Kenwood TM-281A User manual

SERVICE MANUAL
This product uses Lead Free solder.
This product complies with the RoHS directive for the European market.
© 2011-8 PRINTED IN JAPAN
B51-8988-00 (N)217
TM-281A
144MHz FM TRANSCEIVER
Key top
(K29-9497-01)
Modular jack (MIC)
(E58-0535-05)
Panel assy
(A62-1187-13)
Knob (VOL)
(K29-9498-03)
Knob (ENC)
(K29-9499-03)
DISASSEMBLY FOR REPAIR ......................2
CIRCUIT DESCRIPTION ..............................5
COMPONENTS DESCRIPTION...................9
SEMICONDUCTOR DATA.........................11
PARTS LIST ...............................................12
EXPLODED VIEW ......................................18
PACKING....................................................19
ADJUSTMENT ..........................................20
TERMINAL FUNCTION .............................29
PC BOARD
TX-RX UNIT (X57-8130-XX)..................30
SCHEMATIC DIAGRAM ............................36
BLOCK DIAGRAM .....................................40
LEVEL DIAGRAM ......................................42
SPECIFICATIONS ...................BACK COVER
CONTENTS

TM-281A
2
Document Copyrights
Copyright 2011 by Kenwood Corporation. All rights re-
served.
No part of this manual may be reproduced, translated,
distributed, or transmitted in any form or by any means,
electronic, mechanical, photocopying, recording, or other-
wise, for any purpose without the prior written permission
of Kenwood.
Disclaimer
While every precaution has been taken in the preparation
of this manual, Kenwood assumes no responsibility for er-
rors or omissions. Neither is any liability assumed for dam-
ages resulting from the use of the information contained
herein. Kenwood reserves the right to make changes to any
products herein at any time for improvement purposes.
1. Disassembly Procedure
1. When removing the cabinet, first remove the two screws
from the right and left with a phillips screwdriver.
Then, hook your finger on the edge of the cabinet and pull
it out until it is over the chassis protrusion. Remove the
cabinet by prying the cabinet as shown below.
Cabinet
DISASSEMBLY FOR REPAIR
2. To remove the panel assembly, first turn the transceiver
upside down.
Then, insert a flat-head screwdriver into the holes of the
chassis and tilt it in the direction as shown by the arrow.
Panel assembly
Holes

TM-281A
3
3. Disconnect the flat cable from the connector of the panel
assembly.
Flat cable
4. To remove the Display PCB, first remove the volume
knob q.
Then, remove the encoder knob w.
Remove the two screws efrom the Display PCB using a
phillips screwdriver.
Remove the speaker holder of the key top from the
speaker. r
Remove the Display PCB with the speaker from the panel
assembly. t
.
.
;
@
:
=
Speaker holder
of the key top
Panel assembly
Display PCB
5. When removing the TX-RX PCB, first remove the solder
of the antenna hot pin and positive terminal of the DC
cord.
Then, remove the 15 screws from the TX-RX PCB, power
module, and audio amp.
Positive terminal
of the DC cord Antenna hot pin
Power module
TX-RX PCB
Audio amp
Note:
When you supply power to the TX-RX PCB after removing
the TX-RX PCB from the chassis, solder the positive and
ground terminals of the DC cord (recommendation: E30-
3448-25) to the +B and GND terminals of the TX-RX PCB.
TX-RX PCB
Component side
GND
+B
DISASSEMBLY FOR REPAIR

TM-281A
4
6. Pull it out behind the chassis by rotating the bush eof
the DC cord 90 degrees in the direction of the arrow after
the screw qin the negative terminal is removed, and the
positive terminal wis removed from the chassis.
DC cord
:
@
.
2. Precautions for Reassembly
1. Mount the key top and speaker onto the panel assembly.
Then, tighten the two screws qof the Display PCB.
Insert the protrusion area of the key top into the groove
area of the panel assembly using a flat-head screwdriver.
w
Then, mount the encoder knob eand volume knob r.
:@
.
;
:
Key top
Panel assembly
Display PCB
Speaker
2. When mounting the panel assembly, pass the flat cable
through the hole of the chassis as shown below, then
connect the flat cable to connector of the panel assembly.
