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M-795 Power
Service Manual
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Table of Contents
Page
Specifications 3
Block Diagram 14
Schematic Diagram 15
Printed Circuit Board 25
Channel Frequency Charts 26
Test and Alignment Procedure 4
Parts List 38
Semiconductor Voltage Chart 48
Semiconductor Lead Assignment and Block Diagram 49
Exploded View Part list 54
Exploded View 55
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SPECIFICATIONS
1. GENERAL
Channels………………………...…………………………….................. 400 Ch AM/FM 4W
Frequency Range………………………………...……………… ..25.615 to 30.105 MHz
FrequencyControl……………………………………………………..…………..………PLL
Operating Temperature Range……………………..…………………………-10° / +55°C
DC Input Voltage…………………………...……………………………....13.2 V DC ±15%
Size………………………………………….……………….…180(L) X 35(H) X 140(D) mm
Weight………………………………………………………………..……...…………0.850 kg
2. RECEIVER
Receiving System………………………………....Dual Conversion Super Heterodyne
Intermediate Frequency………………………….…1st IF: 10.695 MHz, 2nd IF: 455 MHZ
Sensitivity………………………………………...….0.5 µV for 20 db SINAD in FM mode
Audio Distortion……………………………………..…………….Less Than 8% @ 1 KHz
ImageRejection………………………………………………………………………...65 dB
Adjacent ChannelRejection…………………………………………………………....65 dB
Signal/ Noise Ratio…………………………………………………………………..….45 dB
Current Drain at standby……………………………………………………………..325 mA
3. TRANSMITTER
Output Power……………………………………………………..……...….4W @ 13.2 V DC
Modulation……………………………………………………...………FM: 1.8 KHz ±0.2 KHz
Frequency response………………………………………………...From 400 Hz to 2.5 KHz
Output impedance…………………………………..…………………RF 50 ohm Unbalance
Signal/ Noise Ratio…………………………………………………………………..40 dB MIN
Current Drain…………………………………………………………………….…….1300 mA
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OPERATION INSTRUCTIONS
Having properly installed your batteries and hooked-up the antenna, you are now ready to operate your
radio for optimum reception and voice transmission.
Turn the power “ON” with ON/OFF switch.
Set the desired channel.
Adjust the squelch control knob to reduce any undesirable background noise when no signal is being
received. To do this, select a channel where no signal are present, or wait until signals cease on your
channel. Then, rotate the squelch control knob clockwise to a point where the background noise
disappears.
Note: When the squelch is set properly, the speaker will remain quiet until a signal is
received. In order to receive weak signals, do not set the squelch too high.
Adjust the volume to the desired listening level.
To Transmit
Press and hold the push-to-talk button. Speak slowly and clearly in a normal voice two to three inches
from the microphone. A buit-in modulation control circuit will automatically adjust the microphone input
level. There is no need to speak loudly.
To Receive
Release the push-to-talk button.
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THEORY OF OPERATIONS
TRANSMITTER
A. RF Amplification
The output of double AMP Q301 is fed through tuning IFT L301 and L302 to the base of pre driver AMP
Q302. The output is then supplied through tuning circuit L303 to RF driver AMP Q303. The output of
Q303 is supplied with tuning circuit L304 and C317,C315,C21 and goes to the base of final RF AMP
Q304. The output of Q304 is supplied to the antenna through L-C tuning circuit.
B. Circuit for Suppression of Spurious Radiation
The tuning circuit between the output of final AMP Q304 and antenna, 4-stage “PHI” network L308,
C324, C1, C327, L311, C337, C333, L312, C328, C334, C338, L313 serves as a spurious radiation
suppressor . This network also serves to match the impedance between TX power AMP Q304 and the
antenna.
C. Circuit for Limiting Power
After finished all alignment, the constant voltage supply circuit limits the available power 4 W or slightly
less. RV1 and corresponding three transistors control supply voltage of RF amplifier and other circuits.
Tune all the trimmer parts for maximum indication of RF power meter and adjust RV1 to make 4 w
indication of RF power meter.
The tuning is adjusted so that the actual power is from 3.8 to 4.0 W. There are no other additional
controls for adjusting the TX output power.
D. Modulation Control
a. FM
The mic input is fed to mic audio amplifier IC KIA324 which drives modulation varicap diode
D403 in the VCO circuit. RV401 limits the incoming modulation audio levels to inhibit over modulation.
