KENWOOD TS 940S MODIFS

User Manual: Pdf KENWOOD--TS-940S-MODIFS

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TS 940 Mods

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A Brazilian ham found two factory assembly errors that, if corrected,

may improve sensitivity and AGC performance of your TS-940S.

TS-940S AGC circuit improvements

Expanded xmit for Kenwood TS-940S

1. FromCT1APV

2. 

3. Unplugallexternalradioconnections.



Removetopandbottomcovers.



4. Remove4screwsholdingthefaceplatetothechassisontherightandleftsideofthefaceplate.Gentlypullthe

faceawayfromthechassisaboutaninchforworkingroom,butdon'tunpluganything.



5. Withunitsittingupright,locateDigitalUnitB.Itisontherightfrontcornerrightbehindthefaceplate.Ifyou

haveavoicesynthesizerunitthevoiceunitismountedtothetopofit.Ifnot,thereare4raisedmountingholes

ontoptoholdthevoicesynthesizerunit.Removethe4screwsfromtheVS-1board,andfolditoutofyour

waytotheleft.



6. Removethe5screws(3onleft,2onright)fromtheDigitalUnitBshielding.



Thereisnoneedtodisconnectanyconnectors.TiptheDigitalUnitBshieldingcaseupfromtherightside.Use

somethingtoholditupforyou.



8. Nowsimplylookonthelowerrighthandcorneroftheboard.LocateIC-109.Nowlookupandtotheleftand

youwillfindD-130.SniponeleadofD-130.





9. That'sit.Totest,attachadummyloadandtransmitoutofband.NowGobackthroughthestepsandputthe

unitbacktogether.

*******Original text below*********

 

LocateICnumber109.

Nowfinddiode130andcutitforall-bandtransmit.

IfyouwantjustMARScoverage,locateIC111and112,andsnipdiode135besideit.

Some corrections that may significantly AGC improving functions on the already

very famous TS-940S receiving performance

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ThefollowingchangeswillimprovetheadjacentrejectionoftheTS-940Swhenarelativelystrongnearbysignalispresent

(s-4orabove).

TheAGCswitchshouldremainintheSLOWpositionforallbuthighspeeddatacommunications.MostCWoperations

willalsobenefitfromoperationintheSLOWposition.

Partsrequired:

15Kohm¼wattresistor,(RD14CB2E153J)

1µF16VElectrolytic,(CE04W1C010M)

1S1555diode,(V1S1555)

.047µFcapacitor,(C91-0119-05)

Procedure:

RFunit

1. RemovejumperwireJP7andreplaceitwiththeparallelcombinationofthe15Kohmresistorandthe1S1555diode.

Paycloseattentiontopolarity.

2. Addthe1µFcapacitorasshowninthediagram.

IFunit

ChangecapacitorC75froma1µF50Vcapacitortoa.047µF50Vcapacitor.



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TS-940S Factory Assembly Mistakes

SomecorrectionsthatmaysignificantlyimprovethealreadyveryfamousTS-940Sreceivingperformance

Dedicated to the Detail Lists

AreyouaTS-940SRxlover?Youshouldknowthatitcanbeevenbetterifyoudosomecorrections.TwoerrorsthatI

found,bothalsoshowedintheServiceManual,RevisedEdition.

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See the FETs Q10 (RF Preamp Preamp 2SK125, N-FET, 25V, 0.1A, 0.3W) and

Q4 Q5 (2nd balanced mixer, also 2SK125).

The picture above was scanned from the TS-940S Service Manual, Revised Edition.

In these pictures the FETs are in the wrong position.

IcanassumethatthousandsofotherTS-940SarealsowaitingtobecorrectedandperformRxevenbetter(mistakes

"hidden"foralmost20years!)

Checkyourselfwithinstruments,itcanbeprobablymorethan10dBofgainandabetterAGCperformance.

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See the Level Diagram in the Service Manual. With the FETs on the right position,

we would have 12.5 dB (17 - 4.5) gain in the RF Preamp,

and 17 dB (44 - 27) in the 2nd balanced mixer at the indicated points in the picture above.

It would be interesting to measure these gains before the correction.

With this measurement we would have an idea about the sensitivity losses due to the two errors.

IlaughtalotwhenIconfirmedbotherrors!Almosttwodecadeswaitingtobecorrected!

