Icom Ic-2800 Dual Band Fm Tranciever Service Manual

							A-5570MI-S
© 1999 by Icom Inc.
6-9-16, Kamihigashi, Hirano-ku, Osaka 547-0002, JapanCount on us!
DUAL BAND FM TRANSCEIVERiC-2800h
iC-2800h 
						

							INTRODUCTION
This service manual describes the latest service information for 
the IC-2800H FM TRANSCEIVER at the time of publication.To upgrade quality, all electrical or mechanical parts and   inter-
nal circuits are subject to change without notice or obligation.
DANGER
NEVER 
connect the transceiver to an AC outlet or to a DC 
power supply that uses more than 16 V.  Such a connection 
could cause a fire hazard and/or electric shock.
DO NOT 
expose the transceiver to rain, snow or any liq-
uids.
DO NOT 
reverse the polarities of the power supply when 
connecting the transceiver.
  
DO NOT 
apply an RF signal of more than 20 dBm (100 
mW) to the antenna connector.  This could damage the 
transceiver’s front end.
ORDERING PARTS
Be sure to include the following four points when ordering 
replacement parts:
1. 10-digit order numbers
2. Component part number and name
3. Equipment model name and unit name
4. Quantity required
1110002750 S.IC TA75S01F IC-2800H  MAIN UNIT 1 piece
8810008450 Screw   M4 ´ 8 ZK IC-2800H  CHASSIS 6 piecesAddresses are provided on the inside back cover for your 
convenience.
REPAIR NOTES
1. Make sure a problem is internal before disassembling the
    transceiver.
2. DO NOT open the transceiver until the transceiver is
    disconnected from its power source.
3. DO NOT force any of the variable components.  
    Turn them slowly and smoothly.
4. DO NOT short any circuits or electronic parts.  An insu-
    lated tuning tool MUST be used for all adjustments.
5. DO NOT keep power ON for a long time when the
    transceiver is defective.
6. DO NOT transmit power into a signal generator or a
    sweep generator.
7. ALWAYS connect a 50 dB or 60 dB attenuator between
    the transceiver and a deviation meter or spectrum
    analyser when using such test equipment.
8. READ the instructions of test equipment thoroughly
    before connecting equipment to the transceiver.
MODEL                 VERSION                 SYMBOL
Europe
Italy
U.S.A.
S. E. AsiaIC-2800HUSA
SEA EUR
ITA
CSA C. S. America
6-9-16, Kamihigashi, Hirano-ku, Osaka 547-0002, Japan
Phone :  06 6793 5302
Fax :  06 6793 0013
Communication Equipment
Himmelgeister Str. 100, D-40225 Düsseldorf, Germany
Phone : 0211 346047 Fax : 0211 333639
URL : http://www.icomeurope.comUnit 9, Sea St., Herne Bay, Kent, CT6 8LD, U.K.
Phone : 01227 741741 Fax : 01227 741742
URL : http://www.icomuk.co.ukZac de la Plaine, Rue Brindejonc des Moulinais
BP 5804, 31505 Toulouse Cedex, France
Phone : 561 36 03 03 Fax : 561 36 03 00
URL : http://www.icom-france.com

2380 116th Avenue N.E., Bellevue, WA 98004, U.S.A.
Phone : (
425)
 454-8155 Fax : (
425)
 454-1509
URL : http://www.icomamerica.com
Phone : (
425)
 454-7619A.C.N. 006 092 575
290-294 Albert Street, Brunswick, Victoria, 3056, Australia
Phone : 03 9387 0666 Fax : 03 9387 0022
URL : http://www.icom.net.au 6F No. 68, Sec. 1 Cheng-Teh Road, Taipei, Taiwan R.O.C.
Phone : (02) 2559 1899 Fax : (02) 2559 1874A Division of Icom America Inc.
