Icom IC-725 User manual

HF ALL BAND TRANSCEIVER
IC-725
lcom Inc.

INTRODUCTION
This service manual describes the latest service informa-
tion for the IC-725 HF ALL BAND
TRANSCEIVER at the
time of going to press.
If you require assistance or further information regarding
the operation
and capabilities of the IC-725, contact your
nearest authorized lcom Dealer or lcom Service Center.
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
<SAMPLE ORDER>
1110001310 IC µPC577HA IC-725 MAIN UNIT 5pieces
8810005510 Screw PHM3x6ZKBS IC-725 Topcover 10pieces
Addresses are provided on the inside back cover for your
convenience.
DANGER
NEVER connect the transceiver to an AC outlet or
to a
DC power supply that uses more than 16 V. This
will ruin the transceiver.
DO NOT expose the transceiver to rain, snow or
any liquids.
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.
REPAIR NOTE
1. Make sure a problem is internal before disassembling
the transceiver.
2. DO NOT open the transceiver until the transceiver
is disconnected from a 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 insulated tuning tool MUST be used for all
adjustments.
5. DO NOT keep power ON tor a long time when the
transceiver
is defective.
6. DO NOT tr~nsmit power into a signal generator
or a sweep generator.
7 ALWAYS connect a
40 dB-50 dB attenuator between
the transceiver
and a deviation meter or spectrum
analyzer when using such test equipment.
8. READ the instructions of test equipment thoroughly
before connecting equipment to the transceiver.

TABLE OF CONTENTS
SECTION 1 SPECIFICATIONS.................................................... 1 -1
SECTION 2 INSIDE VIEWS. ... ..... . .. . ... . .. .. . .. . .. .... .... . .... ... . .. .. .. ...... 2 - 1 -·2
SECTION 3 BLOCK DIAGRAM .. .. .. .. . .. .. .. .. .. . .. .... ... ... .... .. .. .. . ... ...... 3 - 1
SECTION 4 CIRCUIT DESCRIPTION............................................... 4-1 -7
4 • 1 RECEIVER CIRCUITS . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . 4- 1
4 · 2
TRANSMITTER CIRCUITS. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . . . . . . 4 - 4
4 • 3
PLL CIRCUITS............................................................. 4-6
4 · 4 LOGIC CIRCUITS. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . 4 - 7
4 • 5
REGULATOR CIRCUITS . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . 4 - 7
SECTION 5 MECHANICAL PARTS AND DISASSEMBLY.................. .. ... ... .. 5 -1 ....., 7
5 · 1 FRAME DISASSEMBLY ................................. : . . . . . . . . . . .. . . . . . . . 5- 1
5 · 2
PA UNIT AND ACCESSORIES . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 5- 3
5 • 3
FRONT AND PLL UNITS CONNECTOR ASSEMBLY. . . . .. . . . . . . . . . . . . . . . . . .. .. . . . . 5 - 5
5 •4
FRONT AND MAIN UNITS CONNECTOR ASSEMBLY . . . . .. . . . . . . . . . . . . . . . .. . . . . . . 5 - 6
5 • 5
PA AND FIL TEA UNITS CONNECTOR ASSEMBLY. . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . 5- 7
SECTION 6 ADJUSTMENT PROCEDURES. . .. .. . .. .. ... .... .. ..... ... . .. .. .. ...... 6 - 1-7
6·1 PREPARATION BEFORE SERVICING;.......................................... 6-1
6 • 2 PLL ADJUSTMENT . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . 6 _.: 2
6 · 3
RECEIVER ADJUSTMENT. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . 6 - 4
6 •4
TRANSMITTER ADJUSTMENT . .. . . . . . . . . . . . . .. . . .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . 6 - 6
SECTION 7 BOARD LAYOUTS .. .. .. . .. ... .. .. .. . .. ..... ... ... ..... .. .. .. .. ... .. .. 7 - 1 ,...., 5
7 · 1 FRONT UNIT . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . .. . . . . . . . 7- 1
7 · 2 MAIN UNIT ....................................... · . . .. . . . . . . . . . . . . . . .. . . . . 7- 2
7 -3
PLL UNIT. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . 7 - 3
7 · 4
PA UNIT. . . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . 7 - 4
7 •5
DDS UNIT .............................................................. ; . 7 - 5
7 · 6
AM • FM UNIT (OPTIONAL) . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . 7 - 5.
SECTION 8 PARTS LIST .......................................................... 8-1-13
SECTION 9 VOLTAGEDIAGRAMS ................................................ 9-1-2
9 • 1 FRONT AND MAIN UNITS . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . 9 - 1
9 • 2
PLL AND PA UNITS . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . . . . . . .. . . . . 9 - 2

