Icom Ic-910h Vhf/Uhf All Mode Tranciever Service Manual

• UT-102 VOICE SYNTHESIZER UNIT
qRemove the bottom cover as shown in the diagram on 
p. 3-1.
• Remove the UX-910 if you have installed it. (p. 3-1)
wRemove the protective paper attached to the bottom of
the UT-102 to expose the adhesive strip.
ePlug UT-102 into J1801 on the MAIN unit as shown in the
diagram at right.
rReturn the bottom cover to its original position.
• UT-106 DSP UNIT
RECOMMENDATION:
When installing only 1 DSP unit, you can install into
either front or rear panel side. However,...

• CR-293 HIGH STABILITY CRYSTAL UNIT
qRemove the bottom cover as shown in the diagram on 
p. 3-1.
• Remove the UX-910 if you have installed it. (p. 3-1)
wRemove 6 screws from the PLL shield cover, then lift up
the PLL shield cover.
PLL shield
cover
eDisconnect the FFC (Flexible Flat Cable) from the DIS-
PLAY unit and the connection cable connectors from
J501 and J1051 on the MAIN unit.
rRemove 5 screws from the PLL unit, then lift up the PLL
unit.
tUnsolder the original reference crystal, then remove...

SECTION 4 CIRCUIT DESCRIPTION
4 - 1
4-1 RECEIVER CIRCUIT
Note: [Main]=Main band, [Sub]=Sub band
4-1-1 VHF TRANSMIT/RECEIVE SWITCHING 
CIRCUIT (PA UNIT)
Received signals from the antenna connector (CHASSIS;
J1) are passed through the low-pass filter (L723–L721,
C728–C726, C728) then applied to the transmit/receive
switching circuit (RL700, D710).
The transmit/receive switching circuit leads receive signal to
the RF circuit from a low-pass filter while receiving.
However, the circuit leads the transmit...

4 - 2
4-1-6 10 MHz IF CIRCUIT (MAIN UNIT)
The 10 MHz IF signal from the mixer circuit is passed
through a monolithic filter (Fl51 [Main], Fl651 [Sub]) to sup-
press out-of-band signals. The filtered signal is amplified at
the IF amplifier (Q51 [Main], Q651 [Sub]). The IF amplifier
provides 20 dB gain.
The amplified signal is then applied to the different circuits
depending on the selected mode.
(1) FM mode
The signal is applied to an FM IF IC pin 16 (IC401 [Main] or
IC951 [Sub]).
(2) SSB and CW mode
The...

4 - 3
4-1-11 NOISE BLANKER CIRCUIT (MAIN UNIT)
The noise blanker circuit detects pulse-type noises, and
stops IF amplifier operation during detection.
A portion of the 10 MHz IF signal from the bandpass filter
(FI51 [Main], FI651 [Sub]) is amplified at the noise amplifier
circuit (Q102, IC101, Q101 [Main], Q702, IC701, Q701
[Sub]). The amplified signal is rectified at the noise detector
(D371 [Main], D701 [Sub]) for conversion into DC voltage.
The DC voltage is amplified at the DC amplifier circuit...

4 - 4 While in SSB mode, the amplified signals from the buffer
amplifier (Q1651) are then applied to the balanced modula-
tor (IC201).
While in AM/FM mode, the amplified signals from the buffer
amplifier (Q1651) are applied to the limiter amplifier
(IC1651b) and splatter filter (IC1651a). The signals are
passed through the buffer amplifier ((IC1652a) and are then
applied to the AM detector (IC1807d, D1652) in AM mode or
to the varactor diode (D253) in FM mode.
4-2-2 MODULATION CIRCUIT (MAIN UNIT)
(1) FM...

4 - 5 low-pass filter and UHF signal are high-pass filter, and then
applied to the VHF and UHF power amplifier circuit sepa-
rately.
4-2-7 POWER AMPLIFIER CIRCUIT (PA UNIT)
The power amplifier circuit amplifies the RF signals to the
specified output power.
(1) VHF power amplifier circuit
The RF signal from the low-pass filter circuit is applied to the
VHF power amplifier circuit (Q651, Q652) to obtain a stable
100 W of RF output power. The amplified RF signal is
applied to the antenna connector (CHASSIS;...

4 - 6 The passed signals are then applied to the RF amplifier cir-
cuit.
4-4-2 1200 MHz RF CIRCUIT (for RX)
Received signals from the transmit/receive switching circuit
are passed through the high-pass filter (L285–L287, L289,
C297–C300) and pre-amplifier (Q281) and are applied to the
RF amplifier circuit (Q271) via the band pass filter circuit
(FI281).
The amplified signals are then passed through the another
bandpass filter (FI271) to suppress unwanted signals. The
filtered signals are then applied to...

SECTION 5 ADJUSTMENT PROCEDURES
5 - 1
4-1 PREPARATION BEFORE SARVICING
REQUIRED TEST EQUIPMENT
DC power supply
RF power meter
(terminated type)
Frequency counter
RF voltmeter
FM deviation meter
Distortion meter
Oscilloscope
Digital multimeterAudio generator
Standard signal 
generator (SSG)
AC millivoltmeter
DC voltmeter
DC ammeter
Spectram analyzer
Attenuator
External speaker
Terminator
EQUIPMENT GREDE AND RANGE EQUIPMENT GREDE AND RENGE
Output voltage : 13.8 V DC
Current capacity : 30 A or more...

5 - 2
5-2 PLL ADJUSTMENTS
30.2 MHz
LEVEL
REFERENCE
FREQUENCY
144M LOCK
VOLTAGE
440M LOCK
VOLTAGE
MAIN BFO
LEVEL
SUB BFO
LEVEL
ADJUSTMENT
ADJUSTMENT ADJUSTMENT CONDITIONMEASUREMENT
VALUEPOINT
UNIT LOCATION UNIT ADJUST
1
1
2
1
2
3
1
2
3
1
2• Display frequency: Any
• Receiving
• Display frequency: Any
• Receiving
This adjustment must be performed
at 5 minutes later after power ON.
• Display frequency: 173.9800 MHz
• Mode : USB
• Receiving
• Display frequency: 136.0200 MHz
• Receiving
• Display frequency:...