Hole of
the chassis
Panel assembly
Panel
assembly
Chassis
Chassis
3. Fit the panel assembly into the two tabs of the chassis
top side first.
Then, fit the panel assembly into the two tabs of the
chassis bottom side by turning the panel assembly.
Panel assembly
Tabs
3. How to Mount the AF PA IC (IC715)
Solder it onto the TX-RX unit (A/2) so that the AF PA IC
part number label “LA4600” is on the reverse side.
Pin 1 marker
IC715
TX-RX UNIT (X57-8130-XX) (A/2)
: TX-RX section
Component side
DISASSEMBLY FOR REPAIR

TM-281A
5
Frequency Configuration
The receiver utilizes double conversion. The first IF is
49.95MHz and the second IF is 450kHz. The first local oscil-
lator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the neces-
sary frequencies. Figure 1 shows the frequencies.
ANT
SW
RF
AMP
1st
MIX
AF
PA
TCXO
MIC
AMP
x3
multiply
RF
AMP
POWER
AMP
CF 450kHz
MCF
49.95MHz
IF SYSTEM
PLL/VCO
16.8MHz
50.4MHz
ANT
RX
TX
SP
MIC
1/2
Fig. 1 Frequency configuration
Receiver System
The receiver is a double conversion superheterodyne.
The frequency configuration is shown in Figure 1.
■Front-end RF Amplifier
An incoming signal from the antenna is applied to an RF
amplifier (Q505) after passing through a transmit/receive
switch circuit (D309, D311, D306 and D307 are off) and
a band pass filter (L515, L514 and varactor diodes: D509,
D508). After the signal is amplified, the signal is filtered
through a band pass filter (L511, L510 and varactor diodes:
D506, D505) to eliminate unwanted signals before it is
passed to the first mixer.
The voltage of these diodes are controlled by tracking the
MCU (IC707) center frequency of the band pass filter. (See
Fig. 2.)
■First Mixer
The signal from the RF amplifier is heterodyned with the
first local oscillator signal from the PLL frequency synthesiz-
er circuit at the first mixer (Q504) to create a 49.95MHz first
intermediate frequency (1st IF) signal. The first IF signal is
then fed through the monolithic crystal filter (MCF: XF501)
to further remove spurious signals.
■IF Amplifier
The first IF signal is amplified by Q503, and then goes to
IC501 (FM processing IC). The signal is heterodyned again
with a second local oscillator signal within IC501 to create
a 450kHz second IF signal. The second IF signal is then
fed through a 450kHz ceramic filter (Wide: CF501, Narrow:
CF502) to further eliminate unwanted signals before it is
amplified and FM detected in IC501.
Item Rating
Nominal center frequency 49.95MHz
Pass bandwidth ±4.0kHz or more at 3dB
50dB stop bandwidth ±25.0kHz or less
Ripple 1.0dB or less
Insertion loss 5.0dB or less
Guaranteed attenuation 80dB or more at fo ± 900kHz
Spurious 50dB or more at fo ± 1MHz
Terminating impedance 270Ω/ 8.5pF
Table 1 Crystal filter (L71-0655-05): XF501
Item Rating
Nominal center frequency 450kHz
6dB bandwidth ±7.5kHz or more
50dB bandwidth ±15.0kHz or less
Ripple 2.0dB or less
Insertion loss 6.0dB or less
Guaranteed attenuation 35.0dB or more within fo ± 100kHz
I/O matching impedance 1.5kΩ
Table 2 Ceramic filter (L72-1034-05): CF501
Item Rating
Nominal center frequency 450kHz
6dB bandwidth ±4.5kHz or more
50dB bandwidth ±10.0kHz or less
Ripple 2.0dB or less
Insertion loss 6.0dB or less
Guaranteed attenuation 60.0dB or more within fo ± 100kHz
I/O matching impedance 2.0kΩ
Table 3 Ceramic filter (L72-0999-05): CF502
ANT
L515,L514
D509,D508
BPF
Q505
RF AMP
Q503
IF AMP
IC705
D/A CONVERTER
Q504
MIX
XF501
MCF
D306,D307
D309,D311
ANT
SW
IC705
D/A
IC706
DC AMP
Q501
x3 multiply
IC2
1/2 divider X1
TCXO
IC501
FM IC
1st local OSC (VCO/PLL)
W/NO
(EVOL2)
CF501 (Wide)
CF502 (Narrow)
TV
MCU
L511,L510
D506,D505
BPF
Fig. 2 Receiver system
CIRCUIT DESCRIPTION

TM-281A
6
■Wide/Narrow Switching Circuit
The Wide port (pin 65) and Narrow port (pin 64) of the
MCU is used to switch between ceramic filters. When
the Wide port is high, the ceramic filter SW diodes (D501,
D502) cause CF501 to turn on to receive a Wide signal.
