Icom 746Pro No-Transmit
Repair Information Summary

A compilation of repair and modification notes for the 746Pro "Sudden Death" transmit problem.
Mark Brueggemann, K5LXP -  Nov, 2004 - k5lxp@arrl.net

Executive Summary
Thanks to the internet and a variety of mailing lists, it's no secret there's a problem with the 746Pro transceiver.  My own 746Pro crapped out in less than 6 months of operation, so this problem was involuntarily foisted upon me. Rather than go through the Icom warranty mill I did my own investigation into, and subsequent repairs of the problem. Since that time I have discovered others who have done a similar effort and devised their own solutions as well.  It only made sense to incorporate all of the known fixes into the radio at one time and take care of the problem, no matter which specific issue is the root cause of the failure.

There are several failure modes suspected as the cause of the "sudden death" 746 transmit problem. They are:

The last one is listed as a bit of levity, but realistically I can't rule out any other possible causes. If someome comes up with some hard data to support another yet-undiscovered problem, I'll be happy to add it to the list. When a radio fails it's difficult to know for sure which one of these was the root cause.  The 746Pro no TX problem always manifests itself in the form of a dead IC151, a broadband medium power amplifier IC. Without a sample of dead parts and subsequent detailed analysis, we may never know exactly what is taking these parts out. The good news is the the fix for the identifed suspected root causes is simple, inexpensive and readily implemented by most hams with simple tools. The group of fixes here, implemented in my own rig, have allowed to it operate longer than it did with just the "protection diodes" Icom installed at the factory.  Time will tell how robust these fixes will be, but to be sure without them you stand a very good chance of joining the "sudden TX death" club.  Yes, the problem should be fixed by Icom, but that's not going to happen. Rather than complain about it, let's just fix it and enjoy this otherwise excellent transceiver.

IC151 Popped, or not?
You turn on your rig one fine day, hit the PTT and no joy.  You check the controls and no matter what you do, there's no transmit power at all.  You get that sinking feeling in the pit of your stomach that you just became a member of the Sudden Death Club.  But how do you know for sure?  The test to see if your IC151 is bad is very simple.  Take the bottom cover off of the unit, and remove the shield covering the back end of the RF Unit PCB.    IC151 is at the back edge of the RF Unit PCB facing out.  Connect the rig to 12V power, turn it on and set the controls as follows:

Frequency: 3.5MHz
Mode: FM
RF Power: Max Clockwise

Using even a basic oscilliscope, set the sweep for 1uS and amplitude for 50mV per division.  Press the [TRANSMIT] button on the rig and you should see about 20mV P-P of RF on pin 1 (RF In), and 145 mV P-P of RF on pin 5 (RF Out) of IC151.  If it's bad, you will see little to no RF at pin 5.  If you do see sufficient RF at this point and your 746 isn't transmitting, the problem is elsewhere and beyond the scope of this discussion.

Technical Data  
I spent a bit of time characterizing the thermal characteristics of IC151 based on the the information found on OZ2M's 746 Page.  It seemed unlikely at first that a part dissipating only a third of a watt could overheat but after reading the component's Data Sheet and Application Note, you discover that the only path for heat to exit the chip is through it's ground leads.  The copper pads for IC151 on the RF Unit PCB aren't nearly enough heatsink for this part. My subsequent bench measurements confirm this.  So connecting a heatsink to the ground legs is the only effective cooling solution.  Merely placing a heatsink on the plastic package itself, while it wouldn't hurt, won't do any good either as the thermal resistance from the die through the plastic package is very high.  OZ2M shows several examples of homebrew heatsinks that fit over the existing IC151.

If you read OZ2M's page you learn that the IC-756Pro uses this same RF amplifier IC as the 746Pro, and you don't hear of any of those transceivers having this sudden TX death problem. It turns out with the 756Pro, power is turned on to the part only during transmit, while in the 746Pro the part is powered up 100% of the time. The part in the 746Pro has to dissipate it's 300mW or so of DC bias current even during RX/standby. Since amateur transmit duty cycles are so low it's no suprise that the IC in the 756Pro isn't failing very often, if at all.  There is also less chance of an unpowered part being damaged than a powered one.  So it seems if you could mimic the 756Pro switched TX VCC circuitry, you might similarly reduce your chances of failure in the 746Pro as well.  There isn't any convenient source of TX switched 5V near IC151, but there is a source of TX switched 8V.  Using a zener diode from the 8V source provides the TX switched VCC to IC151 in the 746Pro.

