Modifications for the Kenwood TR-2400

TR-2400 Volume Control Replacement

When replacing the volume control in a TR-2400, certain precautions should be observed to insure against inducing new problems to either the control or the radio.

When installing a replacement volume control R05-2409-05, soolder the leads to the potentiometer sparingly, to prevent flux or solder from running down the tabs and into the joint between the tab and the deposited resistance material. If this precaution is not observed, future failure of the new control is possible.

Secondly, when reassembling the radio, tape insulate the PCB ground pathe directly adjacent to the power switch tabs on the volume control. This will prevent accidental shorting of the battery to ground, and loss of the B+ RF choke, L101.

Reference page 2, Service Bulletin #808, issued 2/12/80 for a picture of the PCB/Vol control insulation.
TR-2400 Battery Charger Failure

The stock charger (W09-0307-05) supplied with the TR-2400 may be repaired and/or tested as necessary.

Open circuit, the charger should deliver 18-20V. Current when charging sould be 45mA.

Contained in the charger, in series with the output is a 120 ohm 1/2 watt current limiting resistor. Under normal use (including momentary short circuit test) this part should not fail.

If you receive a charger with no output, the unit may be opened by fracturing the glued seam by a sharp moderate hammer blow. Place the charger on its side on a hard, flat surface. A layer of cloth at top and bottom will protect the case from marring. Strike the seam first on one side, and then the other. The case should open in two pieces.

Replace the 120 ohm 1/2 W resistor with a 1 W resistor. After replacing and before closing the unit, perform voltage and current checks. A spot of glue or cement at each corner will reseal the charger.
Speaker Mic for TR-2400

The SMC-25 Speaker Microphone may be adapted for use with the TR-2400 by performing the following:

Parts Required:
Qty  Part No.       Description
1    E30-1628-08    SMC-24 Cable Assembly
1    J42-0429-08    SMC-25 Bushing
  1. Remove the three chrome and one black Phillips head screws from the microphone case.
  2. Seperate the case falfs and remove the PTT switch, the microphone and the speaker assembly.
  3. Carefully pry the brass clamp from the SMC-24 cable assembly, and remove the hard rubber bushing.
  4. Slide the SMC-25 bushing (J42-0429-08) onto the cable and replace the brass clamp.
  5. Remove the SMC-25 cable assembly from the speaker, PTT switch and the microphone. Leave the small resistor (Parallel cap on some models) connected to the mic element for now. Also save the small black and the small brown wire from the SM-25 Cable.
  6. Solder the white wire from the SMC-24 cable to one side of the resistor on the mic element and the small brown wire to the mic element at the other side of the resistor. The resistor and capacitor may now be removed.
  7. Assemble the remaining components: Short black wire to center pin of PTT switch, red wire to switch common, Blue wire to one side of the speaker, and the remaining wires to the other side of the speaker (1 short brown, one long brown, and one short black.
  8. Replace the back cover, and lapel clip, to complete the procedure.
This change may be performed at the owners option and may not be performed in warranty.
TR-2400 Tone Encoder RFI


In some cases, users may report RF interference, or motor-boating, when using a tone encoder in their TR-2400. A secondary complaint of excessive tone level may also be encountered.
  1. For RFI, bypass the B+ terminal to ground with a 100 uf cap.
  2. For excessive tone input to the TR-2400, install a 100 K ohm resistor between the encoder and the TR-2400 Tone input terminal.

TR-2400 Unwanted Noise: Tone or burst at TX


For complaints of a tone burst type noise, or simply a noise, at initial key down, suspect either a broken losse component, or incorrect valued component on the RX TX unit.

On the RX TX unit, Q16 emitter, check C82, a 33 uf tantalum cap for broken connection.

If you find a 22 uf cap at C82 replace it with a 33 uf cap.
TR-2400 Display or Keyboard trouble



An intermittent or blank digit may be caused by either a broken lead, or intermittently shorting lead on the PLL unit.

On the PLL unit, look in the area of Q12, the touchtone generator. Between Q12 and the board edge locate the D1-D4 lines. Inspect this area for either an open (Broken) lead, or shorted (pinched) lead. Repair as necessary, and check lead dress before reassembly.


An intermittent or inoperative keyboard row or column may be caused by a torn or pinched lead, or a cracked diode.

Again, in the area of Q12, inspect the "T" and "Y" lines for shorts or open connections, and inspect diodes D19-D26 for physical breakage.
TR-2400 RX low, Intermittent, or distorted


For any of the above complaints, suspect a broken loose component in the RX IF or audio output circuit on the RX TX unit. Simply pull up on the leads and resolder.

Adjacent to Q9, the audio output IC, find C49, a 1uf cap. If broken loose from the board, audio output may be down, while current consumption may be up from the normal 30mA to about 100mA due to the audio output amp oscillating.

Adjacent to L10, the discriminator, locate C33, a .033uf cap. If this component is broken loose, audio output may be up approximately 6 dB with excessive high frequency audio response.
TR-2400 Scratchy TX


Complaints of scratch audio, noise when flexing the antenna, or noisy audio when handling the unit during transmit may be remedied by tightening the Printed Circuit Board screws.
To disassemble the unit remove the back cover and battery compartment screws. INspec t the VCO shield solder points along the edge and bottom of the PLL board. Resolder if you find a broken solder bond.

Flex the metal frame several times to reseat the board, and retighten the board mounting screws. Be certain the flat metal spring has good tension when installed between the RX TX unit a fram (below the PTT switch).

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