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.
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.
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. 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.
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 kaÃ˜jdn@d… 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.
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.