3071 #5 Road, Unit 9, Richmond, B.C., V6X 2T4, Canada
Phone : (
604)
 273-7400 Fax : (
604)
 273-1900
Crta. de Gracia a Manresa Km. 14,750
08190 Sant Cugat del Valles Barcelona, SPAIN
Phone : (
93)
 590 26 70 Fax : (
93)
 589 04 46
URL : http://www.icomspain.com 
						

							TABLE OF CONTENTS
SECTION 1  SPECIFICATIONS
SECTION 2 INSIDE VIEWS
SECTION 3 DISASSEMBLY INSTRUCTIONS 
SECTION 4 CIRCUIT DESCRIPTION
4 - 1  RECEIVER CIRCUITS ................................................................................................... 4 – 1
4 - 2  TRANSMITTER CIRCUITS  ........................................................................................... 4 – 3
4 - 3  PLL CIRCUIT .................................................................................................................. 4 – 4
4 - 4  POWER SUPPLY CIRCUITS  ........................................................................................ 4 – 5
4 - 5  CPU PORT ALLOCATIONS ........................................................................................... 4 – 5
SECTION 5 ADJUSTMENT PROCEDURES
5 - 1  PREPARATION ............................................................................................................... 5 – 1
5 - 2  PLL AND TRANSMITTER ADJUSTMENTS ................................................................... 5 – 2
5 - 3  RECEIVER ADJUSTMENT ............................................................................................ 5 – 4
SECTION 6 PARTS LIST
SECTION 7  MECHANICAL PARTS AND DISASSEMBLY
SECTION 8 SEMI-CONDUCTOR INFORMATION
SECTION 9  BOARD LAYOUTS 
9 - 1  CONTROL UNIT ............................................................................................................. 9 – 1
9 - 2  MAIN UNIT...................................................................................................................... 9 – 3
SECTION 10  BLOCK DIAGRAM 
SECTION 11  VOLTAGE DIAGRAM
11 - 1  CONTROL UNIT.............................................................................................................11 – 1
11 - 2  MAIN UNIT .....................................................................................................................11 – 2 
						

							MGENERAL
• Frequency range : 
*1Guaranteed 144.000 – 148.000 MHz only
*2Guaranteed 430.000 – 440.000 MHz only; *3Guaranteed 440.000 – 450.000 MHz only
• Mode : FM, AM (118.0 – 135.995; Rx only)
• Nomber of memory channel : 232 (incl. 6 pairs of scan edges, 10 log, 10 repeater and 2 call channels)
• Usable temperature range : –10˚C to +60˚C; +14˚F to +140˚F
• Frequency resolution : 5, 10, 12.5, 15, 20, 25, 30 and 50 kHz
• Frequency stability : ±10 ppm (–10˚C to +60˚C; +14˚F to +140˚F)
• Power supply requirement : 13.8 V DC ±15 % (negative ground)
• Current drain 
(at 13.8 V DC): Receive Standby (squelched) 1.2 A
Max. audio 1.5 A
Transmit  at VHF 50 W/UHF 35 W 12.0 A/11.0 A
at 25 W/25 W (TPE version
)8.0 A
• Antenna connector : SO-239 (50 ½)
• DATA connector : Mini DIN 6 pin
• External VIDEO input : PHONO [RCA (75 ½)]
• Dimensions  : Controller 140(W)
´70(H)´34(D) mm; 51⁄2(W)´23⁄4(H)´111⁄32(D) inch
(projections not included)Main unit 140(W)´40(H)´165.8(D) mm; 51⁄2(W)´19⁄16(H)´617⁄32(D) inch
• Weight : Controller 290 g; 10.2 oz
Main unit 1.15 kg; 2 lb 9 oz
MTRANSMITTER
• Output power : High VHF 50 W/UHF 35 W
Mid-H approx. 20 W
Mid-L approx. 10 W
Low approx. 5 W
• Modulation system : Variable reactance frequency
• Maximum frequency deviation : ±5.0/±2.5* kHz
*[EUR] and [ITA] versions only
• Spurious emissions : Less than –60 dB
• Microphone connector : 8-pin modular (600 ½)
MRECEIVER
• Receive system : Double-conversion superheterodyne
• Intermediate frequency : VHF 1st IF 15.65 MHz 2nd IF 450 kHz
UHF 1st IF 46.05 MHz 2nd IF 450 kHz
• Sensitivity 
(at 12 dB SINAD): Less than 0.18 µV
• Squelch sensitivity 
(threshold): Less than 0.13 µV
• Selectivity 
(wide/narrow): More than 12/6* kHz at –6 dB
Less than 28/18* kHz at –60 dB
*[EUR] and [ITA] versions only
• Spurious and image rejection : More than 60 dB
• Intermodulation rejection ratio : More than 60 dB
• Audio output power 
(at 13.8 V): More than 2.4 W at 10% distortion with an 8½load
• External SP1 connector : 2-conductor 3.5(d) mm (
1⁄8)/8 ½
• External SP2 connector : 3-conductor 3.5(d) mm (
1⁄8)/8 ½
1 - 1
SECTION 1 SPECIFICATIONS 
144 MHz band 440 MHz band
144.000 – 146.000 430.000 – 440.000
Rx: 136.000 – 174.000*
1Rx: 400.000 – 530.000*2
Tx: 144.000 – 148.000*1Tx: 430.000 – 440.000*2
Rx: 118.000 – 174.000*1430.000 – 450.000*3Tx: 144.000 – 148.000*1
Rx: 136.000 – 174.000*1430.000 – 440.000
Tx: 144.000 – 148.000*1
Rx: 118.000 – 174.000*1Rx: 400.000 – 530.000*2
Tx: 144.000 – 148.000*1Tx: 430.000 – 440.000*2
All stated specifications are subject to change without notice or obligation.
