 and yourmeterinternalresistancetocalculatethe newshunt
resistorvalue.Anexample,letssayyourmeterhasan internalresistanceof3,180 ,then the newshunt
resistorshouldhavearesistanceof:Rshunt=1÷(1÷2,452.83 –1÷3,180 )=10,726.5 . Theshunt
resistormakessurethatexactly39.75µA isflowing throughthe meterand shunt resistorwhichis
necessaryforthemetering circuittoworkproperly.
NOTE: Nevereveradjusttheresistorsinsidethe AVOValvetesteraroundthemetering circuitif youare
notabsolutelysureofwhat youaredoing!Changingonevaluewill almost always affectall othervalues
forall otherranges.NeverchangetheAnode circuit measuring resistorat 200 asthataffectsallof the
resistorsonall rangesasthatresistoristheresistorused formeasurementpurposes.
Appendix:
Thecalculation ofthetotal resistanceoftheGridvoltageand mA/Vcontrolcircuit:
The meterhasatotal(internal)resistanceof 3,250 , whichisinparallelwiththeshuntresistor;R9(which
hasaresistanceof10k )resultsinatotalresistanceof2,452.8 .Thetwocalibrationresistorshavea
combined resistanceof1.32 M , addthesetogetherand youhaveatotalresistanceof 1,322,453 inthis
leg.
The otherlegof thecircuithasthe mA/V control,whichhasatotalresistanceof70 ,inparallelwithRV1,
2,500 ;plusthe 2,340 inseries,whichequal2408.1 .Thisresistanceisinparallelwiththe Gridvolts
potentiometer, RV2,whichhasaresistanceof 10k (whichincidentallyisaspeciallaw, not‘log’);this
makesthe totalresistanceof1940.75Ohmforthislegof thecircuit.Whenyouputthisleg inparallelwith
theotherleg,1.322M ,you getatotalresistanceof1937.9 ,Q.E.D.