Modifications for the Kenwood GENERAL

KENWOOD TH-series HT TNC Interconnection

The following is a suggested circuit for connecting a packet TNC to a Kenwood TH-series HT. Included is the TH-21/31/41, the TH- 205/215/315/415, and the new TH-25/45 series HTs.

The connections are as follows:

A sub-miniature plug is used for receive audio. The tip of this plug is audio out; the sleeve of this plug is the common ground for PTT, transmit audio, and receive audio.

A miniature stereo plug is used for transmit audio and PTT. Connect the middle "ring" of the plug to the TNC audio out through a 0.2 uf cap (for DC isolation). Connect the sleeve of the plug to the TNC PTT. The plug's tip is unused.

Set the TNC audio for 50 to 80 mv audio output into a 2K ohm impedance.
R-1000 RF Attenuator 10db Step Option

The RF Attenuator may be converted from the standard 20dB step to 10dB step by replacing resistors on the RX unit X55-1250-00.

          Replace        With           Type
          R1             No Change      
          R2             R3 1.2K        R (Radial)
          R3             1.8K           R
          R4             R7 1.2K        A (Axial)
          R5             4.7K           A
          R6             15K            R
          R7             1.8K           A
          R8             1.2K           R
          R9             No Change
          R10            R16 56 ohm     A
          R11            No change 
          R12            100 ohm        R
          R13            68 ohm         A
          R14            R13 220 ohm    A
          R15            820 ohm        A
          R16            100 ohm        A
          R17            No change
          R18            R12 56 ohm     R
Note: How to solder on the PC board.
  1. Use a LOW WATTAGE PENCIL type iron, 35 w or less.
  2. Use Rosin core solder, small diameter.
  3. Do NOT create solder bridges or splotches.
  4. Do NOT overheat or otherwise lift the foil from the board.
  5. Was excess flux away with FRESH trichlor. and a small flux brush.
Neatness counts - It's your radio.
Service manual rx section level diagrams


This bulletin will explain the development and use of Kenwood Service Manual receiver section level diagrams.
Since they list typical RF and audio levels, the diagrams provide the most efficient means of locating a faulty RX circuit in a transceiver.

In the past, Kenwood has recorded RF levels in volts (V), decibels (dB), and more recently dBu and dBm. Conversion tables and an equation are provided at the end of this text so you can change the RF levels to the form of your choice.

Using the level diagrams require basic test equipment and a couple of test fixtures. One fixture is an 8 ohm non inductive load that replaces the internal speaker. The other is an injection probe. It is used to inject RF from your signal generator into specific sections of the receiver.
Both fixtures are discussed at the end of this text.

How the rx section level diagram is developed

This section will describe how the level diagrams are made for CW and SSB transceivers. It will then discuss the different methods used for FM transceivers.
A working transceiver is tuned to its center frequency. For HF rigs, this would be 14.200MHz. A Standard Signal Generator (SSG) set to the same frequency is connected to the antenna terminal of the transceiver. The SSG is set to a level that will not allow the AGC circuits to function (no S-METER reading). For HF rigs, this would typically be 0dB (0.5V).

An 8 ohm load is inserted into the external speaker jack and the AF gain control is adjusted to obtain 0.68V across the load (measured with an AF VTVM). The AF gain setting is not changed from this point on. The SSG is replaced with a 50 ohm load to terminate the antenna input.

An injection probe is connected to the SSG. The ground clip is attached to the chassis of the transceiver. The probe is placed at the first point listed on the level diagram just past the antenna input. The SSG level is adjusted until the audio output is again 0.68V. The SSG level is then listed on the diagram as a typical reference for that point. This process is repeated until all measuring points are assigned a reference level.


After each mixer stage, the SSG must be adjusted to the respective IF frequency. As an example, after the first mixer on the TS-940S, the frequency would be set to 45.05MHz. After the second mixer, the frequency would be set to 8.83MHz.

The levels in the audio stage of the receiver are measured with an AF VTVM. These measurements are made when the SSG is connected to antenna terminal (just after the AF gain is set).

FM rx section level diagrams

FM transceiver level diagrams are made in the same fashion as CW/SSB diagrams. The transceiver is tuned to its center frequency (or close to it). A low level signal is applied to the antenna terminal. The audio output is held constant while the injection method is used. The differences are as follows:

On some FM transceivers, the injected signal is modulated (typically a 1KHz tone at +/- 3.0KHz deviation is used).
Other transceivers are tested without modulation.