SECTION 1 SPECIFICATIONS
•GENERAL
• Frequency coverage
•Modes
• Number of memory channels
• Antenna impedance
• Usable temperature range
• Frequency stability
• Power supply requirement
• Current drain (at 13.8 V DC)
• Dimensions
•Weight
•TRANSMITTER
• Output power
• Spurious emissions
• Carrier suppression
• Unwanted sideband
•Microphone impedance
•RECEIVER
• Receive system
• Intermediate frequencies
• Sensitivity (preamplifier ON)
• FM squelch sensitivity
• Selectivity
• Spurious response rejection
• Audio output impedance
• Audio output power
• RIT variable range
Receive 500
kHz-30 MHz
Transmit 160-m band
1.8- 2.0 MHz
80-m band 3.5- 4.0 MHz
40-m band 7.0- 7.3 MHz
30-m band 10.1-10.15 MHz
20-m band 14.0-14.35 MHz
SSB (A3J), CW (A 1 ), AM (A3), FM (F3)
17-m band 18.068-18.168 MHz
15-m band 21.0 -21.45 MHz
12-m band 24.89 -24.99 MHz
10-m band 28.0 -29.7 MHz
(Ul-7 AM · FM UNIT is required for AM transmit and FM transmit/receive.)
26
50 a unbalanced
-10 °C-+60 °C (+14 °F-+140 °F)
Less than ±200 Hz up to one hour after power is turned ON.
Less than ±30 Hz after one hour at +25 °C (+ 77 °F).
Less than ±350 Hz at 0 °C-+50 °C (+32 °F-+122 °F).
13.8 V DC ±15 %, negative ground
Receive squelched 1.2 A max. audio output 1.5 A
Transmit
20 A
241
(W) x
94
(H) x 239 (D) mm (Projections not included)
9.5 (W) x 3.7 (H) x 9.4 (D) in
4.6 kg (10.1 lb)
SSB, CW, FM 10-100 W continuously adjustable
AM 10-40 W continuously adjustable
More than 50 dB below peak output power
More than
40 dB below peak output power
More than
50 dB down with 1 kHz AF input
6000
SSB, CW, AM Double-conversion superheterodyne
FM
1st SSB
Triple-conversion superheterodyne
70.4515 MHz
cw 70.4506 MHz
AM, FM 70.4500 MHz
2nd
SSB 9.0115 MHz
CW 9.0106 MHz
AM, FM 9.0100 MHz
3rd
FM 455 kHz
SSB, CW (10 dB S/N) 1.8-30 MHz Less than 0.15 µV (-123 dBm)
AM (10 dB SIN) 0.5-1.8 MHz Less than 13.0 µV (-85 dBm)
1.8-30 MHz Less than 2.0 µV (-101 dBm)
FM (12 dB SINAD)
28-30 MHz Less than 0.5 µV (-113 dBm)
Less than 0.3 µV (preamplifier ON)
SSB, CW More than 2.3 kHz/-6 dB
AM More than 6.0 kHz/-6 dB
FM More than 15 kHz/-6 dB
More than
70 dB
an
Less than 4.0 kHz/-60 dB
Less than 20.0 kHz/-40 dB
Less than 30 kHz/-50 dB
More than
2.6 W at 1o% distortion with an 8 a load
More than ± 1 kHz
All stated specifications are subject to change without notice or obligation.
1-1

