Modifications for the Kenwood TS-940



19-07-1998 TS-940S LCD Clock Display Erratic Operation

We have received a few reports concerning erratic operation of teh LCD clock display when the Main dial is rotated, or the VBT or SLOPE TUNE controls are operated. This appears as if you had pressed the SET, SCROLL, or GRAPH switches.

This erratic operation may be caused by improper timing of the input pulses applied to the auxiliary display microprocessor, IC1.
  1. This symptom is easily corrected by replacing IC2 MBM2764-30T1 with MBM2764-30T2, on Digital Unit A. Early models of the improved IC will carry the old nomenclature but will be distinguished by a white dot on the top of the chip.
  2. After the IC has been changed reset the main microprocessor by pressing and holding the A=B key, and then turning the power switch OFF and then back ON. Then release the A=B key.
  3. After this change is made the sub-display will not immediately track the main display. A slight delay has been introduced int he new IC to prevent any timing errors. It may take approximately 0.5 seconds for the LCD to track the main display.
  4. Please return the old IC2 to the Service Department along with the WSR for Credit.
Perform this modification only if the symptom occurs, as a repair. This change is not required otherwise.

Time required for this modification is 1 hour or less.



19-07-1998 TS-940S SSB Talk Power Improvement
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 921 (22-7-1987)

TS-940S's in the serial number range of 601XXXX through 708XXXX may be limited n SSB peak power as compared to full CW output. The level may be increased by changing the value of capacitors C182 and C183 in the ALC circuit on the Control board. The recommended replacement value for each capacitor is 0.0022 uF, 50v (CK45B1H222K).

Procedure:

  1. Disconnect the power cord and antenna from the transceiver.

  2. Remove the top and bottom covers (16 screws).

  3. Locate the Control board on the bottom of the transceiver.

  4. Locate transistor Q37 on the board (toward the back right hand side.) The two capacitors are mounted on the foil side of the board underneath Q37.

  5. To access the bottom of the board remove the 8 screws that mount the board and heat sink to the chassis (See figure 1.)

  6. Carefully lift the board/heat sink fomr the right hand side and pivot it toward the front of the transceiver.

  7. Replace C182 and C183 with 0.0022 uf, 50v capacitors.

  8. Lower the board back into its original position (don't pinch any wires!) and re-install the 8 mounting screws.

  9. Put the covers back on the TS-940S and secure them with the 16 case screws.

  10. Connect the antenna and power cord then test the transceiver for normal operation.

This is an optional change and may not be performed under warranty.
Time required for this change is 1 hour or less.



19-07-1998 TS-940S Erratic display
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 951 (20-1-1989)

Intermittent control or display problems that are difficult to correct with the reset procedure may be caused by a faulty ROM socket. The socket should be checked to insure that it makes good electrical contact with each pin of the ROM. If it is found to be intermittent, remove the socket and solder the ROM directly to the board. It should be noted that units between serial numbers 701XXXX to 811XXXX and from 903XXXX to 909XXXX are not likely to have this failure. In addition, units with a serial number of 909XXXX and above do not incorporate a socket.

CAUTION

Removing the socket requires good soldering skills. The ROM is mounted on the Digital A unit and is designated as IC2. The board, being double sided, has solder connections on both component and foil sides. When the socket is removed, it is very important to insure that each pin is completely solder free. Having to pry up on the socket means that it is not completely desoldered and will cause the circuit foils to tear.

When performing any work on the Digital A board, CMOS handling techniques must be observed. Such techniques include using a grounded or isolated soldering tip, avoid touching the pins of IC chips with your fingers, and ground yourself with a wrist ground strap.

To remove the socket:

  1. Disconnect the power cord and antenna coax.

  2. Remove the top and bottom covers from the transceiver.

  3. Remove the 2 flat head screws from each side of the front panel chassis.

  4. Loosen the round head screw on each side of the front panel chassis.

  5. Carefully rotate the front panel forward. It will be necessary to unplug the VS-1 cable from the transceiver.

  6. Remove the 4 screws from the speaker mount.

  7. Carefully pull up on the mount and rotate it toward the front panel. Swing the mount toward the right side of the transceiver and allow it to rest on the Digital B unit shield.

  8. Remove the 8 screws from the Digital A unit shield plate.

  9. Lift the plate and rotate it to the left side of the transceiver.

  10. While avoiding contact with the pins, remove the ROM (IC2) and set it aside on anti-static foam.

  11. Remove the 6 screws that mount the Digital A board. Rotate the board toward the front panel to expose the bottom side of the board.

  12. Carefully desolder the socket and remove it from the board. Do not pry up on the socket. If it does not easily pull off the board, the top foils are still soldered to the socket.

  13. Install the ROM in the board and solder it in place.

  14. Assemble the transceiver by reversing steps 1 - 11. Do not pinch the power switch cables between the front panel and the body of the transceiver.

Time required for this modification is 1.5 hrs or less.



19-07-1998 RX AUDIO

  1. On Kenwood TS-904's that do not have the optional Voice Synthesizer module VS-1 installed, you may notice a hum in the RX audio. The already present molex connectored cable for this module should be dressed to the top of the shield case where the voice synthesizer mounts. However, on some units they have been left hanging free in the vicinity of the PLL and digital A units and are quite good at picking up noise from these circuits.
  2. The Fix, is simply to secure this cable to the top of the shield case where the VS-1 would mount. Plastic tie straps, or even simple electrical tape works very nicely.