While reading the modulation factor on the modulation analyzing equipment, adjust
RV401 shall not exceed +-1.8 KHz/Dev. After 20 dB up from 1.25 KHz/1.2 KHz/Dev. Audio level
b. AM
Modulation signals are filtered with RC network and goes to the audio power AMP IC IC103 to make
nominal signal level to achieve wanted modulation. To control incoming audio signal, diode D203 and
corresponding ALC circuit limits the modulation shall not exceed +/-80% adjust RV201 +/-80%
modulation under 1.0 KHz AF 60% mod plus 20 dB of audio signal.
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E. Receiver
CB receiver is dual conversion supper-heterodyne type with the first IF 10.695 MHz and second IF 455
KHz. Receiver is separated two blocks, 1
The PLL synthesizer supplies first local frequency 16.270 ~ 16.710 MHz.(for EU) and 16.90625 MHz ~
17.29625 MHz (for UK) With the provided first local frequencies Q105,Q106 mixes the incoming RF
signal to generate first IF signal. Mixed signals were filtered with the XF101 (10.695 MHz) crystal filter
and other tuning circuits. Output signal of mixer is filtered with CF101 (455 KHz ceramic filter). The 455
KHz signal from the 2
quadrature detector loop L104. Then we can see the recovered audio signals on Pin 11 for FM of
IC102. With the amplitude of recovered signals, Q112 serves as an audio amplifier. For AM signal Will
be pass filter CF101 and induced to Q116, Q117 respective and detected to voice signal by D111.
nd
IF filter was amplified and limits internaly. After amplification the signals fed the
st
IF section and 2nd IF section.
TROUBLESHOOTING HINTS
Symptom
• Defective power switch VR102.
Unit does not
work at all
No output from
speaker at all
No noise on
speaker
• Blown fuse.
• Broken DC power cord.
• Defective IC101.
• Defective external speaker jack.
• Poor connection on microphone connector
• Defective push switch on microphone.
• Defective internal speaker.
• Defective VR102, IC103 other components.
• Measure all the voltage of IC102 with voltage
chart
• Defective squelch circuit components (RV102,
VR1, IC102)
Probable Cause Remedy
• Replace
• Replace
• Replace
• Replace
• Repair or replace
• Repair or replace
• Repair or replace
• Replace
• Replace the defective
components.
• Replace
• Replace
Squelch does not
work
• Defective VR1, RV2, IC2.
• Replace the defective
components.
• Re-adjust
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No modulation
• Defective microphone.
• Poor audio output and defective modulation
microphone amplifier components (IC103).
• Defective microphone connector components.
• Defective ALC circuit D413, D414.
• Replace
• Replace the defective
components.
• Replace
• Replace the defective
components.
LCD meter does
not work
LCD display does
not work
Channel selector
does not work
EMG CH9.19
does not work
• Defective D5.D3
• Defective IC101.
• Defective RV101.RV202
• Defective RED wire fuse.
• Defective IC101, Q414.
• Defective IC101, SW105. • Replace
• Defective EMG SW.
• Defective IC101.
• Replace
• Replace
• Replace
• Replace
• Replace
• Replace
• Replace
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ALIGNMENT PROCEDURE
Step Setting
Frequency adjustment
MIC : Receive
Volume : optional
1
Squelch : optional
CH selector : 19 EU
CH9 : off
VCO Voltage
adjustment
MIC : Receive
2
Volume : optional
Squelch : optional
CH selector : 1A
CH9 : OFF
Connection Adjuster Adjust for
Frequency counter
to dummy load
(Figure 1).
Connect DC
voltmeter between
R406, C411 and
GND.
(Figure 2).
CT201
L501 1V at TX.
27.185MHz±300Hz
Figure 1 Figure 2
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TRANSMITTER SECTION
Test Equipment Required
• RF power meter (RF SSVM) • DC power supply (13.2 volt, 3 amp)
• 50 ohms dummy load (non-inductive) • Spectrum analyzer
• RF attenuator (50 ohms non-inductive) • Frequency counter
• Oscilloscope • Coupler
• Audio generator
ALIGNMENT PROCEDURE
Step Setting
RF power stage
MIC : Transmit
Volume : optional
Squelch : optional
1
CH : selector : 19
CH9 : OFF
Second harmonic
check
MIC : Transmit
Volume : optional
Squelch : optional
CH : selector : 19
CH9 : OFF
2
Connection Adjuster Adjust for
Connect dummy load
and RF power meter to
the EXT-ANT jack on the
set (Figure 3).
Connect RF power meter
With dummy load to
spectrum analyzer
through coupler /-40 dB
Attenuator to EXT-ANT
jack on the set
(Figure 4).