Version
EUR
ITA
USA
SEA
CSA 
						

							2 - 1
SECTION 2 INSIDE VIEWS
• CONTROL UNIT
• MAIN UNIT
* : Located underside of this point
* : Located underside of this point
Q15
Q19
Q20Q14
V-VCO circuit 
Main CPU
(IC19: HD6433876N) 
EEPROM*
(IC17: X25320SI) PLL IC*
IC2 M64076AGP
U-VCO circuit 
UHF FM IF IC*
(IC8: TA31136FN)
MIC amplifier*
(Q88: 2SC4081)
PLL reference oscillator
(X1: CR-549 15.2 MHz) UHF antenna 
switch 
(D27*, D37, 
 D39, D72)
UHF 1st mixer 
(Q42, Q43, D32)
VHF antenna 
switching
(D9*, D16, D18)
VHF power module*
(IC1: M67746) VHF APC DET*
(D7, D8: RB706F)
VHF RF amplifier
(Q16: 3SK166)
High/Low control IC
(IC5: TA75S01F)
VHF 1st mixer
(Q15: 3SK166)
AF amplifier
(IC12: LA4445)
VHF FM IF IC*
(IC28: TA31136FN)
UHF power module*
(IC4: SC-1318) UHF APC DET*
(D25, D26: RB706F)
SUB CPU
(IC10: HD6433032SK)
T-data buffer amplifier*
(Q12: 2SC4081) R-data buffer amplifier*
(Q13: 2SA1576) CPU reset IC*
(IC11: S-80945ANMP-DD9)
LCD controller
(IC8: SED1354F0A) CFL drive
(Q14, Q15, Q19*, Q20*) 
						

							3 - 1
SECTION 3 DISASSEMBLY INSTRUCTIONS
• Removing the MAIN unit
qUnscrew 1 screw, A, and remove the cover.
wUnplug J4 to separate the fan motor from the MAIN unit.  
eRemove 2 main shield, 
B, and TR-A clip, Cin the direc-
tion of the arrow.  
• Removing the CONTROL unit
qUnscrew 4 screws, A, and remove the cover.
wUnplug J5 to separate the speaker and front panel.  
eRemove rear panel in the deirection of the arrow.  
rRemove 6 knobs, 
B.  
tUnscrew 5 screws, 
C, and remove the CONTROL unit in
the direction of the arrow.  
yUnscrew 2 screws, 
D, when remove the LCD.   rUnsolder 3 points, D, and unscrew 11 nut, E.
tRemove the MAIN unit in the direction of the arrow.  
A
B
B
C
Cover
J4
E
E
E
E
MAIN UNIT
Chassis
D
A
Rear panel
J5
Front panel
A
CONTROL 
UNIT
D
D
B
B LCD
C
C 
						

							4 - 1
SECTION 4 CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUITS
4-1-1 DUPLEXER CIRCUIT
The transceiver has a duplexer (low-pass and high-pass fil-
ters) on the first stage from the antenna connector to sepa-
rate the signals into VHF and UHF signals. The low-pass fil-
ter (L15, L16, L78, C70–C72) is for VHF signals and the high-
pass filter (L44, L45, L82, C189, C190, C493) is for UHF sig-
nals. The separated signals are applied to each RF circuit.
