I worked for Icom from 1989 until 2001 and Never knew the serial number had any reference to the date of manufacture. The serial number just identifies the product. There is usually a version number close to the serial number. Depending upon the part of the world you live the version number will be different. Gary K8IZ Washington State Resident. Jan 27, 2004 I worked for Icom from 1989 until 2001 and Never knew the serial number had any reference to the date of manufacture. The serial number just identifies the product. There is usually a version number close to the serial number. Depending upon the part of the world you live the version number will be different. Gary K8IZ Washington State Resident. Add your own review for the ICOM IC-756 Pro III Dec 31, 2010. Review the ICOM IC-756 Pro III. Aug 04, 2020 One end has a 3.5mm jack plug which connects to the CI-V socket on the Icom radio. The other end has a USB plug connects to the PC's USB port. 1 x USB CI-V CAT interface cable. If you require a long length, see my other item for extension cables.
Improvements | Specifications | Accessories | Mini Scope | RTTY | Rear Panel
The Icom 756 Pro III was the last member of the venerable '756' family that started with the 756, 756 Pro, 756 ProII. This latest version offers some incremental improvements over the previous 756 Pro II:
Improved Third Order Intercept point of +30dBm
Real time spectrum scope (with mini-scope)
The IC-756 Pro II spectrum scope display occupied the lower half of the screen. The IC-756Pro III provides a selectable mini-scope display mode. This allows simultaneous display of the filter, memory-keyer and other menus along with a half-size spectrum-scope display.
60 Meter 5 MHz Band
Improved 1st filter and BPF for reduced distortion
Preamp changed to a noiseless feedback type to reduce second order distortion.
New ITU Morse '@' symbol
The new '@' symbol is provided even with the memory keyer enabled.
Programmable SSB transmit bandwidth
The IC-756 Pro II provided three selectable TOBW settings: WIDE/MID/NAR. The lower and upper -6dB points of the transmitted audio-frequency response are fixed for each setting. In the IC-756Pro III, it is possible to program each of the three settings individually with the following preset cutoff frequencies: Lower -6dB point: 100, 300 and 500 Hz., Upper -6dB point: 2.5, 2.7 and 2.9 kHz.
This permits the operator to configure his own custom WIDE/MID/NAR selections by mixing and matching low and high roll-off points.
Clock function displays local time and UTC time with time difference indication
Screen saver function to prolong LCD life and reduce burn-in.
AccessoriesImproved Third Order Intercept point of +30dBm
Real time spectrum scope (with mini-scope)
The IC-756 Pro II spectrum scope display occupied the lower half of the screen. The IC-756Pro III provides a selectable mini-scope display mode. This allows simultaneous display of the filter, memory-keyer and other menus along with a half-size spectrum-scope display.
60 Meter 5 MHz Band
Improved 1st filter and BPF for reduced distortion
Preamp changed to a noiseless feedback type to reduce second order distortion.
New ITU Morse '@' symbol
The new '@' symbol is provided even with the memory keyer enabled.
Programmable SSB transmit bandwidth
The IC-756 Pro II provided three selectable TOBW settings: WIDE/MID/NAR. The lower and upper -6dB points of the transmitted audio-frequency response are fixed for each setting. In the IC-756Pro III, it is possible to program each of the three settings individually with the following preset cutoff frequencies: Lower -6dB point: 100, 300 and 500 Hz., Upper -6dB point: 2.5, 2.7 and 2.9 kHz.
This permits the operator to configure his own custom WIDE/MID/NAR selections by mixing and matching low and high roll-off points.
Clock function displays local time and UTC time with time difference indication
Screen saver function to prolong LCD life and reduce burn-in.