AsIneverheardaboutthisproblem,Idecidedtorevealittothewholeworld.

Myradiowasworkingapparentlywell.Thisisthe1stTS-940SthatIcouldobserveandanalyzeindetailitscircuits.

FirstIfoundtheRFPreampFETinaoddposition,butonlyfewdaysafterIcouldfocusandseethatthecascodecircuit

wascorrectintheschematicdiagramandwronginthePCboard.ItwasalsowronginthePCboardviewpicturesinthe

ServiceManual.

IalsonoticedthataFETwashotterthantheother.ItwasassimpleasdisorderedandturntheFET180°andresolderit

alloveragain.

SomeFETscanbeusedeitherwayaroundwithoutanynoticeablechangeinbehaviour.However,mostFETsarebuilt

withachannelwideratoneendthattheother.So,althoughyoucanuseaFETwiththedrainandsourceleadsswapped

around,mostofthemdon'tworkverywellifyoutryit.ThereforetrytocorrectyourTS-940S.Whoknowsthatthe

2SK125canbeswapped?

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

This picture shows the FET in the correct position.

TheRFgainincreasedandthenoisedecreased.Veryfine!But,inthemiddleofthenight,excitedwiththisamazing

discovery,Ikeptonthinkingifthiserrorwastheonlyone.

IgotbacktotheServiceManual,andfollowingthecircuitdiagram,checkingthePCboards,Ifoundasecondmistake:in

the2ndbalancedmixer,intheIFunit,anotherinvertedFET(Q4),harmingthemixing/conversion,thegain,andthe

AGCperformance.



This picture shows the FET in the correct position.

Byeliminatingthese"losses",myfamousTS-940S"won"10dBmore!Becauseofthat,IhadtomadeS-Meter

adjustments.

IsuggestyoutorevisethefamousTS-940Sreceivingperformancetobeevenbetter,ofcourse!

OurnextstepwillbeidentifydifferentN-FETforreplacement.The2SK125isdiscontinuedsince1996.Probablyby

usingmodernFETs,thereceivingperformancewillincrease.Whoknows?

Irecommendvoltagemeasurementsbeforeandafterthecorrection.

Themodificationswereverifiedandtestedbymyself,soifyoutrythismodonyourownequipment,it'sonyourown

risk.

Havefun,reviseyourTS-940S!Smile!

NelsonRicardo-PY1NR

Donothesitatedtocontactusifyouhavequestions

Thisarticlecanalsobefoundathttp://www.guisard.com/Index_Choose_Language.htm.

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TS-940S remote relay elimination

AftermanyyearsofservicetherelayRL2thatactivatesthePTTtoalinearamplifiercanbecomeunserviceableandits

replacementisanelaborateprocessinvolvingthecompleteremovaloftheControlPCB.

AllthiscanbeavoidedbysubstitutingtherelaywithatransistorthatcanbemountedphysicallyontheRemoteSocket

andonlyoneresistorlegneedstobecutontheControlPCB:



ForusewithaTL922amplifieritisnecessarytouseahigh-voltagetransistorsuchastheMJE340andazenerdiodeof

150Vasithasa100VDCrelaycoilthatmustbegroundedforTXoperation.Thismodificationwillworkonany

amplifierrequiringpull-downforTX.Ifthepull-

downisonlyfor12V,thezenerdiodevoltagecanbeloweredto15Vor

eliminatedalltogetherifthereisnorelay.

COLLECTORSWITCHINGWAVEFORM

TS-940 PA drivers replacement

AnyownerofTS-940knowsthatthisradiohasverypourPAdrivers,MRF485fromMotorola.Incaseofdamage,to

replacethesetransistorsyouhavetofollowsometrickysteps.Touselowgaintransistors,ortousehighgaintransistors

(differentdotcolors)andmodifythebias,etc.

Inanysituationyouhavebigchancestoblowupagainthedrivers.Thefirstreasonisbecausethistransistorsarehigh

voltagetransistors(29V)WeknowthathighvoltageRFtransistorshashighergainthen12Vtransistors,andthe

possibilityofoscillationsitisalsobigger.InthesametimethedesignofthePAlayoutisnotoneofthebest,whichhelp

tooscillate.

WhatIdid,IreplacedMRF485with2SC2509(oranyequivalent10to15W,12VRFtransistorsinTO220package).