4-1-2 VHF ANTENNA SWITCHING CIRCUIT
The antenna switching circuit functions as a low-pass filter
while receiving. However, its impedance becomes very high
while transmitting by turning ON diode (D18). Thus transmit
signals are blocked from entering the receiver circuits. The
antenna switching circuit employs a 1/4
ltype diode switch-
ing system. The passed signals are then applied to the VHF
RF amplifier circuit.
4-1-3 VHF SQUELCH ATTENUATOR CIRCUIT
The attenuator circuit attenuates the signal strength to a
maximum of 10 dB to protect the RF amplifier from distortion
when excessively strong signals are received.
The current flow of the antenna switching circuit (D18) is con-
trolled by the [SQL] control via Q33. When the [SQL] control
is rotated clockwise deeper than 12 o’clock, the current of
D18 is increased. In this case, D18 acts as an attenuator.
4-1-4 VHF RF CIRCUIT
The RF circuit amplifies signals within the range of frequen-
cy coverage and filters out-of-band signals.
The signals from the antenna switching circuit pass through
the tunable bandpass filter (D15, L25, L26, C115–C117). The
filtered signals are amplified at the VHF RF amplifier (Q16)
and are then enter another 3-stage tunable bandpass filter
(D11–D14, L20–L21, C94, C96–C105) to suppress unwant-
ed signals. and improve the selectivity. The filtered signals
are applied to the VHF 1st mixer circuit (Q15).The tunable bandpass filters (D11–D13, D15) employ varac-
tor diodes to tune the center frequency of the RF passband
for wide bandwidth receiving and good image response
rejection. The PLL lock voltage is used for control voltage of
these varactor diodes. The PLL lock voltage is amplified at
the DC-amplifier (Q18) and then applied to the CPU (IC19,
pin 99). The CPU outputs the control signal to the varactor
diodes via the D/A converter (IC3).
4-1-5 VHF 1ST MIXER CIRCUIT
The 1st mixer circuit converts the received signal to a fixed
frequency of the 1st IF signal with a 1st LO (V-VCO output)
frequency.
The signals from the VHF RF circuit are mixed with the 1st
LO signal at the 1st mixer circuit (Q15) to produce a 15.65
MHz 1st IF signal.
4-1-6 VHF 1ST IF CIRCUIT
By changing the PLL frequency, only the desired frequency
will pass through a pair of crystal filters at the next stage of
the mixer.
The 1st IF signal from the VHF 1st mixer circuit is applied to
a pair of crystal filters (FI1) to suppress out-of-band signals
via a matching circuit (R61, C88). The filtered signal is ampli-
fied at the IF amplifier (Q40) and is then applied to the VHF
2nd mixer circuit (IC28).
4-1-7 VHF 2ND IF AND DEMODULATOR CIRCUITS
The 2nd mixer circuit converts the 1st IF signal to a 2nd IF
signal. A double superheterodyne system (which converts
receive signals twice) improves the image rejection ratio and
obtains stable receiver gain.
The FM IF IC (IC28) contains the 2nd local oscillator, 2nd
mixer, limiter amplifier, quadrature detector, and noise detec-
tor circuits, etc.
• VHF 2nd IF AND DEMODULATOR CIRCUITS
Mixer
16 Limiter
amp.2nd IF filter
450 kHz
PLL IC
IC2
X1
15.2 MHz
X2(15.2 MHz)
RSSIIC28 
TA31136F
14
1st IF (15.65 MHz) from Q40
VSQL signal to the CPU pin 97 11 10 9 875 3
AF signal
VR8V Squelch level
adjustment R19621
19
Active
filter
FI4
(FI5)
Noise
detector
FM
detector 
						

							4 - 2
The 1st IF signal from the 2nd IF amplifier is applied to the
2nd mixer section of IC28 (pin 16), and is mixed with a 15.2
MHz 2nd LO signal generated by the reference oscillator cir-
cuit (X1, IC2) to produce a 450 kHz VHF 2nd IF signal.
The 2nd IF signal from the 2nd mixer passes through the 2nd
IF filter (FI4) (during wide channel spacing selection, or pass-
es through FI5 during narrow channel spacing selection;
[EUR], [ITA] versions only), where unwanted signals are
suppressed. It is then amplified at the limiter amplifier section
(IC28, pin 5) and applied to the FM detector section (X2,
IC28, pins 10, 11) for demodulation the 2nd IF signal into AF
signals.