SECTION 2 INSIDE VIEWS
• PLL UNIT
A/D converter------
(IC12: MB4052M-G)
Cl-V interface-------=
Cl-V diode matrix-----
CPU (IC8: HD63A01YOG83P)
Lithium backup battery---
+5 V regulator-------=
(IC10: TA78005AP)
Sensor
circuit------
Band voltage buffer----....;:;;,:..,.,.-""""
(IC9: M54562P)
•MAIN UNIT
Rx bandpass filters ------=-
Rx 1st mixer circuit-----+
Tx 2nd mixer circuit---_...
MCF (Fl1: FL-71)-----,
Noise blanker circuit-----
Rx 2nd/Tx 1st mixer----:
(IC1: ND487C1-3R)
MCF (Fl2:
FL-23)----~====!-i..--I
9 MHz SSB • CW filter
(Fl3: FL-30)
2-1
-----1st LO amp circuit
Phase detector/Prescaler
------Sub loop VCO
c---------DDS UNIT
-:::-------2nd LO circuit
'---------=;----Reference crystal
(X2: CR-21 30.72 MHz)
Divider (IC15: M74ALS74AP)
Space for Ul-7 AM • FM UNIT
+------APC/ALC circuit
'+-------- BFO circuit
;.;------SSB modulator
(IC6: µPC1037HA)
~----Product detector
(IC5: µPC1037HA)
----Rx 2nd IF amplifier
.------9.01 MHz AM filter
(Fl4: FL-116)

• PA AND FILTER UNITS
Tx/Rx switching relay-----;;.-=:-;;-
(RL13: DS1-M-DC12V)
--------Predrive amplifier
(01: 2SC1971)
~----Drive amplifier
(02, 03: 2SC3133)
'=+----Final amplifier
(05, 06: 2SC2904)
-'------Thermal switch
(81: OHD-3 90M)
2-2

c.>
I
....
a:~Tr~N 1 .-1 , • ,,. <
022
2SK241
I 14 I~ ~I
+•
TB
••·••)b ~
SW -+~
DELAY
..
MAIN UNIT
!lb 2nd mlur.1
Tx !ti mixer
TXC
"" ~I '.
RF POWl!:R
CTRL
13.BV
o----<--------- 13.81/
0.151"\..1,IMHz
ATT SW
T1 2nd mlur
PA UNIT
~
"~8MH;
~
,, ..,----L:..:._jJ 3.5MH~
~
., ....------L::._jj- --7.0MH;
07.8
IK&OX2
ANT
--------------~~;~
015, 17, ,., 21 -r::;;l I
~-1·--·- --- -
VCOI • 70.H15"'- 71.4514MH1
VC02 '78.4&15"" 85.4&f4MHz
VC03 '85.4514.,,. 92.4514MHr
VC04 '92.40115" 100.451!5MHz
PLL UNIT
IC17
pPCI037H
2SK112A )f4 PA LPF BUFF
Lt ... LI I
I
01
MVO
Q4
2801406
D2,3
MV5X2
', QT 2SBOl2
I
··~ 0
~
L---------
•••
DATA
CPU
tea
HD83AOIY
IC12 M84052
MAIN
DUL
RIT
I- s & • VFO. MHr •IC.
DISPLAY
D~
IC4 pPD4069
IC6
BA222
~
-151/ 13.81/
v