On some transceivers, the constant output is measured at 12dB SINAD. On others it is measured at 20dB of noise quieting (NQ).

Each Service Manual level diagram contains footnotes that describe the method used. Due to the variations and available test equipment, it may be necessary to change the initial set up and then write your own level diagram.

Using the level diagram to troubleshoot a faulty receiver

It is not practical to set the audio output to a specified level by injecting a signal into the antenna terminal of a transceiver that has low sensitivity. In many cases, the input signal would be so strong that it would attack the AGC circuits. This would cause all measurements in the receiver to vary from the typical levels in the diagram. The best approach to this situation is to start at the end of the receiver circuit and work towards the antenna.

Connect a 50 ohm load to the antenna terminal and an 8 ohm load to the external speaker jack. Inject a signal at the end of the IF stage. The IF frequency and required level are listed on the Service Manual level diagram. Adjust the AF gain control to obtain the specified audio level at the 8 ohm load. Do not change the AF gain setting from this point on. Inject the next signal in the middle of the receiver chain (again refer to the level diagram for the frequency and level). If the audio comes up to the specified level, the faulty component is somewhere between the antenna terminal and the middle of the receiver. If the audio does not come up, the bad component is between the two injection points.

The next injection point would again cut the chain in half in order to work towards the faulty component. For instance, if the problem is between the antenna terminal and the middle of the receiver, inject the next signal in the middle of those two points. Continue in this manner until the problem can be isolated to a small portion of the circuit.


Do not forget that D.C. measurements around an isolated circuit are necessary. For instance, an amplifier will not have any gain if an open resistor cuts off the supply voltage.

Injection probe

An injection probe is a coax that connects to your SSG, has a 0.01uF capacitor on the center conductor of the coax, and a tip connected to the other end of the capacitor. The shield of the coax connects to a ground clip. Many electronic supply shops sale injection probes. If you decide to purchase one, make sure it contains a 0.01uF capacitor between the tip and center conductor of the coax (used to block D.C.). If you decide to fabricate your own cable, it can be done with a few simple parts as shown in Figure 1.

8 OHM load

An 8 ohm non inductive load is used to substitute the internal speaker on the transceiver. The load is connected to the external speaker jack for convenience. For the purpose of troubleshooting a faulty receiver, it is not necessary to purchase a precision 8 ohm resistor (also, precision resistors are inductive). Six carbon composite 47 ohm, 1/2 watt resistors connected in parallel make an adequate load. Figure 2 shows a typical 8 ohm load fabrication.


As mentioned in the introduction, Kenwood Service Manuals have listed levels in V, dB, dB, and dBm. Unless otherwise specified in the footnotes on the level diagram, dB and dBu levels are equal.

Aside from the conversion tables on page 6, a conversion equation and examples are provided.

EQUATION:   dB = 20 log --------
In the equation, solving for dB would yield a value that is the difference between the two voltages. In other words, suppose you already know that 2uV = 12dBu and 15.8uV = 30dBu. You can see that the difference is 18dBu. Plugging these uV values into the equation would yield the same result. With this in mind, one can plug in a known voltage from a conversion table, plug in a voltage not listed in the table, and find the dB difference. To make the answer simplistic, use 0.5uV for dB/dBu and 224mV for dBm. Since they equal 0dB/0dBu/0dBm respectively, the difference is the actual value of the unknown voltage.

                Japanese "SSG"                  American "SSG"

                -6dB ------------------------------ 0.25V
                 0dB ------------------------------ 0.5V
                 6dB ------------------------------ 1V
                12dB ------------------------------ 2V
                24dB ------------------------------ 8V
                30dB ------------------------------ 15.8V
                40dB ------------------------------ 50V
                50dB ------------------------------ 158V
                60dB ------------------------------ 500V
                70dB ------------------------------ 1.58mV
                80dB ------------------------------ 5mV
                90dB ------------------------------ 15.8mV
            100dB ------------------------------ 50mV
            120dB ------------------------------ 0.5V

                   TABLE 1 (dB or dBu to V conversion)

          Japanese "SSG"                         American "SSG"