The FM detector circuit employs a quadrature detection
method (liner phase detection), which uses a ceramic dis-
criminator (X2) for phase delay to obtain a non-adjusting cir-
cuit. The detected signal from IC28 (pin 9) is applied to the
AF circuit.
4-1-8 VHF AF AMPLIFIER CIRCUIT
The AF amplifier circuit amplifies the detected signals to drive
a speaker. The AF circuit includes an AF mute circuit for the
squelch.
AF signals from FM IF IC (IC28, pin 9) pass through the AF
selector (IC21, pins 9, 8), and are then applied to the low-
pass (Q83, R370–R373, C406–C409) and high-pass (Q84,
R375–R379, C415–C418) filters. The filtered signals are
level adjusted at  the volume control IC (IC10), and are ampli-
fied at the AF power amplifier (IC12, pin 2) passing through
the V-AF mute switch (Q70). 
The output signal from IC12 (pin 11) drives the external or
internal speaker.
4-1-9 VHF SQUELCH CIRCUIT
• NOISE SQUELCH
A noise squelch circuit cuts out AF signals when no RF sig-
nals are received. By detecting noise components in the AF
signals, the squelch circuit switches the AF mute switch.
Some of the noise components in the AF signals from the FM
IF IC (IC28, pin 9) are passed through the active filter section
(IC28, pin 8, 7), and then applied to the noise detector sec-
tion (IC28). The variable resister (R196) adjusts the input
level of the active filter, and the level is used for squelch
threshold reference. The detected noise signals are applied
to the CPU (IC19 pin 97) via the “VSQL” line.
The [SQL] (CONTROL unit; R154) controls the input level of
the sub-CPU (CONTROL unit; IC10, pin 59) in DC voltage.
The sub-CPU reads the angle of the [SQL] rotation, then
send the squelch data to the CPU incorporated in the RDATA
line. Then the CPU controls V-AF mute switch (Q70) via the
“VAMUTE” line. 
Even when the squelch is closed, the V-AF mute switch
(Q70) opens at the moment of emitting beep tone.
• TONE SQUELCH
The tone squelch circuit detects AF signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS). When tone squelch is in use, and
a signal with a mismatched or no subaudible tone is
received, the tone squelch circuit mutes the AF signals even
when noise squelch is open.
A portion of the AF signals from the FM IF IC (IC28, pin 9)
passes through the active filter (IC20) to remove AF (voice)
signals via the AF selector (IC29, pins 5, 4), and is then
applied to the CTCSS decoder inside the CPU (IC19, pin 1)
via the “TONEIN” line to control the AF mute switch.
Q83, Q84
Q85, Q86
IC8 IC28
VDET
UDETVA F
UAFCommon
MUTE
UMUTE IC21
98
6
4
312
8
7211
5
7
5SP
• AF amplifier circuit
VHF FM
Detector
UHF FM
Detector
VDMUTE
UAMUTE VAMUTE
UDMUTE
VMUTE
AF power
Amplifier Volume
controller AF selector
IC10 IC12Q70
IC11, Q69
Q71J1
BPF
BPFJ2 
						

							4 - 3
4-1-10 UHF RF CIRCUIT
The UHF RF signals are passed through part of a duplexer
(high-pass filter; L44, L45, L82, C189, C190, C493). The sig-
nals are then passed through the low-pass filter (L42, L43,
C187, C463), antenna switching circuit (D37, D39, D72), and
then amplified at the RF amplifier (Q44). A bandpass filter
(FI3) is used at the next stage of the RF amplifier. The RF
switch (D35, D33) turns on the UHF RF circuit when UHF sig-
nals are received.
4-1-11 UHF 1ST MIXER AND 1ST IF CIRCUITS
The filtered signals from the bandpass filter (FI3) are applied
to the 1st mixer circuit (Q43). The applied signals are mixed
with a 1st LO signal which comes from the U-VCO circuit
(Q20, Q21) to produce a 46.05 MHz 1st IF signal.
The 1st IF signal passes through the 1st IF filter (FI2) to sup-
press out-of-band signals via a matching circuit (R149,
C226). The filtered signal is amplified at the 1st IF amplifier
(Q41) and is then applied to the 2nd mixer circuit (IC8).