When the Narrow port is high, the ceramic filter SW di-
odes (D501, D502) cause CF502 to turn on to receive a Nar-
row signal. (See Fig. 3.)
Narrow
IC707 64pin
IF_IN MIX_O
IC501
FM IC
CF502
(Narrow)
CF501
(Wide)
R505 R507
R506
R504
D501 D502
Wide
IC707 65pin
Fig. 3 Wide/Narrow switching circuit
■AF Signal System
The detection signal from the FM IC (IC501) goes to the
D/A converter (IC705) to adjust the gain and is output to the
AF filter (IC709) for characterizing the signal. The AF signal
output from IC709 and the DTMF signal, BEEP signal are
summed and the resulting signal goes to the D/A converter
(IC705). The AFO output level is adjusted by the D/A con-
verter. The signal output from the D/A converter is input to
the audio power amplifier (IC715). The AF signal from IC715
switches between the internal speaker and the speaker jack
(J701) output. (See Fig. 4.)
AF
Filter
D/A
CONV.
D/A
CONV.
IC705 IC709 IC705
W/NO
(EVOL2) AF PA
IC715 SP
FM IC
IC501
Fig. 4 AF signal system
■Squelch Circuit
The detection output from the FM IC (IC501) passes
through a noise amplifier (Q502) to detect noise. A volt-
age is applied to the MCU (IC707). The MCU controls the
squelch according to the voltage (SQIN) level. The signal
from the RSSI pin of IC501 is used for the S-meter. The
electric field strength of the receive signal can be known
before the SQIN voltage is input to the MCU, and the scan
stop speed is improved.
Q502
NOISE AMP D503IC501 IC707
FILOUT
RSSI
DET
MCUFM IC
SQIN
RSSI
Fig. 5 Squelch circuit
PLL Frequency Synthesizer
The PLL circuit generates the first local oscillator signal
for reception and the RF signal for transmission.
■PLL
The frequency step of the PLL circuit is 5 or 6.25kHz. A
16.8MHz reference oscillator signal is divided at IC1 by a
fixed counter to produce the 5 or 6.25kHz reference fre-
quency. The voltage controlled oscillator (VCO) output sig-
nal is buffer amplified by Q9, then divided in IC1 by a dual-
module programmable counter. The divided signal is com-
pared in phase with the 5 or 6.25kHz reference signal in the
phase comparator in IC1. The output signal from the phase
comparator is filtered through a low-pass filter and passed
to the VCO to control the oscillator frequency. (See Fig. 6.)
■VCO
Double the operating frequency is generated by Q6 in
transmit mode and Q4 in receive mode. The oscillator fre-
quency is controlled by applying the VCO control voltage,
obtained from the phase comparator, to the varactor diodes
(D4 and D6 in transmit mode and D3 and D5 in receive
mode). The TX/RX pin is set high in receive mode causing
Q7 and Q5 to turn Q6 off, and turn Q4 on. The TX/RX pin is
set low in transmit mode. The outputs from Q4 and Q6 are
amplified by Q9 and sent to the RF amplifiers through a 1/2
divider (IC2). (See Fig. 6.)