Thistransistorischeap(usedinmostofCBradios).Becarefultoconnections,2SC2509hasdifferentpinconfiguration,

BEC,(MRF485hasBCE).

YouhavetolifttheL7inductor(FinalUnit)fromthe29Vbar,andconnecttotheoutputofa12V/1.5Aregulator.This

couldbeaLM7812(TO3type).Theinputof12Vregulatorwasconnectedto29V,fromthesamepointwhereyou

disconnectedtheL7inductor.IinstalledLM7812regulatoronasmallheatsink,outsideofthePAcompartment.

Nowyouhavetoadjustthebiasofthedrivers.PutlowpowerinSSBmodeandadjustVR2(FinalUnit)tohave0.7Von

thebaseofdrivers.Finallyiftheoutputpowerisnot100WyoucanadjustVR2(ControlUnit),andVR3(ControlUnit)

forALC.

GoodluckandIamnotresponsibleforanymistakesyoumake!



TS-940S MRF-485 Driver Transistor News

SomeMRF-485transistorsarebeingsuppliedbyMotorolawithaGreenorBluecolorranking.Ifthesetransistorsare

installedwithoutmodifyingthedriverbiascircuitthereisastrongpossibilitythattheywillfailwithinaveryshorttime

frame.

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Thesehighgaintransistorscausethecircuittobecomeunstablewhichcancausethecircuittobreakintoselfoscillation,

andthereforeself-destruct.

Recommendations:

UseofRed,Orange,orYellowhfecolorrankingsisrecommend.Theselowergaintransistorsworkjustfineanddonot

sufferfromthecircuitinstabilityproblem.Ifyouareonlyabletoobtainthehighgaintransistorsyouwillneedtomodify

thevaristor/temperaturecompensationcircuitonthefinalunit(X45-1400-00)bychangingR16from1.2Kohmto2.2

Kohm.

DuringitsproductiontheTS-940Susedtwodifferentvaristorvalues.TheoriginalpartwasanSTV3H(O).Itwas

changedinmid-productiontoanSV-03YS.R15waschangedfroman820ohmresistortoa1Kohmresistoratthesame

time.Thereforethecountermeasurediffersdependingupontheserialnumberoftheradio.



Caution:IfusingaGreenorhigherhferankoneoftheabovecountermeasuremustbetakendependingupontheserial

numberoftheset.Afterreplacingthedriverscheckthebiascurrent.Werecommendtransmittingfor1hourinSBB

modewithnomodulationintoadummyload.Afterthistimeframecheckthebiascurrent.Itmustnotexceed300mA

ontheoriginalradio.IfthecurrentchangesyoumustchangeR16from1.2Kohmto2.2Kohm.





Serial number lot Varistor R15 R16 MRF485 Green or Higher rank

106xxxxorearlier STV3H(O) 820 1.2K See"Cautionbelow"

107xxxxorlater SV03YS 1K 2.2K ChangeR16from1.2to2.2K

TS-940S VCO carrier to noise ratio improvement

TheCarriertoNoiseratiooftheTS-940Smaybeimprovedbythefollowingchanges.Thisbulletinsupersedesbulletin

number911datedSeptember15,1986.

Partsrequired:

PLLUNIT(X50-2020-00)

R120,1293.3Kohm1/6wattcarbonresistorRD14CB2C332J

R121,124680ohm1/6wattcarbonresistorRD14CB2C681J

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PLL UNIT

 



RF UNIT

 

C180,181.01µFdiscceramiccapacitorC91-0117-05

C184,185.33µF35VTantalumcapacitorCS15E1VR33M

RFUNIT(X44-1660-00)

R154,1553.9Kohm1/6wattcarbonresistorRD14CB2C392J

C193,1941µF25VTantalumcapacitorCS15E1E010M

Procedure:

OnthePLLunit(X50-2020-00)removecapacitorC176,C180,andC181.ChangeresistorR120andR129from470

ohmsto3.3Kohm.ChangeresistorR121andR124from1Kohmto680ohms.ChangecapacitorsC184andC185

from.22µFto.33µF35Vtantalumcapacitors.InstallC180andC181inthepositionsshownintheaccompanying

diagrams.Theuseofnewcapacitorsisrecommendedtoimprovereliability,don'ttrytoreusetheoldcapacitors!

