KB2LJJ Radio Mods Database and Manuals
Contents
1. 1 2 to 2 2 K 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 http www kb21jj com data kenwood ts 940 htm 6 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals HAF AAT hie Calar Caden oG 25c34191 1 X da 8 o2 DOUDE 1 2K czh ngm te 2 2K LTLTTITDE ij 5 n 2281 E QA C il Damal agues Steeet debs CR 90401 5745 TE Oagrn SEP 465 Dsi ear Transi toe m im 85n 9A25 A Zl 9z hae 0 2 24 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 RDIA4CB2C332J R121 124 680 ohm 1 6 watt carbon resistor RDIACB2COSIJ C180 181 01 uF disc ceramic capacitor C91 0117 05 C184 185 33 uF 35V Tantalum capacitor CSISEIVR33M RF UNIT X44 1660 00 R154 155 3 9 Kohm 1 6 watt carbon resistor RD14CB2C392J C193 194 1 uF 25V Tanta2. 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 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 O3 YS 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 ee Varistor R15 R16 MRF485 Green or Higher rank 106xxxx or earlier STV3H O 820 12K See Caution below Caution below 107xxxx or later SV03YS 1K 2 2K Change R16 from
3. KB2LJJ Radio Mods Database and Manuals Radio Niods Database B2LJJ TS 940 Mods Expanded xmit for Kenwood TS 940S From CTLAPV Unplug all external radio connections Remove top and bottom covers 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 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 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 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 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 exi Original text beloy iiit Digital Umt B IC 107 IC 112 e kK p 130 IC 111 IC 110 IC 109 J4 455 135 9 IC 105 Locate IC number 109 Now find diode 130 and
4. ce manual http www kb21jj com data kenwood ts 940 htm 9 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals pur p T Ife jal your face iE E 2 UEM CIS MM pus TS zu m CT E T Change 166 from 55 pF to 13 pF This change is applicable to units prior to serial number 606xxxx ALC time constant This Mod will change the TS 940 time constant from approximately 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 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 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 k
5. cut it for all band transmit If you want just MARS coverage locate IC 111 and 112 and snip diode 135 beside it A Brazilian ham found two factory assembly errors that if corrected may improve sensitivity and AGC performance of your TS 940S Some corrections that may significantly AGC improving functions on the already very famous TS 940S receiving performance http www kb21jj com data kenwood ts 940 htm 1 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals S 940S AGC circuit improvements he 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 watt resistor RDI4CB2EI53J uF 16V Electrolytic CE04W 1CO10M 181555 diode V1S1555 047 uF capacitor C91 0119 05 Procedure RE unit Remove jumper wire JP7 and replace it with the parallel combination of the 15 Kohm resistor and the 181555 diode Pay close attention to polarity Add the 1 uF capacitor as shown in the diagram IF unit Change capacitor C75 from a 1 uF 50V capacitor to a 047 uF 50V capacitor Remove Jumper ER 15K 52 1 4 Ww and 151555 diode ob ipF 15v p1491 tpF 0NF i IF Unit RF Unit IF Uni RF UNIT http www kb21jj com da
6. ith 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 28K 125 can be swapped 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 i r 7 z ol di v M F A ax i i E o in ee E c a 4 A a r 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 28K 125 is discontinued since 1996 Probably by using modern FETs the receiving performance will increase Who knows http www kb21jj com data kenwood ts 940 htm 4 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals I recommend voltage measurements before and after the correcti
7. lum capacitor CSISEIE010M 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 uF to 33 uF 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 http www kb21jj com data kenwood ts 940 htm 7 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals PLL UNIT E120 R123 C184 LIBE Eamave Rigi PLL UNIT A59 2020 0 Component side TEW R124 Foil Side View RF UNIT Add 01853 184 au Foil Side View http www kb21jj com data kenwood ts 940 htm 8 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals N r mta TS 940S PLL unlock HLZ t Cheng Valua RLL
8. nife 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 uF capacitor to a 5600 uF 25 V capacitor C90 2037 05 http www kb21jj com data kenwood ts 940 htm 10 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals AVR UNIS X43 1590 DU Component side vigw This change is applicable to serial numbers prior to 701xxxx ATTENTION he KB2LJJ takes no responsibility for any damage during the modification or for any wrong information made on this modification http www kb21jj com data kenwood ts 940 htm 11 di 11 30 08 2009 19 53 08
9. on 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 PYINR 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 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 o pl any leg MIE 340 brawn hi red ROB remote socket 15 RLY2 T5940 rear panel 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 150V COLLECTOR SWITCHING WAVEFORM py ense a TS 940 PA drivers replacement http www kb21jj com data kenwood ts 940 htm 5 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Da
10. performance RX SECTION RF UNIT I KA 1 60 Q0 do MF UeITIXAB 0a30 200 o b s Jib 4 bak C ata Tal 44h Prai LEET ai i i I i 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 http www kb21jj com data kenwood ts 940 htm 3 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals 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 w
11. rig Charge Valus d r Rice i WEL 14 RF Unir ie Ta PLL Umie C128 Hem Laterian a claa Ales ura E Bic d f ee n cist Te FLL CiB t 18 Change Value ER Uai r ___ 3 AFE 1018 rb i i l I i i WOE e RE dnir I ji EE EEE ten asd LDkhkaagr Malas m i i j 4 i E L CiM T PLL Uait xX 2580 rp Hhego brBH RF Unit xu4 1558 223 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 Set the Dial frequency for approximately 1 8 MHz inside the band Using an RF probe at TP5 adjust L22 L23 and L24 for a maximum reading on the meter You should see approximately 250 mV Adjustment of L24 will produce the greatest change which may be up to a 90 adjustment from its present position E E PLL ligit X50 2020 00 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 On the RF unit X44 1660 00 change C168 from a 33 pF capacitor to a 15 pF 50V capacitor Readjust the VCO BPF RF unit L74 76 according to the instructions contained in the TS 940 Servi
12. ta kenwood ts 940 htm 2 di 11 30 08 2009 19 53 08 KB2LJJ Radio Mods Database and Manuals 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 Li zl E EFF TE um Jii m RE ED s qi Pa Low dx a S CHEM amm ds H mm m m af m M rus gifs mE mese Uem r E v V h asiria r LA ES Mia E T Rd He ek UN REL Po a TE rrr RT Le v Are r S Fw mar y j i il 5d a as Jif L Ae m E is T t m Pr Tg E a OPE CUm ir RE Ks Ps T Y CETT e TA P P i 1 E 1 i E Pet XTT nm mod 1 J im a IPLT E tales E d LE m ELT E T 2 T us ec hau E T m er ae gpe m a 1 vet j e LEE cin a E ar 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
13. tabase and Manuals 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
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