          -119dBm ------------------------------ 0.25V
          -113dBm ------------------------------ 0.5V
          -107dBm ------------------------------ 1V
          -101dBm ------------------------------ 2V
          -88.9dBm ----------------------------- 8V
          -83dBm ------------------------------- 15.8V
          -73dBm ------------------------------- 50V
          -63dBm ------------------------------- 158V
          -53dBm ------------------------------- 500V
          -43dBm ------------------------------- 1.58mV
          -33dBm ------------------------------- 5mV
          -23dBm ------------------------------- 15.8mV
          -13dBm ------------------------------- 50mV
            0dBm ------------------------------- 224mV
            7dBm ------------------------------- .5V
                        TABLE 2 (dBm to V conversion)


 -   Kenwood models:

                TS-140   TS-440   TS-450   TS-680   TS-690   TS-711
                TS-721   TS-811   TS-850   TS-940   TS-950

                        KENWOOD RS-232 INTERFACE

                                                               ( P.C.)
                                                            DB-25S (DB-9S)
                        +---------------------+         +----- 7     (5)
                        |                     |         |
                        |           MAX-232   |        +++     6
                        |        + +-------+  |
              78L05     |   +--|+--|1    16+--+            +-- 5     (8)
             +-----+    |   |    + |       |               |
     +12V ---|I   O+----+------|+--|2    15+--+            +-- 4     (7)
             |  G  | | +|   |      |       | +++   RxD 
             +-----+ | ---  +------|3    14+------------------ 3     (2)
                |    | ---       + |       |       TxD
                |    |  |   +--|+--|4    13+------------------ 2     (3)
                |    |  |   |      |       |
               +++   | +++  +------|5    12+--------+          1
                     |        +    |       |        |
                     |      +--|+--|6    11+-----+  |
                     |      |      |       |     |  |
                     |      |      |7    10+     |  |           ( RADIO )
                     |     +++     |       |     |  |
                     |             |8     9+     |  |         DIN
                     |             +-------+     |  |
                     |                           |  |          6
                     |                           |  |
                     +------------------------+  |  |      +-- 5 (RTS)
                                              |  |  |      |
                                    74LS04    |  |  |      +-- 4 (CTS)
                                   +-------+  |  |  |
                                   |1    14+--+  |  |  +------ 3 (/RXD)
                                   |       |     |  |  |
                                   |2    13+------------------ 2 (/TXD)
                                   |       |     |  |  |
                                   |3    12+-----+  |  |   +-- 1 (GND)
                                   |       |        |  |   |
                                   |4    11+--------+  |  +++
                                   |       |           |
                                   |5    10+-----------+
                                   |       |
                                   |6     9+
                                   |       |
                                +--|7     8+
                                |  +-------+
  1. All capacitors are 22uF 16v
  2. Radio connector is a 6 PIN DIN

How to remove Display-Scratches

hello, kenwood-fans!

Since a few minutes i'm happy about the new outlook of my ts440's display: it's free of scratches now!
During the last two years i used it as a mobile rig and had a lot of fun with it. now it is replaced by a ts50 (works very well!).
During that mobile application the plastic cover of the display was damaged by little scratches- not too deep at all, but unpleasant to look on. may be, i touched it with the mike or car key.

So, back in the shack, i looked for a method to get it clear.


With tooth paste!
No joke, it really works!

That's the way:
  1. Clean the display with a wet(!) clean cloth
  2. Cover areas not to be treated with insulating tape or s.e.
  3. Take one wet edge of a cloth (i used that one from the kitchen reserved for to dry the finest dishes) and put it over your forefinger
  4. Take a piece of tooth paste (the old fashioned white one, that contents polishing elements, i used "thera med", made by henkel") on that finger
  5. Polish the dispaly's surface by wiping in rotations -not strictly in one direction only- about 2 or 3 minutes
  6. Clean the area with a fresh, but wet(!) cloth
  7. Dry it with the other -dry- part of the clean cloth
  8. Remove tapes
  9. Injoy perfect brand new looking display!

Honestly said, it's not originally my idea. I was inspired by a friend of mine, who gave me this advice for a similar application (glass cover of my watch).

