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:
Qty Part No. Description
1 E30-1628-08 SMC-24 Cable Assembly
1 J42-0429-08 SMC-25 Bushing
Remove the three chrome and one black Phillips head screws from the
Seperate the case falfs and remove the PTT switch, the microphone and
the speaker assembly.
Carefully pry the brass clamp from the SMC-24 cable assembly, and
remove the hard rubber bushing.
Slide the SMC-25 bushing (J42-0429-08) onto the cable and replace the
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.
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
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.
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.
For RFI, bypass the B+ terminal to ground with a 100 uf cap.
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
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
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
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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