SECTION 4 CIRCUIT DESCRIPTION
4·1 RECEIVER CIRCUITS
4-1·1 RF SWITCHING CIRCUIT
(PA AND MAIN UNITS)
RF signals from the antenna connector pass through the
transmit/receive switching relay
(RL13) and a low-pass
filter,
and are applied to the MAIN UNIT via P2 (MAIN
UNIT:
J12).
The signals from the PA UNIT either bypass or are
attenuated at
20 dB attenuator (R102, R103, RL1). There
are no non-linear components between the antenna
connector
and attenuator to prevent distortion caused
by strong signals. The signals are then applied to
RF
filters.
4·1·2 RF BANDPASS FILTER CIRCUIT
(MAIN UNIT)
The RF UNIT has 7 RF. bandpass filters (BPF) for signals
above 1.6
MHz and 1 low-pass filter (LPF) for signals below
1.6
MHz. The signals pass through one of the bandpass
or low-pass filters depending
on the receive frequency
range.
(1) 0.5-1.6 MHz
A diode
is not used at the low-pass filter entrance
removing diode distortion from very strong signals.
Signals bypass a preamplifier by the bypass switch (012).
(2) 1.6 MHz AND ABOVE
Signals are applied to a high-pass filter consisting of
L42,
L43, C143-C146. This filter suppresses strong signals
below 1.6
MHz such as a broadcasting station.
The filtered signals are applied to one of 7 bandpass filters
depending
on the frequency of the signals and then to the
preamplifier circuit (08, 09).
(3) FILTER SWITCHING CIRCUIT
An RF bandpass filter is selected with BPF switching
voltage (BO-B7) from the
CPU via IC16 current amplifier.
The switching voltage of the BPF entrance is higher than
the
BPF exit to improve multi-signal and strong signal
characteristics.
4·1·3 PREAMPLIFIER CIRCUIT (MAIN UNIT)
The preamplifier circuit uses low-noise junction FETs
(2SK125 x 2) to provide 1O dB gain over a wideband
frequency
range.
When the [PRE] switch is turned ON, the signals from
the
RF filter are amplified by the preamplifier circuit (08,
09).
When the [PRE] switch is turned OFF, the signals
bypass the preamplifier through 030
and 032. When the
operating frequency is below
1.6 MHz, 012 turns ON and
the signals bypass the preamplifier regardless of the
[PRE] switch.
4-1
Amplified or bypassed signals are applied to the 1st mixer
circuit via the low-pass filter. The low-pass filter cuts
off at
35 MHz to suppress image frequency at the 1st
mixer circuit (013, 014).
PREAMP CIRCUIT
RF signal
from BPF
030
Q10
Fig.1
4·1·4 1ST MIXER CIRCUITS (MAIN UNIT)
The signals from the low-pass filter enter the 1st mixer
circuit (013, 014) to
be converted to a 70.45 MHz tst IF
signal.
EXACTNESS 1ST IF FREQUENCY
MODE
FREQUENCY (MHz)
SSB
70.4515
cw
70.4506
AM, FM
70.4500
The 1st mixer circuit employs a balanced mixer using
low-
noise junction FETs (2SK125 x 2) to expand the dynamic
range.
The 1st LO signal (70.9515-100.4515 MHz) enters the
MAIN UNIT from the PLL UNIT via J5. The signal is
amplified at 04, filtered by a low-pass filter, and then
applied to the 1st mixer circuit (013, 014). The low-pass
filteremploys a ring core inductortoprevent 1st LO leakage
signals.
The output level from 04 is approx. 25 dBm.
The 1st IF signal is applied to
an MCF (Monolithic Crystal
Filter;
Fl 1) to suppress out-of-band signals. The signal is
amplified at the 1st IF amplifier (015), and then applied
to the 2nd mixer
(IC1).

4·1·5 IF CIRCUITS (MAIN UNIT)
The 1st IF signal from 015 is converted to a 9 MHz 2nd
IF signal at the 2nd mixer (IC1). IC1 is a DBM (Double
Balanced Mixer).
EXACTNESS 2ND IF FREQUENCY
MODE FREQUENCY (MHz)
SSB 9.0115
cw 9.0106
AM, FM
9.0100
The 2nd LO signal (61.44 MHz) from the PLL UNIT via J4
is applied to the 2nd mixer. The converted 2nd IF signal
passes through D4 (D35 for transmitting) and is applied
to the
MCF (Fl2) to suppress unwanted signals.
The signal output from Fl2 passes through the noise
blanker gate
(D5-D8) and is amplified at the 2nd IF am-
plifier (021). The signal enters one of the three 9 MHz
filters (Fl3, Fl4, optional
CW narrow filter) or optional AM
· FM UNIT via D52. The filters are selected with mode
selecting signals (SSB ·CW, AM, CW-N) and the "TB"
voltage line.
The signal from a 9 MHz filter is amplified at the 2nd IF
amplifiers (027-029) and applied to the demodulator
circuit.
Dual-gate
FETs are used on the 1st and 2nd IF amplifiers
(015, 021, 027). The 2nd gates of 015, 021 and 027 are
controlled by
AGC bias voltage. A rapid time constant
is used for 027 to prevent raising the edge distortion of
receive signals.
R140, connected to the gate of 028, improves the
temperature characteristics of the receiver gain.
R138
adjusts the receiver gain.
IF CIRCUIT
AF signal SQL
IC8
BFO
SSB DET
AM OET
062
4·1·6 NOISE BLANKER CIRCUITS
(MAIN UNIT)
The IC-725 uses a noise trigger noise blanker circuit that
cuts out pulse-type noise signals at the noise blanker gate
(D5-D8).
A portion of the signals from Fl2 is amplified at the noise
amplifiers (016,
IC2) and detected at the noise detector
(D12,
D13). The detected voltage from the noise detector
is applied to the noise blanker switch (019).
The threshold level of the noise blanker switch
is set at
0.9 V. When the detected voltage exceeds the threshold
level, 020 outputs a blanking signal to activate the noise
blanker gate
(05-08).
A portion of the detected voltage is applied to the noise
AGC circuit (018) and fed back to the noise amplifier (IC2)
as noise AGC voltages. The time constant of the noise
AGC circuit is determined by R43, R47 and C60. This
AGC circuit does not operate to detect pulse-type noise.
When the operating frequency or mode is changed, the
"ONB" signal line becomes "LOW," turning 020 ON. The
noise blanker gate prevents PLL click noise.
4·1·7 BFO CIRCUIT (MAIN UNln
A 9 MHz signal oscillated at the BFO circuit (031, X1)
is buffer-amplified at 042 and used at the balanced
modulator
(IC6) and a product detector (IC5). The BFO
frequency is shifted with a mode signal using 067-069.
In USB mode, the "USB" signal line becomes "HIGH,"
turning ON 069. The frequency is then adjusted with
C294 to set the
USB carrier point.
At
CW mode transmitting, the "CW" signal line becomes
"HIGH" and 033 becomes OFF, turning ON 068. The
frequency is then adjusted with L83 to set the CW
transmit carrier point.
In LSB mode, the "LSB" signal line becomes "HIGH,"
turning ON 067. The frequency is then adjusted with
L82 to set the
LSB carrier point.
NOISE
BLANKER
Fig. 2
059, 060
4-2