Photo | Item | Description | List Price | Your Price | Order |
---|---|---|---|---|
#3702 | AH-4 | Automatic antenna tuner 120 watts, 80 to 6 meters. INFO | $345.00 | $299.95 |
CT-17 | CI-V Level Converter for connection to PC RS-232C port. INFO | Discontinued | ||
HM-36 | Hand mic [as supplied]. | Discontinued | ||
#4219 | HM-219 | Hand mic [replaces HM-36]. | $55.00 | $44.95 |
#4309 | OPC-025D | DC Power cord 9 foot (3m). [Same as supplied]. | $45.00 | $34.95 |
PS-125 | Switching Power Supply. 25 Amp. INFO | Discontinued | ||
SM- 6 | Desk microphone with flexible neck. | Discontinued | ||
SM-20 | Uni-directional electret condenser-type, Up/Down desktop microphone with low cut function. INFO | Discontinued | ||
#1696 | SP-23 | External Speaker with dual inputs, audio filtration, speaker switch and headphone jack. Click here for details. | $330.00 | $219.98 |
UT-102 | Voice synthesizer announces frequency and mode. | Discontinued | ||
#0229 | CASE 756-3 | Icom commemorative canvas carry bag VIEW | $35.00 | $21.98 |
MINI MANUAL | Nifty spiral bound mini-manual contains 19 laminated pages [4.5 x 8 inch]. | Discontinued |
Note: This info applies mainly to early-production units that do not have the factory upgrades.
Icom 756 Pro Iii Ebay
I don’t know the exact serial number when the upgrades began. But from what I’ve seen, serial numbers from 02023xx and up have all the updates.
So far, serial number 02021xx is the highest I’ve seen that did not have the factory upgrades. And all 5-digit serial numbers I’ve seen did not originally have the upgrades.
Later on, Icom re-designed the PA board. And from what I’ve seen, serial numbers from 0207xxx have the new board.
Chip IC151:
![Icom 756 pro iii serial numbers 1 Icom 756 pro iii serial numbers 1](https://www.ab4oj.com/icom/ic756pro3/images/ic756pro3_med.jpg)
Chip IC151 on the RF Unit can fail due to electro-static discharge (ESD).
Icom released a service bulletin (#920), which recommends adding two PIN diodes to the HRX-line. This can help prevent ESD-related failure.
Icom Ic 756 Pro Iii
Late-production units have a couple of diodes soldered on top of IC151 (see the picture below). I’ve drawn a diagram of the factory modification. The SMD diode is an MA77. The other diode is 1SS53. This upgrade is to prevent damage from ESD.
In the latest-production units, Icom revised the RF Unit board to accommodate three extra SMD diodes in the IC151 circuit (for additional ESD-protection). Its basically the same as the circuit described above, except it uses the smaller version of the MA77 diode (MA2S077). Two of these MA2S077 diodes are in series, in place of the large amber diode in the previous circuit. The other MA2S077 is used in place of the MA77 in the previous circuit. A picture and schematic are shown here:
PA Pre-Driver Circuit:
The pre-driver FET (Q1 on PA Unit) will sometimes become intermittent or fail completely. This is a 2SK2973, and it runs fairly warm. When I replace the FET, I solder a small heat-sink to its source tab. A 2AG-style fuse-clip fits perfectly by cutting off one of its legs, then soldering the remaining leg to the FET tab. See the pictures below:
Some of the later IC-746PRO’s were manufactured with the wrong value resistor in the predriver circuit. R3 on the PA board should be 10 ohms, but some boards had a 4.7 ohm resistor instead. Game warrior orochi 2 pc rip. This can cause failure of the predriver FET (Q1).
PA Driver Circuit:
In early-production units, the two driver FET’s (2SK2975) run quite hot, because the factory set the idling-current (bias) too high. The excessive heat can eventually cause their solder connections to become intermittent, which causes fluctuations of the transmitter RF output. This affects all bands and all modes, and it is most noticeable by observing the ALC meter.
