Abell TH-308GB UHF Building instructions

TH-308GB UHF SERVICE MANUAL
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Abell Industries Co., Ltd
TH-308GB UHF SERVICE MANUAL
V1.0
Contents
GENERAL………………………………………………………………………………1
REALIGNMENT ………………………………………………………………………2
DISASSEMBLY FOR REPAIR ………………………………………………………3
CIRCUIT DESCRIPTION ……………………………………………………………4
PARTS LIST……………………………………………………………………………5
EXPLODED VIEW ……………………………………………………………………6
PACKING………………………………………………………………………………7
ADJUSTMENT…………………………………………………………………………8
PC BOARD VIEWS
TX-REX UNIT(X57-6030-XX)…………………………………………………9
SCHEMATIC DIAGRAM……………………………………………………………10
SPECIFICATIONS ………………………………………………………………… 11

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GENERAL
INTRODUCTION
SCOPE OF THIS MANUAL
This manual is intended for use by experienced technicians familiar with similar types of
commercial grade communications equipment. It contains all required service information for
the equipment and is current as of the publication data. Changes which may occur after
publication are covered by either Service Bulletins or Manual Revisions. These are issued as
required.
ORDERING REPLACEMENT PARTS
When ordering replacement parts or equipment information, the full part identification
number should be included. This applies to all parts: components, kits, or chassis. If the part
number is not known, include the chassis or kit number of which it is a part, and a sufficient
description of the required component for proper identification.
PERSONAL SAFETY
The following precautions are recommended for personal safety：
DO NOT transmit until all RF connectors are verified secure and any open connectors are
properly terminated.
SHUT OFF and DO NOT operate this equipment near electrical blasting caps or in an
explosive atmosphere.
This equipment should be serviced by a qualified technician only.
SERVICE
This radio is designed for easy servicing. Refer to the schematic diagrams, printed circuit
board views, and alignment procedures contained witin.
Destination Number of CH RF power output
DH 16 3W
DL 16 3W

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REALIGNMENT
1 Modes
MODE FUNCTION
User Mode Use this mode for normal operation
PC Mode Use this mode, to make various
settings by means of the FPU
through the RS-232C port.
Manufacture
Mode
Use this mode, to realign the various
settings through the RS-232C port
during manufacture work
2 How to enter each mode
MODE PROCEDURE
User Mode Power ON
PC Mode Connect to the IBM PC compatible
machine and controled by the FPU
3 Setting acquainted
User Mode PC Mode Manufacture Mode

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○
1PTT(Push-To-Talk)switch
Press this switch, then speak into the microphone to call a station. Release the switch to
receive.
○
2Monitor Key
Press and hold to monitor how busy the current channel is and to monitor signals being
received that don’t contain the matched QT code.
○
3Channel selector
Rotate to select channels 1-16.
○
4LED indicator
Lights red while transmitting, green while receiving a signal. Flashes red when the battery
voltage is low while transmitting.
○
5Power switch/Volume control
Turn clockwise to switch the transceiver ON. Turn counterclockwise until a click sounds, to
switch the transceiver OFF. Rotate to adjust the volume level.
PC MODE
Preface
The transceiver is programmed by using a personal computer, programming interface
(provided by ABELL) and programming software. Pic 1 shows how the devices work.
Connection procedure
1．Connect the TH-308G to the personal computer with the interface cable(CNX-02).
2．When data is transmitted from the transceiver the red LED lights.
When data is received by the transceiver the green LED lights.
Notes:
·The data stored in the personal computer must match the Model Name when it is written
into the EEPROM. Run the software and connect the transceiver with the personal computer,
and then turn ON.
·DO NOT press the [PTT] key during data transmission or reception.

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·CNX-02 description
(PC programming interface cable: Option)
The CNX-02 is required to interface the TH-308G with the computer . It has a circuit in its
D-sub connector(9-pin) case that converts the RS-232C logic level to the TTL leverl. The
CNX-02 connects the side panel jacks of the TH-308G to the computer’s RS-232C serial
port.
·Programming software description
By installing a programming software of TH-308G, users are allowed to program the
transceiver with CNX-02.
·Programming with PC
If data is transferred to the transceiver from a personal computer with software provided
by Abell, the destination data (basic radio information) for each set can be modified.
Normally, it is not necessary to modify the destination data because their values are
determined automatically when the frequency range (frequency type) is set.
The values should be modified only if necessary.
Data can be programmed into the EEPROM in RS-232C format via the SP MIC plug.
DISASSEMBLY FOR REPAIR
Seperating the case assembly from the chassis
1．Remove the two knobs①and round nut②.
2．Remove the two screws③.
3．Expand the right and left sides of the bottom of the case assembly, lift the chassis, and
remove it form the case assembly④.

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4．Taking care not to cut the speaker lead⑤, open the chassis and case assembly.
Seperating the chassis from the unit
1．Remove the eleven screws⑥.
2．Remove the solder from the antenna terminal using a soldering iron then lift the unit off⑦.
3．Remove the two screws⑧and remove the antenna connector.
Note: When fixing the unit in the chassis, be sure to solder the antenna terminal.

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CIRCUIT DESCRIPTION
1.Frequency configuration
The receiver utilizes double conversion. The first IF is 21.70MHz and the second IF is
450KHz. The first local oscillator signal is supplied from the PLL circuit.