D4,D6
IC2
Q6
TX VCO
Q9
BUFF
AMP
D3,D5
Q4
RX VCO
Q5,Q7
T/R SW
1/2
divider
Charge
pump
LPF
Phase
comparator
1/M
1/N 5kHz/6.25kHz
5kHz/6.25kHz
REF
OSC
16.8MHz
PLL
DATA
IC1 : PLL IC
Q3
AMP
RF amplifiers
Q1,Q2
TX/RX (MCU)
Fig. 6 PLL circuit
CIRCUIT DESCRIPTION

TM-281A
7
■Unlock Circuit
During reception, the 8RC signal goes high, the 8TC
signal goes low, and Q708 turns on. Q710 turns on and a
voltage is applied to 8R. During transmission, the 8RC sig-
nal goes low, the 8TC signal goes high and Q709 turns on.
Q711 turns on and a voltage is applied to 8T.
The MCU monitors the PLL IC (IC1) LD signal directly.
When the PLL is unlocked during transmission, the PLL LD
signal goes low. The MCU detects this signal and makes
the 8TC signal low. When the 8TC signal goes low, no volt-
age is applied to 8T, and no signal is transmitted. (See Fig. 7.)
IC707
MCU
Q708
SW
Q710
SW
IC1
PLL
Q709
SW
Q711
SW
LD
8RC
8C
8R 8T
8TC
PLL lock
: LD “H”
Fig. 7 Unlock circuit
Transmitter System
■Outline
The transmitter circuit produces and amplifies the desired
frequency directly. It FM-modulates the carrier signal by
means of a varicap diode.
■Power Amplifier Circuit
The transmit output signal from the VCO passes through
a 1/2 divider (IC2) and the transmission/reception selection
diode (D9, D10) and is amplified by Q303. The amplified
signal goes to the power module (IC302) through a low-pass
filter. The low-pass filter removes unwanted high-frequency
harmonic components, and the resulting signal is transmit-
ted through the antenna terminal. (See Fig. 8.)
■APC Circuit
The automatic transmission power control (APC) circuit
detects part of a final amplifier output with a diode (D313,
D314) and applies a voltage to IC303. IC303 compares the
APC control voltage (PC) generated by the D/A converter
(IC705) and DC amplifier (IC706) with the detection output
voltage. IC303 generates the voltage to control the Vgg of
IC302 and stabilizes the transmission output.
The APC circuit is configured to protect over current of
IC302 due to fluctuations of the load at the antenna end and
to stabilize transmission output at voltage and temperature
variations. (See Fig. 9.)
Fig. 8 Transmitter system
Fig. 9 APC circuit
Q10
RF AMP
Q303
ANT
DRIVE
AMP
IC302
POWER
MODULE
IC2
1/2
DIVIDER
IC705 Q6
D/A
CONVERTER
D/A
CONVERTER
X1
TCXO
16.8MHz
VCO
IC1
PLL
Q9
BUFFER
Q3
RF AMP
IC706
BUFFER
IC101
IC705
MIC KEY
INPUT MCU
IC706
SUM
AMP
IC704IC703
MIC
MIC/IDC SPLATTER
FILTER
DRIVE
AMP
Q303
POWER
MODULE
IC302
DC
AMP
IC706
ANT
SW
D306,D307
D309,D311
LPF
ANT
POWER
DET
D313,D314
IC303
APC
CONTROL
D9
IC705
(PC/TVO)
(PC)
CIRCUIT DESCRIPTION

TM-281A
8
Control Circuit
The MCU carries out the following tasks (See Fig. 10.):
1) Controls the WIDE, NARROW, and TX/RX outputs.
2) Controls the display circuit.
3) Controls the PLL IC (IC1).
4) Controls the D/A converter (IC705) and adjusts the vol-
ume, modulation and transmission power.
IC705
D/A
converter
IC1
PLL
IC707
MCU
EVLLD
DT
CK
PLLE
Fig. 10 Control circuit
■Memory Circuit
The transceiver has an 64k-bit EEPROM (IC708). The EE-
PROM contains adjustment data. The MCU (IC707) controls
the EEPROM through three serial data lines. (See Fig. 11.)
EEPCK
IC707
MCU
IC708
EEPROM
EEPSDT
EEPWP
Fig. 11 Memory circuit
■ Display Circuit
The MCU (IC707) controls the display LCD and LEDs.
When power is on, the LCD driver will use the BL line to
control the LCD illumination and key backlight LEDs.