I'm not sure, if it works with icom or yaesu devices, so don't tell them (hi).
Kenwood 4 pin microphone connections to TNC

                                 1           4

                                 2           3

    Pin 1 = MIC input

    Pin 2 = PTT (Push To Talk)

    Pin 3 = Ground

    Pin 4 = Ground

Please make sure that you are familiar with the pin numbers on the mic sokets (fitted to end of mic leads) before you make any connections.
Kenwood 6 pin microphone connections to TNC

                                 5           1


    Pin 1 = MIC input

    Pin 2 = PTT (Push To Talk)

    Pin 3 =

    Pin 4 =

    Pin 5 =

    Pin 6 = Ground

Please make sure that you are familiar with the pin numbers on the mic sokets (fitted to end of mic leads) before you make any connections.
Kenwood 8 pin microphone connections to TNC

                                 1           7

                               2       8       6

                                 3           5

    Pin 1 = MIC input

    Pin 2 = PTT (Push To Talk)

    Pin 3 =

    Pin 4 =

    Pin 5 =

    Pin 6 =

    Pin 7 = Ground

    Pin 8 = Ground

Please make sure that you are familiar with the pin numbers on the mic sokets (fitted to end of mic leads) before you make any connections.
Kenwwod mic modification

The following concerns modificatio of some Kenwood Microphone.

Note: What follows is just suggested, proceed at your own risk.

If you have a Kenwood TM-xxxE (European version) trans', Than your mic, probably has 1750 Hz baton, Which is useless in some countries (not if you are a jammer Hi). This baton can Probably be transformed to have another transceiver function.

I myself, tested the above & succeeded with both: TM-241E & TM-741E (of which the mic's slightly differ).

GENERALLY (at least in the two above) the 1750 Hz baton provides aprox' 7.5v (when depressed), in order to generate the tone. While the other 3 control baton on the mic provide lower voltage levels (aprox' 1.5v & 3.5v if i remember correctly). The idea is simply to disconnect the 7.5v at the baton input & to connect the other voltage level (1.5v or 3.5, I forgot which).
To do this follow the next modif' stages.

  1. Disconnect the 7.5 voltage level by cutting the only jumper on the print. (to be sure measure 7.5v relative to ground on it, on the 741E mic it is symbolized by J1)
                 the TM-741E                      the TM-241E
            3             ZDDDDDD? 3         3             ZDDDDDD? 3
            3             3 ptt  3 3         3             3 ptt  3 3
            3             3switch3 3         3             3switch3 3
            3             @DDDDDDY 3         3             @DDDDDDY 3
            3                 DD   3         3            DD        3
            3                 J1   3         3            J3        3
            3                      3         3                      3
            3                      3         3                      3
  2. Short the 2 connections on:
                 the TM-741E                      the TM-241E
            3  ZDDDD?              3         3    ZDD?              3
            3  3    3              3         3    3  3              3
            3                      3         3                      3
            3  3 3  3 3  3 3  3 3  3         3  3 3  3 3  3 3  3 3  3
            3   O    O    O    O   3         3   O    O    O    O   3
            3   3    3    3    3   3         3   3    3    3    3   3
            3   3    3    3    3   3         3   3    3    3    3   3
            3                      3         3                      3
            3 1750  VFO   MR   PF  3         3 1750  VFO   MR   PF  3
            3                      3         3                      3
            3                      3         3                      3
            3Microphone T91-0398-XX3         3Microphone T91-0398-X53
     The new function becoms:
                ^                                 ^
                3                                 3
                3                                 3
        default:call                              call
    programable:PF4                               ----
Optional narrow ceramic filters

Author: Trio-Kenwood Communication, inc.

Service Bulletin no. 49

The optional filters listed below are for a narrower bandpass. Some will have different input & output impedance, & some will require new mounting holes. Only one will require a compoment change.

TR-2200DACFM-455ECFR-455GJust replace
Just replace
Modify PCB mounting
Just replace
Just replace
TR-7400ACFR-455FCFR-455GJust replace
Replace C101 470pF to .00uF
Replace C101 with .001uF &
modify PCB mounting.
TR-8300CFM-455RCFM-455FJust replace


-3 dB Down
-6 dB Down
CFM-455EL72-0025-05More than
More than
1500 ohm
CFR-455FL72-0037-05 4.5KHz 6KHz70dB
2000 ohm
CFR-455GL72-0302-05---- 4KHz70dB
2000 ohm
CFS-455GL72-0303-05---- 4KHz80dB
2000 ohm
CFT-455F2L72-0309-05 5KHz 7.5KHz60dB
1500 ohm
CFL-455FL72-0306-05 4.2KHz 6KHz70dB
2000 ohm
CFM-455FL72-0000-00 4.2KHz 6KHz 12 KHz2000 ohm

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