If the FET’s are still good, they can be re-soldered. But re-soldering the FET’s is no easy task. It requires proper tools and skills. The trick is to melt the solder on all three of an FET’s strips at same time, while holding the FET firmly against the mounting plate, so it doesn’t slide out of place. If you’ve re-soldered without the FET becoming loose, you didn’t get it hot enough, and the joints will fail again. To get an idea of how the FET’s are soldered, here are some photos:
Personally, I prefer not to remove the DRV Board. Instead, I remove the entire PA Unit. This may be more labor-intensive, but it eliminates the risk of damaging the traces of the DRV Board, as it requires considerable heat to unsolder its mounting pins. Removing the PA board only requires unsoldering the VHF SO-239 connector and two of the Varistor Boards.
Note: After the FET’s are re-soldered or replaced, the the PA idling current adjustment (R11) should be set to 1.5-amps (instead of 2.5-amps, as stated in the service manual).
PA Driver Idling Current:
Some folks have set the driver idling current lower than the latest factory-recommended setting of 1.5 amps. I don’t recommend doing so, as this can increase IMD (inter-modulation distortion). Here is spectrum analysis of an IC-746PRO with idling current set to 1.5 amps, compared to 0.5 amps:
Here are the changes made to the late-production PA Units:
- The two mounting screws on the DRV Board have been replaced with longer ones that have larger washers.
- Inductor L2 has been changed from 100uH to 10uH.
- Inductor L6 has been changed from a ferrite-bead type to a toroidal type.
- Resistors R41 and R42 have been changed to 6.8-ohm.
- Resistor R11 on the DRV board has been removed.
- The PA idling current adjustment (R11) is set to 1.5-amps (instead of 2.5-amps, as stated in the service manual).
HRX-Muting Circuit:
Another common problem is failure of the HRX-muting circuit. This is usually caused by RF overload, high SWR, or ESD. If this circuit fails, the transmitter will tend to oscillate intermittently. This is most noticeable on the higher HF bands (6, 10, or 12 meters). 2-meters will not be not affected. The symptoms are: unusually high current demand from the power supply, fluctuating RF output, and/or erratic antenna-tuner operation.
Another common problem is failure of the HRX-muting circuit. This is usually caused by RF overload, high SWR, or ESD. If this circuit fails, the transmitter will tend to oscillate intermittently. This is most noticeable on the higher HF bands (6, 10, or 12 meters). 2-meters will not be not affected. The symptoms are: unusually high current demand from the power supply, fluctuating RF output, and/or erratic antenna-tuner operation.
To get an idea if the HRX-muting circuit is at fault, turn the receive attenuator (ATT) on. If the problem goes away when the ATT is on, the HRX-muting circuit is probably at fault. This test works because the attenuator interrupts the path of oscillation.
Suspect one or more of the following components on the CTRL Unit:
Q25 (2SC4213)
D21, D22 (MMBV3700LT1)
D24 (MA77)
D21, D22 (MMBV3700LT1)
D24 (MA77)
Note: To check the DC voltages at Q25, transmit on SSB with the mic-gain at minimum. The base voltage should be 0-volts RX, and 0.7-volts TX. The collector voltage should be 13.8-volts RX, and 0-volts TX.
CTRL Unit Factory Upgrades:
In the latest-production units, Icom has made changes to the CTRL Unit. Here are the changes made to the latest CTRL Units:
![Icom 756 pro iii serial numbers 1 Icom 756 pro iii serial numbers 1](https://www.n1eq.com/wp-content/uploads/2018/03/20180320_120005-e1521660321376-700x543.jpg)
- Two PIN diodes were added back-to-back across the HRX-line, from the junction of L27/R160/RL4 to ground.
- Resistor R154 has been changed to 1K.
Here is a photo of the added PIN diodes:
NOTE: If you don’t have the skills or equipment to work with static-sensitive surface-mount components, please leave it to an experienced technician.
DISCLAIMER: Attempting these modifications on your own equipment is at your own risk. If you have any doubts, don’t try it.
Mike Nadeau – N1EQ
www.n1eq.com
www.n1eq.com
Revised: March 16, 2009