The brightness function is controlled by switch Q904.
The LCD driver (IC901) and MCU (IC707) communicate
through the CE, CL, DI, and DO lines. (See Fig. 12.)
Q901
SW
Q902
SW
IC707
MCU
D903~D905
Q903
SW D906~D911
Q904
SW
IC901
LCD
driver
CE
CL
DO
DI
BL
COM0~
COM3
SEG0~
SEG30
LCD
BRI
Fig. 12 Display circuit
■Key Matrix Circuit
The front panel has function keys. Each of them is con-
nected to a cross point of a matrix of the KI1 to KI3 and KSI
to KS2 ports of the LCD driver.
The LCD driver monitors the status of the KI1 to KI3 and
KS1 to KS2 ports. If the state of one of the ports changes,
the LCD driver assumes that the key at the matrix point cor-
responding to that port has been pressed.
IC901
LCD
driver
KI1
KI2
KI3
KS2
KS1
FUNC
REV
CALL
VFO
MR
MHz
(Encoder)
Fig. 13 Key matrix circuit
■Encode
The CTCSS and DCS signals are output from TONE of the
MCU (IC707). The DTMF signal is output from DTMF of the
MCU (IC707). These signals are summed with a MIC signal
by the summing amplifier (IC706), and the resulting signal
goes to the D/A converter (IC705) to adjust the MOD level,
and then goes to the buffer amplifier (IC706).
Part of the signal goes to the TCXO, and the rest goes to
the D/A converter (IC705) in order to adjust the modulation
balance between the VCO and TCXO.
X1
TCXO
VCO
IC706
SUM
AMP
IC705
D/A
IC705
D/A
IC706
BUFFER
IC1
PLL
Amp
MIC
IC707
MCU
TONE
DTMF
Fig. 14 Encode
■Decode
• CTCSS/DCS
The signal (W/NO (EVOL2)) goes to DTMF IN (pin 95) of
the MCU (IC707) through the amplifier (IC709). The CTCSS/
DCS signal will pass through the low-pass filters in the MCU
(IC707) and be decoded within the MCU (IC707).
IC707
MCU
IC709
AMP
DTMF IN W/NO (EVOL2)
95
Fig. 15 Decode
CIRCUIT DESCRIPTION

TM-281A
9
■D/A Converter
The D/A converter (IC705) is used to adjust the MO mod-
ulation, AF volume, TV voltage, FC reference voltage, and
PC POWER CONTROL voltage level.
Adjustment values are sent from the MCU as serial data.
The D/A converter has a resolution of 256 and the following
relationship is valid:
D/A output = (Vin – VDAref) / 256 x n + VDAref
Vin: Analog input
VDAref: D/A reference voltage (Approx. 2.2V)
n: Serial data value from the microprocessor (MCU)
Power Supply Circuit
When the power switch on the display unit is pressed,
the power port on the display unit which is connected to
port 17 (POWER), goes low, then port 82 (SBC) goes high,
Q713 turns on, SB SW (Q714) turns on and power (SB) is
supplied to the radio.
When the DC power is supplied to the radio, the volt-
age regulator IC (IC713) is supplied to the MCU VDD and
reset voltage detect IC (IC714). IC714 will generate a signal
(RESET) into the reset terminal of the MCU (IC707) to carry
out a power ON reset. If DC power is less than approxi-
mately 9.5V, the radio is unable to power on.
When the DC power voltage deceases from the normal
voltage, the INT voltage detector IC (IC712) will set to high
on MCU port 18 (INT) if B line becomes less than approxi-
mately 9.5V. The MCU will then send the backup data to the
EEPROM (IC708) and enter stop mode.
This circuit has an overvoltage protection circuit. If a DC
voltage of 18V or higher is applied to the base of Q707, this
voltage turns Q707 on and will set to high on MCU port 18
(INT). The MCU will then send the backup data to the EE-
PROM (IC708) and enter stop mode. (See Fig. 16.)