L301
L302
L303
At no modulation,
Maximum indication
on the power meter (4
watts). If indication is
not in 4 watts range,
adjust L301, L302,
L303.
compare the level o
fundamental
frequency to the level
of harmonic
frequency.
Suppression of the
nd
2
harmonic
frequency level must
be lower than –60 dB.
Check for the other
channels.
Frequency check
MIC : Transmit
Volume : optional
Squelch : optional
3
CH : selector : 19
CH9 : OFF
Connect dummy load
and frequency counter
though coupler to RF
powermeter. Connect RF
powermeter to EXT-ANT
jack on the set
(Figure 5).
CT201 Be sure that the
indication of the
transmitter frequency
is 27.185MHz±300Hz
on the frequency
counter.
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TX power level
adjustment
Volume : optional
Squelch : optional
4
CH : selector : 1
CH9 : OFF
Connect dummy load
and oscilloscope through
Coupler to RF
powermeter connect
audio generator to
microphone jack
(Figure 6).
RV202 Adjust until the 5
appears on the
channel display LCD.
th
bar
Figure 3
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Figure 4
RECEIVER SECTION
Test Equipment Required
• RF Signal generator (RFSG)
• SSVM
• Distortion meter
• DC power supply
ALIGNMENT PROCEDURE
Step Setting
Audio output adjustment
MIC : Receive
Volume : Fully clockwise
Squelch: Turn to-
Counter clockwise
CH selector : 19
RF gain : Fully clockwise
1
CH9 : OFF
RFSG:27.185 MHz,1kHZ
1µV , 1.2 K Dev.
Connect RF signal
generator to EXT-ANT
jack. Connect SSVM
and distortion meter
with 8 ohm dummy
load
(Figure 7).
Connection Adjuster Adjust for
L101
L102
L103
L104
L106
L401
L402
L403
Maximum indication
on SSVM. Reduce
output from RFSG
until the audio output
becomes about
500mW (2V).
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Squelch adjustment
MIC : Receive
Volume : 50mW (2V)
Squelch : Clockwise
2
CH selector : 19
RFSG:27.185MHz, 1kHZ
1mV, 1.2K DEV.
Connect RF signal
generator to EXT-ANT
Jack. Connect SSVM
and distortion meter to
EXT speaker jack with
8 ohm dummy load
(Figure 7).
RV1 Adjust until the audio
output appears.
RF signal meter adjustment
MIC : Receive
Volume : 500mW (2V)
Squelch : Fully counter
3
Clockwise
RFSG: 27.185 MHz, 1kHZ,
1µV, 1.2K Dev.
Connect RF signal
generator to EXT-ANT
Jack. Connect SSVM
and distortion meter to
EXT speaker jack with
8 ohm dummy load
(Figure 7).
RV101 Adjust until the 3
LED on the S/RF
meter lights up.
rd
Figure 5
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Figure 6
Figure 7
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PLL CIRCUIT BLOCK DIAGRAM
1. INTRODUCTION
The frequencies for transmitter and receiver first local frequencies are all derived from a single 4.5 MHz
crystal by means of a phase locked loop. The first local oscillator frequencies are 16.270 MHz (CH 1) to
16.710 MHz (CH 40) for EU and 16.90625 MHz (CH 1) to 17.29625 MHz (CH 40) for UK . The second
local frequency is fixed at 10.240 MHz to generate second IF 455 KHz. During transmit, The VCO of
the PLL operates 13.4825 MHz (CH 1) to 13.7025 MHz (CH 40) for EU ,13.800625 MHz (CH 1) to
13.995625 MHz (CH 40) for UK the VCO frequency gose to the double circuit Q301,L301,L302 which
doubles the frequency to generate 26.965 MHz (CH 1) to 27.405 MHz (CH 40) for EU and 27.60125
MHz (CH 1) to 27.99125 MHz (CH 40) for UK
13.4825 MHz (CH 1) EU 26.965 MHz (CH 1)
13.7025 MHz (CH 40) EU 27.405 MHz (CH 40)
13.800625 MHz (CH 1) UK 27.60125 MHz (CH 1)
13.995625 MHz (CH 40) UK 27.99125 MHz (CH 40)
The VCO operating frequency for the receiver is 16.270,16.90625 MHz (CH 1) to 16.710,17.29625 MHz
(CH 40) as the first local oscillator, injected through the buffer AMP Q506 into the first fed balanced
mixer Q107,Q108
Q 408
VCO
Q 301
Doubler
To Transmitter
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