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The PLL circuit in the transmitter generates the necessary frequencies.
2.Receiver
The receiver is double conversion superheterodyne, designed to operate in the frequency
range of 450-470MHz, 400-420MHz, 350-370MHz, 370-390MHz.
1）Front-end RF amplifier
An incoming signal from the antenna is applied to an RF amplifier (Q4) after passing
through a transmit/receive switch circuit (D19 off) and a band pass LC filter. After the signal
is amplified by RF amplifier (Q4), the signal is filtered through a band pass filter to eliminate
unwanted signals before it is passed to the first mixer.
The voltage of these diodes are controlled by to track the CPU(IC2) center frequency of the
band pass filter.
2）First Mixer
The signal from the RF amplifier is heterodyned with the first local oscillator signal
from the PLL frequency synthesizer circuit at the first mixer (Q24) to create a 21.70MHz first
intermediate frequency (1st IF) signal. The first If signal is then fed through two monolithic
crystal filters(XF1 and 2) to further remove spurious signals.
3）IF amplifier
The first IF signal is amplified by Q48, and then enters IC10 (FM prcessing IC). The signal
is heterodyned again with a second local oscillator signal witin IC200 to create a 450KHz
second IF signal. The second IF signal is then fed through a 450KHz ceramic filter (CF2) to
further eliminate unwanted signals before it is amplified and FM detected in IC200.
4）AF amplifier
The recovered AF signal obtained from IC200 is amplified by IC300C, filtered by the
high-pass filter IC300B and low-pass filter Q14. And de-emphasized by R43 and C31. The
processed AF signal passes through an AF volume control and is amplified to a sufficient
level to drive a loud speaker by an AF power amplifier(IC11).
5）Squelch
Part of the AF signal from IC17 enters IC17 again, and the noise component is amplified
and rectified by a filter and an amplifier to produce a DC voltage corresponding to the noise
level.
The DC signal from the FM IC goes to the analog port of the microprocessor(IC2). IC2
determines whether to output sounds from the speaker by checking whether the input
voltage is higher or lower than the preset value.
To output sounds from the speaker, IC2 sends a high signal to the AFCC lines and turns IC4
on through Q33, Q39, Q6 and Q5.

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6）Receive signaling
QT/DQT
300Hz and higher audio frequencies of the output signal from IF IC are cut by IC300A and
IC300D. The resulting signal enters the microprocessor (IC2). IC2 determines whether the
QT matches the preset value, and controls the AFCC and the speaker output sounds
according to the squelch results.
3.PLL frequency synthesizer
The PLL circuit generates the first local oscillator signal for reception and the RF signal for
transmission.
1）PLL
The frequency step of the PLL circuit is 5 or 6.25kHz. A 12.8mhz reference oscillator
signal is divided at IC3 by a fixed counter to produce the 5 or 6.25kHz reference frequency.
The voltage controlled oscillator (VCO) output signal is buffer amplified by Q47, then devided
in IC3 by a dual-module programmable counter. The devided signal is compared in phase
with the 5 or 6.25kHz reference signal in the phase comparator in IC3. The output signal
from the phase comparator is filtered through a low-pass filter and control the oscillator
frequency through VCO.
2）VOC
The operating frequency is generated by the oscillator with Q3. The control voltage is
applied to the varactor diodes D27 and D28 to rectify the oscillator frequency. The RX pin is
set low in transmit mode, Q30 and Q20 are turned off to adjust current frequency to transmit
frequency.
3）UNLOCKED DETECOR
If a pulse signal appears at the LD pin of IC3, an unlocked condition occurs,and the DC
voltage obtained form D7, R1 and C291 causes the voltage applied to the LOCK pin of the
microprocessor to go low. When the microprocessor detects this condition, the transmitter is
disabled and the error indicating alarm occurs, ignoring the push-to-talk switch input signal.
4.Transmitter
1）Transmit audio
The modulation signal from the microphone is amplified by IC5A, passes through a
preemphasis circuit, and amplified by the other IC5B to perform IDC operation. The signal
then passes through a low-pass filter (splatter filter)(Q13 and Q9) and cuts 3kHz and higher
frequencies. The resulting signal goes to the VCO through the VCO mudulation terminal for
direct FM modulation.
2）QT/DQT encoder

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A necessary signal for QT/DQT encoding is generated by IC2 and FM-modulated to the
PLL reference signal. Since the reference OSC does not mudulate the loop characteristic
frequency or higher, modulation is performed at the VCO side by adjusting the balance.
3）VCO and RF amplifier
The transmit signal obtained from the VCO buffer amplifier Q45, is amplified by QY1. This
amplified signal is passed to the power amplifier, QY4 and Q29, which consists of a 2 stage
FET amplifier and is capable of producing up to 3W of RF power.
4） ANT switch and LPF
The RF amplifier output signal is passed through a low-pass filter network and a
transmit/receive switching circuit before it is passed to the antenna terminal. The
transmit/receive switching circuit is comprised of D17, D19. D1 and D19 turned on
(conductive) in transmit mode and off (isolated) in receive mode.
5） Terminal protection circuit
When the thermistor(TH2) reaches about 80°C, the protection circuit turns on Q50 to
protect transmitting final amplifier (Q29) from the over heating.
5.Power supply
A 5V reference power supply [5V] for the control circuit is derived from an internal battery.
This reference is used to provide a 5V supply in transmit mode [TXC], a 5V supply in receive
mode [RXC], and a 5V supply common in both modes [PLC] based on the control signal sent
from the microprocessor.
6.Control system
The IC2 CPU operates at 7.37 MHz. This oscillator has a circuit that shifts the frequency
according to the EEPROM data.