19-07-1998 Amtor switching time

  1. Most stock TS-940S' require 30 to 50 mS to switch back from TX to RX. This can be reduced to less than 20 mS by cutting the center lead of Transistor Q68 on the Control Unit PCB.
  2. After the modification, a slight "click" will be heard in the speaker during switching. However, this is no big deal to the AMTOR nut because of the improved switching time.




19-07-1998 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.




19-07-1998 TS-940 Mod for MARS and all band operations

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.



15-04-2000 TS-940S PLL unlock
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 900 (18-10-1985)

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.

Time required for the procedure is .5 hour or less.



15-04-2000 TS-940S Antenna Tuner Relays
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 907 (4-4-1986)

Occasionally during operation of the AT-940 a VSWR greater than 1.5 to 1 might cause overloading of the relay contacts in the antenna tuner. This may be especially noticeable when switching BANDS, or if TUNINS is performed too frequently. The following procedure should reduce or eliminate the tendency.

Parts Required

Q69: Digital transistor, (DTC124ES)
R304: 5,6 Kohm 1/6 Watt Resistor, (RD14BB2C562J)

Procedure:

  1. Add the circuit as shown below.
  2. After the circuit has been installed adjust VR4 for a 10-15 watt output when the antenna tuner operates.



Time required for the modification is 1 hour or less.



15-04-2000 TS-940S PLL unlock
Author: Trio-Kenwood Communication, inc.


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.

This change is applicable to units prior to serial number 606xxxx.
Time required for this modification is 1 hour or less.



15-04-2000 TS-940S AVR unit capacitor change
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 909 (8-4-1986)

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 F capacitor to a 5600 F 25 V capacitor (C90-2037-05)

This change is applicable to serial numbers prior to 701xxxx.
Time required for this modification is 1 hour or less.



15-04-2000 TS-940S AGC circuit improvements
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 910 (3-6-1986)

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 watt resistor, (RD14CB2E153J)
1 F 16V Electrolytic, (CE04W1C010M)
1S1555 diode, (V1S1555)
.047 F 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 F capacitor as shown in the diagram.

IF unit

Change capacitor C75 from a 1 F 50V capacitor to a .047 F 50V capacitor.

Time required for this modification is 1 hour or less.



15-04-2000 TS-940S Signal to noise ratio improvement
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 913 (28-6-1986)

The following change will reduce the tendency of the Zener diode in the Noise Blanker circuit to cause an increase in the ambient noise level. This change measures that no current flows thru this diode in the normal receive position.

Parts required:

560 ohm 1/ watt resistor, (RD14CB2E561J)

Procedure:

On the IF unit (X48-1430-00) change R23 from 330 ohms to 560 ohms.

This change will be incorporated in all models beginning with serial number 603xxxx.

Time required for this modification is 1 hour or less.



15-04-2000 TS-940S VCO carrier to noise ratio improvement
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 917 (2-3-1987)

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
C180, 181     .01 F disc ceramic capacitor         C91-0117-05
C184, 185     .33 F 35V Tantalum capacitor         CS15E1VR33M

RF UNIT (X44-1660-00)

R154, 155     3.9 Kohm 1/6 watt carbon resistor     RD14CB2C392J
C193, 194     1 F 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 F to .33 F 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.

PLL UNIT

PLL UNIT

RF UNIT

RF UNIT





15-04-2000 TS-940S Squelch switching noise
Author: Trio-Kenwood Communication, inc.


Service Bulletin no. 918 (21-7-1987)

In some TS-940S below S/N 711xxxx, a "popping" or "clicking" noise may be heard when the squelch circuit is opened or closed. This may be more noticeable when an optional CW filter is installed. The following procedure will minimize the noise.

Parts required:

R194        560 Kohm 1/6 watt             RD14CB2C564J
R197        220 ohm 1/6 watt              RD14CB2C221J
R355, 356   220 Kohm 1/6 watt             RD14CB2C224J
C284        0.47 F, 50V electrolytic      C90-0484-05

Procedure:

  1. Disconnect the power cord and antenna from the transceiver.

  2. Remove the top and bottom covers (16 screws).

  3. Locate the I.F. board on the bottom of the transceiver.

  4. Remove the 7 screws from the I.F. board.

  5. Carefully unplug connector # 17 (right side of optional CW filter) by prying up on the plug. Do not pull on the wires.

  6. Move the 4 wires (yel, org, brn, red) that run across the far side of the I.F. unit out of the way so the board may be removed.

  7. Carefully lift and rotate the board toward the left side of the transceiver to expose the foil side of the board.

  8. Using a small craft knife, cut the foil at the 3 points shown in figure 1. (top right corner of the board).

  9. Unsolder and discard resistors R194 and R197. They are mounted on the component side of the board and soldered on the foil side.

  10. Install a 560 Kohm resistor on the component side of the board as shown in figure 2. This resistor is now designated as R194.

  11. On the foil side of the board, add the 3 resistors (R197, 355, 368) as shown in figure 3.

  12. Add a small jumper wire as shown in figure 3.

  13. Add capacitor C282 as shown in figure 3. Observe polarity and insulate the positive (+) lead with shrink tubing.

  14. Carefully place the board in its original position (do not pinch wires) and secure it with the 7 screws.

  15. Plug in connector # 17. Move the 4 wires on the far side of the board back to their original position.

  16. Put the covers back on the TS-940S and secure them with the 16 case screws.

  17. Connect the antenna and power cord then test the transceiver for normal operation.

This is an optional change that may not be performed under warranty. Time required for this modification is 1 hour or less.



21-04-2000 TS-940S MRF-485 Driver Transistor Notes
Author: Kenwood Communication, inc.


Service Bulletin no. 988 (16-1-1992)

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-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.

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

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.