4-1-12 UHF 2ND IF AND DEMODULATOR CIRCUITS
The 1st IF signal from the IF amplifier is applied to the 2nd
mixer section of the FM IF IC (IC8, pin 16). The signal is
mixed for producing a 450 kHz 2nd IF signal with a 45.6 MHz
2nd LO signal whitch generated by the tripler circuit (L68,
L69, C208–C212) using the PLL reference frequency.
The 2nd IF signal from IC8 (pin 3) is passed through the 2nd
IF filter (FI6), and is then applied to the limiter amplifier sec-
tion in IC8 (pin 5). The signal is applied to the FM detector
section in IC8 to demodulate into AF signals.
4-1-13 UHF AF AMPLIFIER CIRCUIT
AF signals from IC8 (pin 9) pass through the AF selector
(IC21, pins 3, 4), low-pass filter (Q85, R381–R384,
C415–C418) and high-pass filter (Q86, R386–R390,
C419–C421).
The filtered signals pass through the volume control IC
(IC10). And the level adjusted signals are applied to the AF
power amplifier (IC12, pin 5) via the U-AF mute switch (Q71). 
The output signal from IC12 (pin 7) drives the external speak-
er (connected at J2), or it is fed back to the input line of the
AF power amplifier (IC12, pin 2: VHF AF line).
4-1-14 UHF SQUELCH CIRCUIT
A portion of the AF signals from the FM IF IC (IC8, pin 9) are
applied to the active filter section (IC8, pin 8, 7). The active
filter section amplifies and filters noise components. The fil-
ered signals are applied to the noise detector section. The
variable resister (R229) adjusts the input level of the active
filter, and the level is used for squelch threshold reference.
The detected noise signals are output from pin 14 as the
“USQL” signal, and are then applied to the CPU (IC19, pin
95).
The [SQL] (CONTROL unit; R148) controls the input level of
the sub-CPU (CONTROL unit; IC10, pin 61) in DC voltage.
The sub-CPU reads the angle of the [SQL] rotation, then
send the squelch data to the CPU incorporated in the RDATA
line. Then the CPU controls U-AF mute switch (Q71) via the
“UAMUTE” line. 
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
The microphone amplifier circuit amplifies audio signals from
the microphone to a level needed at the modulation circuit.
The microphone amplifier circuit is commonly used for both
the VHF and UHF bands.
The AF signals from the microphone pass through the MIC
sensitivity control circuit (IC25, D66) and MIC mute switch
(IC26), and are then amplified at the microphone amplifier
(Q88). The amplified signals are applied to the IDC limiter
amplifier (IC23b, pin 6). The output signals from the IDC lim-
iter amplifier (IC23b, pin 7) are passed through the splatter
filter (IC23a, pin 3, 1) and then applied to each VCO circuit
via the deviation adjustment pot.
4-2-2 VHF MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The audio signals (MOD) from the splatter filter (IC23a)
change the reactance of D3 to modulate the oscillated signal
at the V-VCO circuit (Q4, Q5) after passing through the fre-
quency deviation control (R2). The modulated signals are
amplified at the buffer amplifiers (Q6, Q7), and are then
applied to the drive amplifier circuit via the T/R switching cir-
cuit (D4).
4-2-3 VHF DRIVE AMPLIFIER CIRCUIT
The drive amplifier circuit amplifies the VCO oscillating signal
to a level needed at the power amplifier.
The RF signals from the buffer amplifier (Q7) pass through
the low-pass filter (L5, C35, C36), T/R switch (D4) and atten-
uator (R33–R35). The Tx signal from the attenuator is ampli-
fied at the pre-drive (Q11) and drive (Q12, D5, D6) amplifiers
to obtain an approximate 400 mW signal level. The amplified
signal is then applied to the RF power amplifier (IC1).
4-2-4 VHF POWER AMPLIFIER CIRCUIT
The power amplifier circuit amplifies the driver signal to an
output power level.
IC1 is a power module which has amplification output capa-
bilities of about 70 W. The RF signal from the drive amplifier
(Q12) is applied to IC1 (pin 1).
The amplified signals from the power amplifier (IC1, pin 4)
pass through the APC detector (D7, D8), antenna switching
circuit (D9) and low-pass filter (L15, L16, L78, C70–C72),
and is then applied to the antenna connector.