CapacitorsC181andC180shouldbeattachedtothefoilsideofthePLLunit.

OntheRFunit(X44-1660-00)installtheseriesRCcircuitcomposedofR154andC193,andR155andC194,asshown

intheaccompanyingdiagrams.AsthediagramsillustrateitiseasiesttomoveC132andC193tothefoilsideofthe

boardandinstalltheseriesRCcircuitonthecomponentside.

Thisisanoptionalchangeandmaynotbeperformedunderwarranty.Timerequiredforthismodificationis1.5houror

less.

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

TS-940S PLL unlock

SomeusersoftheTS-940Shavereportedablankingofthedisplayaccompaniedbyalossoftransmitandreceive.

ReadjustmentofthePLLunitwillcorrectthistendency.

Procedure:

1. SettheDialfrequencyforapproximately1.8MHz(insidetheband).

2. UsinganRFprobeatTP5adjustL22,L23,andL24foramaximumreadingonthemeter.Youshouldsee

approximately250mV.



3. AdjustmentofL24willproducethegreatestchange,whichmaybeuptoa90°adjustmentfromitspresent

position.





TS-940S PLL unlock



ServiceBulletinno.908(8-4-1986)



ReportsofaPLLunlockonthelowerfrequenciesmaybeduetolowVCOlevels.Thefollowingchangewillincrease

theVCOlevelandshouldpreventreoccurrenceofthissymptom.

Partsrequired

C168,changeto15pF50V(CC45SLIH150J)

Procedure:

1. OntheRFunit(X44-1660-00)changeC168froma33pFcapacitortoa15pF50Vcapacitor.

2. ReadjusttheVCOBPF(RFunitL74-76)accordingtotheinstructionscontainedintheTS-940Servicemanual.



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

Thischangeisapplicabletounitspriortoserialnumber606xxxx.



ALC time constant

1. ThisModwillchangetheTS-940timeconstantfromapproximately1secondto.022second.Thismeansthatthe

ALCwillnolongerimposeitsowncharacteristicsonyouraudioresponse;theALCwillnowfollowyourown

sylabbicrateandemphasis.Usuallytheaverageoutputwillincrease,whichwilldrivealinearamplifierharder(or

atleastthosemeterswillswinghigher!!).UsecautioninthismodandhaveanotherHam-friendcheckaroundyour

TXfrequencyforsplatteringwhileyouareQRV.

2. TheTS-940hasa10uFcap(C31)and100Kresistor(R104)whichmakeupthetimeconstantfortheALC.This

tendstoreducetheoutputpowerforthedurationoftheALCtimeconstant(ortillthecircuitchargesupagain).

Thenitstartsalloveragainonthenextword.

3. ThisprocedureallowsmodificationtothecontrolPCB(X53-1420-11)intheTS-940WITHOUThavingto

removetheboard.RemovethebottomcoverandlocatethecontrolPCB.LocateR137andR104.Theseare

locatedintheupperrighthandcornerastherigfacesyouupsidedown(nearVR-3).Aservicemanualishelpfull

forlocatingparts!!!

UsinganExactoknife,VERYCARFULLYscrapeoffsomeinsulationfromthetopofthesetworesistors.Now

tack-soldera1/8watt2.2kohmresistorfromthetopofoneoftheseresistorstothetopoftheother.Thetime

constantisnowchanged.Reassembleandenjoyasuperbrig.

TS-940S AVR unit capacitor change

Inthoseareasofthecountrythataresubjecttolargepowerlinevoltagefluctuations,excessivevoltagemaybeappliedto

capacitorC12ontheAVRunit(X43-1500-00).Thesevoltagefluctuationsmightexceedthecapacitorbreakdown

voltage.ReplacingcapacitorC12withacapacitorhavingahigherbreakdownvoltagewillhelppreventthispotential

problem.

Procedure:

OntheAVRunit(X43-1500-00)changeC12froma6800µFcapacitortoa5600µF25Vcapacitor(C90-2037-05)



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ATTENTION

TheKB2LJJtakesnoresponsibilityforanydamageduringthemodificationorforanywronginformationmadeonthis

modification.



Thischangeisapplicabletoserialnumberspriorto701xxxx.





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KENWOOD TS 940S MODIFS

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