BFO FREQUENCY IN EACH MODE
MODE
FREQUENCY (MHz)
USB
9.0130
CW (Tx)
9.0106
LSB
9.0100
CW (Rx)
9.0098
AM
NO OUTPUT
4·1·8 DEMODULATOR CIRCUITS
(MAIN UNIT)
The IC- 725 has 2 detector circuits, a product detector and
a diode detector to demodulate the SSB, CW signal and
AM signal respectively.
In SSB or CW mode, the 2nd IF signal from the IF amplifier
(029) is mixed with the BFO signal at the product detector
(IC5) to demodulate the 2nd IF signal into an AF signal.
The detected signal passes through the AF input mode
selector switch
(IC8).
In AM mode, the 2nd IF signal from 029 passed through
C121 is detected at D62 and passes through the AF input
mode selector switch (IC8).
4-1·9 AF INPUT MODE SELECTOR SWITCH
(MAIN UNIT)
The AF signal from a detector circuit or the optional AM
· FM UNIT is applied to the AF input mode selector switch
(IC8). IC8 consists of 4 analog switches and they are
selected with a mode signal from IC15 and the squelch
control signal. The AF signal
is applied to the AF amp
circuit.
IC8 AF INPUT MODE SELECTOR SWITCH
MODE
ACTIVATING
CONTROL
PIN NUMBERS
PIN NUMBER
USB, CW 2-1 13
AM 3-4
5
FM 10-11 12
ANY MODES
9-8 6
(for S-meter)
AGC, S-METER AND SQUELCH CIRCUIT
Squelch signal
av
S.meter
signal FM S·meter
signal
SQLS
to [RX)
indicator
Fig. 3
4-3
4·1·10 AF AMP CIRCUIT (MAIN UNIT)
The AF signal from the AF input mode selector switch is
applied to the AF preamplifier (035, 036). The CW
sidetone signal is applied to 036.
The output from the AF preamplifier is applied to the [AF]
control
(FRONT UNIT, R1 b) and the 2.8 kHz cut-off active
low-pass filter (037). The
AF signal is power-amplified at
IC9 to drive the speaker.
4·1·11 AGC AND S·METER CIRCUIT
(MAIN UNIT)
The receiver gain is determined by the voltage on the AGC
line (030, collector). When strong signals are received,
the
AGC circuit decreases the voltage on this line.
The IF signal from the IF amplifier (029) passes through
C117, is detected at D59 and 060, and applied to the base
of 030. A time constant
(C113, R120) is connected to
the AGC line that determines the AGC release time.
The time constant is controlled by the [AGC] switch. When
the [AGC] switch is pushed OUT, C112 and R119 are
connected
in parallel with the AGC line to obtain a slow
AGC release time.
The AGC bias voltage is applied to the differential amplifier
(IC4, pin 6) where the difference between the bias and
reference voltages is detected. The resulting S-meter
signal passes through the meter switching circuit
(IC8)
and is then applied to the meter on the front panel. The
reference voltage is adjusted with R116. IC8 pins 8 and
9 are connected inside the IC in receiving.
The FM S-meter signal from the optional AM · FM UNIT is
applied to the meter switching circuit (IC8) via 057. The
signal is also applied to the squelch circuit (IC4 pin 2).