Collector voltage for the driver (Q12) and control voltage for
the power amplifier (IC1, pin 2) are controlled by the APC cir-
cuit to protect the power module from a mismatched condi-
tion as well as to stabilize the output power. 
						

							4 - 4
4-2-5 VHF APC CIRCUIT
The APC circuit protects the power amplifier from a mis-
matched output load and stabilizes transmit output power.
The APC detector circuit (L12, D7, D8) detects forward sig-
nals and refrection signals at D7 and D8 respectively. The
combined voltage is at a minimum level when the antenna
impedance is matched at 50 ½and is increased when it is
mismatched. 
The detected voltage is applied to the APC amplifier (IC5, pin
3) and compared with a reference voltage which is supplied
from the CPU (IC19, pin 68–pin 75) as a D/A control signal. 
When antenna impedance is mismatched, the detected volt-
age exceeds the reference voltage. The output voltage of the
APC amplifier (IC5, pin 4) controls the bias voltage of the
power module (IC1) and drive amplifeir (Q12) to reduce the
output power via the APC controller (Q30, Q31).
4-2-6 UHF MODULATION CIRCUIT
Audio signals from the splatter filter (IC23a) pass through the
frequency deviation control (R78), and are then applied to
the modulation circuit (D20) to change the reactance of D20
and modulate the oscillated signal at the U-VCO circuit (Q20,
Q21). The VCO output is amplified at the buffer amplifiers
(Q22, Q24), and is then applied to the T/R switching circuit
(D23) via the low-pass filter (L33, C153, C154).
4-2-7 UHF DRIVE AMPLIFIER CIRCUIT
The VCO signals from the T/R switch (D23) are amplified at
the buffer-amplifier (Q27), pre-drive amplifier (Q28) and drive
(Q29, D24) amplifier to obtain an approximate 400 mW sig-
nal level. The amplified signal is then applied to the RF power
amplifier (IC4).
4-2-8 UHF POWER AMPLIFIER CIRCUIT
IC4 is a power module which has amplification output capa-
bilities of about 50 W.
The RF signal from the drive amplifier (Q29) is applied to IC4
(pin 5). The amplified signal from the power amplifier (IC4,
pin 1) is passed through the antenna switching circuit (D27)
and is then applied to the antenna connector via a bandpass
filter (L42–L45, L82, C186–C190, C467, C493).
4-2-9 UHF APC CIRCUIT
The APC detector circuit (D25 and D26) detects forward sig-
nals and refrection signals respectively. The combined volt-
age is at a minimum level when the antenna is matched at 50
½and increases when it is mismatched.
The combined voltage is applied to the APC amplifier (IC5,
pin 3), and the power setting voltage from the CPU (IC19, pin
68–pin 75) as a D/A control signal is applied to the other input
(IC5, pin 1) for the reference.
The output voltage from IC5 (pin 4) is applied to the APC
control circuit (Q30, Q31) to control the bias voltage of the PA
module (IC4) and drive amplifier (Q29).
4-3 PLL CIRCUITS
4-3-1 GENERAL
A PLL circuit provides stable oscillation of the transmit fre-
quency and the receive local frequency. The PLL circuit com-
pares the phase of the divided VCO frequency to the refer-
ence frequency. The PLL output frequency is controlled by a
crystal oscillator and the divided ratio of the programmable
divider. IC2 is a dual PLL IC which controls both VCO circuits
for VHF and UHF.
4-3-2 VHF LOOP
The generated signal at the V-VCO (Q4, Q5, D3) enters the
PLL IC (IC2, pin 6) via buffer-amplifiers (Q6, Q8) and is divid-
ed at the programmable divider section and is then applied to
the phase detector section.
The phase detector compares the input signal with a refer-
ence frequency, and then outputs the out-of-phase signal
(pulse-type signal) from pin 8.
The pulse-type signal is converted into DC voltage (lock volt-
age) at the loop filter (Q99, Q100, R531, C476–C478), and
then applied to the V-VCO to stabilize the oscillated frequen-
cy.
• VHF APC circuit
D7D8 L12
HV
VTXAPC
Q30 APC control
Q32 Q31D9
Power
module Drive
APC amplifierto antenna antenna
switch
APC
detector
POWCQ12 Q11 IC1
IC5
Pre-drive RF signal
from PLL circuit