Q714
SW
Q713
SW Q707
SW
IC713
AVR
D712D710
B
IC714
RST
SBC
R824R825
INT
5M
IC707
MCU
POWER
SW
POWER
RESET
5M
IC712
INT
SB
Fig. 16 Power supply circuit
CIRCUIT DESCRIPTION
TX-RX UNIT (X57-8130-XX)
Ref. No. Use / Function Operation / Condition / Compatibility
IC1 PLL IC Frequency control
IC2 Divider Frequency divider
IC302 Power module TX final amplifier
IC303 APC controller TX power control
IC501 FM IC FM detector/ 2nd mixer/ RSSI/
Noise amplifier
IC703 OP amplifier MIC amplifier/ IDC
IC704 OP amplifier Splatter filter
IC705 Digital
potentiometer
Frequency adjustment
Modulation balance adjustment
AF level adjustment
Backlight control
Modulation limit adjustment
TX power/ BPF control
RX signal level control
Ref. No. Use / Function Operation / Condition / Compatibility
IC706 OP amplifier Buffer amplifier/ SUM amplifier/
DC amplifier/ 1/2 VCC
IC707 MCU
IC708 EEPROM Tuning data/ Memory data
IC709 OP amplifier Audio filter
IC710 5V AVR 5C
IC711 8V AVR 8C
IC712 Voltage detection MCU INT voltage detection
IC713 5V AVR 5M
IC714 Voltage detection MCU reset voltage detection
IC715 Audio amplifier Audio final amplifier
IC901 LCD driver Display control
Q1,2 Switching Charge pump
Q3 RF amplifier PLL Fin
COMPONENTS DESCRIPTION

TM-281A
10
Ref. No. Use / Function Operation / Condition / Compatibility
Q4 Oscillator RX VCO
Q5 T/R switch On when RX
Q6 Oscillator TX VCO
Q7 T/R switch
Q8 Ripple filter
Q9 Buffer amplifier Output of VCO
Q10 RF amplifier Output of VCO
Q303 Drive amplifier TX driver
Q501 Tripler 16.8MHz × 3
Q502 SQL amplifier Noise amplifier
Q503 IF amplifier
Q504 Mixer 1st mixer
Q505 RF amplifier LNA
Q506 PC/TV switch On when TX
Q701 AGC/ MIC mute
Q702,703 Beat shift switch Active while beat shift is ON
Q704,705 AF mute Active while AFM is “H”
Q706 Buffer amplifier RX audio
Q707 Over voltage
detection
Active when PS voltage is more
than 18V
Q708 DC switch (8R) Active while RX
Q709 DC switch (8T) Active while TX
Q710 DC switch (8R) Active while RX
Q711 DC switch (8T) Active while TX
Q712 AF mute Active while SPM is “H”
Q713,714 DC switch (SB) Active when power is ON
Q715 AF mute Active while AMPSW is “H”
Q901 Key backlight
switch
Q902 DC switch
Q903 LCD backlight
switch
Q904 Brightness
function switch
D1 Voltage dropped
D2 Ripple filter
D3 VCO frequency
control RX
D4 VCO frequency
control TX
Ref. No. Use / Function Operation / Condition / Compatibility
D5 VCO frequency
control RX
D6 VCO frequency
control TX
D7 VCO modulation
D8 Ripple filter
D9,10 RF switch TX/RX
D301 Voltage protection 5.1V Zener diode
D305 Protection PC/TV
D306,307 ANT switch On when TX
D309 ANT switch On when TX
D311 ANT switch On when TX
D312 Temperature
compensation
D313,314 APC voltage
detect
D316 Protection
D501,502 IF switch Wide/Narrow
D503 Detection Noise squelch
D504 Protection
D505,506 RF BPF tuning
D508,509 RF BPF tuning
D702 Surge absorption CM
D704 Surge absorption MIC key
D705 Surge absorption RXD
D706 Surge absorption PTT
D708 MIC mute
D709 Detection AGC
D710 Protection
D711 Limiter
D712 Over voltage
detection
On when PS voltage is more
than 18V
D714 Reverse connec-
tion protection
D901 Reverse connec-
tion protection
D902 Surge absorption 6.2V Zener diode
D903~905 LED Key backlight
D906~911 LED LCD backlight
COMPONENTS DESCRIPTION