Lastly, based on the experiments performed by Steve, NB30 that he graciously shared with the 746 Yahoo Group and gave me permission to quote here, shows there is some validity to the ESD sensitivity claimed by Icom.  Adding the diodes to the HRX line (the 'protection diode fix') while a step in the right direction, is a poor stopgap to mitigate this problem.  ESD can contain a significant amount of energy, certainly more than what it would take to blow the PIN diodes, and the next part in line, IC151.  A better solution is to use a choke, which is virtually impossible to damage with ESD.  Additionally, Steve identified a potentially damaging transient to the input of IC151, along with a suggested fix.  The text of his tests and conclusions can be found here: 

Email and ListServe Discussions

Tiny Heatsink
IC151 undergoing thermal testing using a mockup heatsink.
A view of the competed SSOIC installation on the SOIC pads, along with the homebrew heatsink.

If you weren't already aware that the 746Pro is comprised mostly of 0603 (60x30mil) size surface mount parts, you're about to discover it.  While performing the fixes below are basically no more difficult than installing the optional high stability oscillator module, not every ham posesses the skills required to perform them.  If SMT makes you long for the days of Heathkits then maybe this isn't for you.  If you go ahead anyway and hose it up, don't blame me, I'm not the one holding the iron.  And no, I'm not in the business of fixing ham radios but I'll gladly help folks any other way I can.  The fixes described here worked fine on my own rig but due to circumstances beyond my control I cannot guarantee their efficacy in any other unit.  Proceed at your own peril.  (Scary, eh?)

What you'll need.
You're going to need the following parts, Digi-Key part numbers are shown but they may be alternately sourced as necessary.

   1ea  IC151 RF Amp:  Icom# 1110001890, DK# UPG1678GVCT-ND
   2ea SOT PIN Diodes: 1SV252, Icom# 91213606, Substitute- DK# BAT54SWT1OSCT-ND
   1ea .01uF 0603 X7R Cap:  DK#399-3189-1-ND
   1ea 1 millihenry molded axial leaded Choke: DK# TK4312-ND
   1ea 2.7V Zener:  DK# MMSZ5223BDICT-ND

The UPG1678 from Digi-Key is an SS08 package, not an S08 as originally used in the radio.  If you're careful, you can bend the leads out and get them to fit on the S08 pads.  Alternatively you can get the exact replacement from Icom.  If you're reading this before your radio died and are doing the fix as a preventative measure, you can leave it alone.

I couldn't find a retail (or even OEM) source for the 1SV252 diodes, so the most convenient source becomes Icom for those. If you don't care about using "genuine" Icom replacement parts, the  BAT54 diodes from Digi-Key are a form/fit/function replacement and I think a better substitute than the original.  While it may seem superfluous, I would order at least two sets of everything.  This will cover at least one botched attempt or lost part.  Having spares on hand will save you much time and effort than if you'd only ordered one, had a problem, and had to repeat the order and wait.  It's only a few extra bucks.

Something you may consider doing while having your 746Pro on the operating table is to install the CR-282 High Stability Oscillator module.  If you operate much on 2 or 6M weak signal modes, the 7ppm frequency (in)stability of the 746Pro is pretty noticeable.  You'll have the radio apart to that point anyway, and you can fix two problems at once (drift and sudden TX death).  Just a thought.

Component Installation Details
Refer to the diagrams and photos for placement of the new/replacement parts.  Photos are clickable for a high resolution view.

The 1SV252 SOT diodes are the ESD protection diodes.  If they aren't present in your radio, it would be a good idea to install them at this point.  Even if your radio already has these installed from the factory, it might not be a bad idea to replace them.  If your IC151 was blown by ESD, it's a given that one or both of the diodes are fried or weakened and should be replaced.  Even though their overall effectiveness is questionable, it doesn't hurt to put them in.   You will have to remove the RF Unit PCB to get to the backside of the board to install/replace the diodes.  It will help to remove the small shield that covers one of them.  Details on how to remove the RF Unit PCB can be found on page 91 in the 746Pro Instruction Manual, in the section that describes how to install the high stability oscillator module.

After the diodes are in and the shield soldered back in place, trim the leads of the 1mH choke to about 1/8" and solder it on the bottom of the PC board connected between the pad for the center pin of the HRX connector (J1) and the PCB groundplane.  You will have to scrape away some solder mask on the groundplane to expose the copper.  That should shunt any errant DC and static that should come down the HRX pipe. This is the intended function of the Icom diode fix, but a choke is a lot less likely to get blown out as the diodes are.  This completes work on the bottom side of the PCB.

This diagram shows the location of
the "protection diodes" and the
soldering points for the choke. 
Here is a view of the PCB with the
choke soldered in place.  There is
plenty of room for the choke even
standing up like it is.

On the top side of the PCB, R&R C154 with the .01 cap. That takes care of the RX-TX switching transient issue.

