I really want an IC-746PRO. AES recently dropped the price of this radio to $1300, and for that price, I think it’s the best deal on the market.
At least it would be if not for that nagging IC151 problem.
- In the IC-746/PRO displays, there’s a wider opening, so I add a spacer. The IC-746/PRO series requires 3 sections of this LED strip. And the IC-756PRO series requires 7 sections. (Update: I now use BD437TG instead of 2N6121.) IC-756PROII with LED conversion at 100% brightness: IC-746 with LED conversion at 100% brightness.
- Positive the number, the better, and TOI figures can also be negative. Although the PROIII Instruction Manual doesn’t specify the advertised TOI number, an ICOM Product Guide, origi-nally in Japanese, spells out the measure-ment conditions: 100 kHz spacing (wider than our Lab’s widest 20 kHz spacing mea-surement), preamps off and a 2.4 kHz fil.
- Here's my settings with 756 PRO 3 Time Owned: 0 to 3 months. Bought this mic used, to be connected to my newly acquired ICOM 756 PRO III. Great audio reports and the mic is both stylish and well made. Beware the LOCK button as it's kind of easy to hit. My SUGGESTED settings with a 756 PRO III is: Microphone gain on mic = 30% or less.
- AH-2B AH-4 CT-17 HM-36 IC-PW1 SM-20 SP-23 UT-102: Mobile antenna element, 10-80 m with AH-4 External automatic antenna tuner, HF/6 m, 120 W CI-V level converter.
Hindi typing apps download. Brand new in box Icom IC756 Pro III. Basically unboxed, inspected, and then placed back in the box and in the closet. Serial number is: 3204610. The item “Icom IC 756 PROIII Radio Transceiver NIB New in box late serial number” is in sale since Thursday, April 28, 2016.
There actually seems to be two problems with IC151. Both cause the rig to quit transmitting.
The first problem seems to be a thermal problem. Some IC-746PRO owners say that IC151 gets hot, causing it to fail prematurely. If you read the IC-746PRO reviews on eHam, it looks as though quite a high percentage of the units fail, and many fail more than once.
![Serial Serial](https://www.eham.net/data/classifieds/images/250783.jpg)
What’s even more confusing, or dismaying, is that the IC-756PRO uses this same IC in the same application, but IC-756PRO owners don’t report anywhere near the number of failures that owners of the IC-746PRO do. Many hams point to the difference in the two circuits. Gubelin watch serial numbers. Telecharger patch jonah lomu rugby challenge ps3. OZ2M, an electrical engineer by training with five years of experience with Motorola as a Senior RF Design Engineer, points out on his IC-746PRO web page that in the IC-756PRO, the VCC line is switched on and off, with the IC only being powered when the rig is transmitting. This will, of course, keep the part much cooler. OZ2M also shows on this page various heatsinks that some hams have designed and installed in attempts to keep the chip cooler.
K5LXP also experimented with heatsinks, but he took his fix a step further. In addition to adding a heatsink to IC151, he also found a way to switch the VCC to the chip. What he did was find an 8 VDC line that was switched on only during transmit and used that to power IC151. He suggests that perhaps you don’t need the heatsink with the VCC fix, as this modified circuit is similar to the circuit in the IC-756PRO, which does not seem to have an IC151 problem.
![Icom 756 pro iii serial numbers lookup Icom 756 pro iii serial numbers lookup](https://www.ab4oj.com/icom/ic756pro3/images/mosfet_pa/mosfet_pa1.jpg)
The second problem is an ESD problem, and Icom claims that if there is a problem it is an ESD problem, not a thermal problem. The Icom America website has a page titled, IC-746PRO – IC-151 Protection Diode Fix states,
There are rumors on the Internet that the circuit around IC-151 is incorrectly designed causing overheating of this part and that the protection diode fix is an attempt to mislead customers by ICOM. Nothing could be further from the truth. Measurements show that the surface temperature of IC-151 is low, even after 4 hours of continuous operation. Our service technicians do not see any discoloration of the circuit board or the deformation of parts that are the symptoms of overheating. As stated above, protection diodes have been shown to resolve the issue and no heating calculations are needed. ICOM America service records show that the failure rate of this part has been reduced to 1/10th of the previous value.
Note that they don’t say that the fix eliminates the problem completely. It is OZ2M’s opinion that the fix may indeed prevent some units from failing, but that the fix does nothing to prevent thermal failures.
It also appears that there’s more to this ESD problem than meets the eye. As one message to the IC-746PRO mailling list mentions, Burghardt Amateur Center is preparing a more comprehensive ESD fix. The message quotes someone at Burghardt, who says,
“Most of the cases when we see IC151 bad is due to static discharge. Not only will IC151 of the RF unit be bad but also D21,D22,D24, and Q25 of the Tuner control unit. Theses other components will cause the receiver to be noisy. Changing the supply line to IC151 will have no effect on this since the surge is coming down the receiver line and damaging the outputs in IC151. This will occur whether the IC151 is powered or not. There are a lot of misconceptions out there about what is the main cause of this failure. We are currently in the process of having a surge suppression board built for the IC-746,746PRO,756,756PRO, and 756PROII. It is installed on the tuner control board and will help protect all receiver components from static and excessive RF fields. It will be available 8/1/04 and sells for $69.95. If you have any questions please E-mail me at kaØ[email protected]… or call 605-886-7314 and ask for David.”
All this is very troubling for me. I’d really like to buy this radio, but I’m afraid it will be just one hassle after another if I do. It’s especially troubling because I really like my IC-735, and have never had any trouble with it in the almost 20 years that I’ve owned it. Airfoil 5.6.0.
I swapped a few e-mails with K5LXP about this, and he was very kind to give me his take on the situation. He noted that hams have a long tradition of modifying gear, and while modern rigs are certainly more complicated than the older stuff, these modifications are not impossible to do. He also reminded me that at $1300, the IC-746PRO is a really great deal, and the cost of the modifications is very small.
In the end, I have the feeling that I’m going to bite the bullet and buy the IC-746PRO. Current production units already have the ESD protection diodes installed, and if IC151 does fail, I’ll apply K5LXP’s VCC fix.I really want an IC-746PRO. AES recently dropped the price of this radio to $1300, and for that price, I think it’s the best deal on the market.
Related posts:
Note: This info applies mainly to early-production units that do not have the factory upgrades.
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.
Victor matthews warfare manual. Later on, Icom re-designed the PA board. And from what I’ve seen, serial numbers from 0207xxx have the new board.
Chip IC151:
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.
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.
Icom 756 Pro Iii Serial Numbers 1
Haynes flute serial number list. 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. 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).
Icom Ic 756 Pro Iii
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)
Icom 756 Pro 3 Reviews
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:
- 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: U he plugins pack.
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