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IC751.8
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1990-10-14
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199 lines
Model: IC-751A
Division: Amateur
Note: This service bulletin is meant for technical per-
sonnel with experience working on solid state com-
munications equipment. Damage caused by im-
properly installing this modification may cause
ICOM to charge for subsequent repairs to the
product. ICOM does not warrant this modification.
Bulletin #: 28Oct86-08
Date: October 28, 1986
Subject: Gain improvement of noise blanker in narrow mode
Procedure
1. Replace R42 (currently 150K) on the main board with a 100K
resistor.
Bulletin #: 28Oct86-09
Date: October 28, 1989
Subject: Passband tuning improvement
Procedure
1. Replace R335 (currently 1M) on the main board with a 3.3 M
resistor.
Bulletin #: 22987-001A
Subject: Correction for IC-751A service manual
Corrections
1. Changes to section 6-3
Adjustment Points
BFO Frequency Current Correction
Step 2 LSB Mode L34 L33
Step 3 CW Mode L33 L34
Switch Settings
Current Correction
Step 9 RTTY Mode S1 S2
S2 S1
Step 10 S2 S1
Bulletin #: 13889-010
Date: May 1, 1989
Subject: Reduce intermodulation from FM stations (70-108
MHz) and TV stations (165-245 MHz)
Procedure
Write to ICOM's customer service department for a copy of
this service bulletin.
Bulletin #: 13889-010
Date: May 1, 1989
Subject: Can not change modes (USB, LSB, FM, AM, or CW)
Procedure
1. Locate the green wire labeled W148 that goes from one side
of the board to the other. Re-crimp the connections on the
end of this wire.
Bulletin #: 87-002
Date: December 2, 1987
Subject: Improvement of transmitter intermodulation distor-
tion figure in the 18 MHz band
Procedure
1. Remove the covers from the radio.
2. Replace the following parts on the indicated boards.
Current New ICOM
Designator Part Board Part P/N
C27 10pf Filter Board 12pf 500V 918-01516
C29 47pf Filter Board 12pf 500V 918-01516
Q4 PA Section 2SC2904 906-00434
Q5 PA Section 2SC2904 906-00434
3. Readajust and verify the PA idle current (set to 100 ma) and
PA drive current (set to 600 ma) are correct.
4. Replace covers and check all functions.
Bulletin #: 5889-001
Date: February 27, 1989
Subject: Some 751As exhibit a slight chirp on CW when
operating in SPLIT mode.
Information: This modification speeds up the transmitter "start
time" circuit on the main board
Procedure
1. Remove the top cover from the radio.
2. Remove R32 (currently 180K) and replace it with a 82K 1/8
watt resistor (ICOM P/N 915-01123).
3. Replace the cover and return the radio to service.
Bulletin #: 12589-001
Date: April 1, 1989
Subject: Power amplifier oscillates at 1.9 MHz
Procedure
1. Remove the top and bottom cover from the radio.
2. Remove the PA Section screws and Remove the PA section from
the radio. NOTE: Make sure that the location of each wire
going to the PA section is noted so they may be reconnected
properly when the PA section is reinstalled.
3. Remove the filter board from the radio.
4. Replace the components listed below on the filter board.
Current New ICOM
Part Part P/N
L2 LR137 LR147 924-02138
L2 LR137 LR147 924-02138
LR35 LR136 924-02206
C1 15pf 18pf Dip Mica
C3 2700pf 2200pf Dip Mica 918-01822
C4 220pf 120pf 500V 918-01559
C5 1500pf 1200pf Dip Mica
C56 330pf 918-01575 Note: Add this
part to the foil
side of the board
5. Reinstall the filter board in the radio
6. Reinstall the PA section in the radio.
7. Replace the covers on the radio.