If your IC151 is dead you will have to remove and replace it. If you order it from Icom you'll get an exact form/fit/function S08 replacement. If you order it from Digi-Key or Mouser you'll get one in a different package, as the original SO8 version was obsoleted by NEC/CEL some time ago. The two versions are functionally and electrically interchangeable.  It is possible to fit the SS08 version on the S08 pads, but will take some lead bending/extending and careful soldering.   If anyone has a source for real S08 UPG1678's other than Icom I'd love to hear about it.  The only sources I've found are obsolete parts brokers that will only sell them by the reel.

You may consider adding a heatsink at this point as described on OZ2M's webpage, though fabricating one to work with the tiny SS08 version can be a challenge.  I made a couple attempts at making a heatsink before I finally came up with one that was good both electrically and mechanically. Unfortunately it's not something most hams would probably be comfortable in dealing with. This IC is barely 1/8" square, including the pins.  It fits comfortably on the end of a pencil eraser.  I used some 18ga copper sheet from a hobby store for heatsink material.   It had to be made out of several small pieces, otherwise it worked too well as a heatsink and I couldn't get it hot enough to melt solder on it.  I made the base by filing away the areas where the SO8's input, output and Vcc pads were on the PCB. Then I bent the SSO8's input, output and Vcc leads up so they wouldn't touch the copper ground/heatsink.  Kapton tape went over the PCB where IC151 resides to insulate a hot via there. I tinned everything, fluxed it and in one step using an 800F iron I flowed the copper heatsink base to the PCB ground pads, and the SSO8 IC to the copper heatsink base all at once.  Then in a second step, I soldered two square copper fins to the tabs on the base.  Now, since the group of fixes detailed here includes switched TX VCC just like the 756Pro, which doesn't have a heatsink and doesn't suffer the sudden TX death problem, it may be overkill to heatsink the part.  It's up to you.

The last step is to install the zener which will switch power to IC151 only during transmit. Remove R157 and solder it standing up on it's pad closest to IC151. Solder the anode end of the zener to the top end of R157. Using some 30ga wire wrap wire, connect the cathode end of the zener to the end of R156 closest to IC151 (the TX switched 8V point) as shown in the PCB diagram.  Between the voltage drop of the zener diode and R157, IC151 sees about 4.7VDC during transmit, which is perfect.

This diagram shows where the various affected components are located on the top side of the PCB.  The red dot indicates the TX 8V connection point. 
This is a view of the competed mods done to my own 746Pro..


Smoke Test
That completes all the mods that are known to date.  Button it back up, put the fire to it, and if everything went right your 746Pro is once again a happy camper.  It shouldn't require any alignment or other adjustments.  Any slight differences between the old and new amplifier IC is automatically compensated for since this stage is within the ALC loop.  A while back, Allen AE0Z posted a question on the 746Pro Yahoo Group page wondering if the switched TX VCC modification contributed to any QSK CW bit truncation.  To verify this I "unmodified" mine, tested the CW keyup delay, then tested it again with the switched TX voltage in place. No measureable difference was noted between the two (about 12mS).  Now, rather than push your luck again, this would be a good time to install some quality DC shunt type arrestors on the feedlines coming into the shack, such as the ones available from Array Solutions.

It mystifies me why Icom won't issue a service bulletin regarding this defect. The official company position is that it's not a common problem, but if you have a problem with your rig under warranty it will be repaired at no charge. While this is a safe position for the company to take, it is little consolation for someone who just dropped in the neighborhood of 1.5 kilobucks on a new rig, and now has to pay UPS $30 in shipping and insurance to ship the rig to Bellevue, WA.  Plus be without it for a number of weeks until they get it repaired and returned.  There is a page at Icom's website detailing the ESD diode fix but they don't explain why units with the ESD diodes installed are still failing.  Interestingly, they state, "ICOM America service records show that the failure rate of this part has been reduced to 1/10th of the previous value."  Gee, they almost fixed it.  What I found especially amusing is their mention about the heat issue- "Our service technicians do not see any discoloration of the circuit board or the deformation of parts that are the symptoms of overheating."  It's pretty ignorant of them to associate melted components and discolored copper before suspecting an overheated component. Why they are so afraid of admitting any problem with the rig is a mystery. With such a complex piece of equipment and limited development and testing budget (and likely limited profit margin to boot), it's no surprise something like this could sneak past the development engineers. I can accept that but repeated denying there is any fault is insulting the intelligence of myself and other hams out here that do design work and have reasonable evidence that there is a design deficiency.  It says on the back of my Icom manuals, "Count on us!".   I wonder what for, certainly not for customer satisfaction.  Makes you want to buy a different brand.  Problem is, it's no greener on the other side of that fence either.

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