IC151

Allot of opinions on this. This is what I have found after a few years of servicing these radio's.
Heatsinks may help, but I now have doubts on thier actual need. Device heat... Ok it is documented the device runs warm, but I am finding it is within device tolerances.

ESD diodes, I have had allot of repeat failure with the ESD diodes in place, either by Non-ICOM installations, or the factory, technically this should have no support for IC151, since IC151 is isolated by other diodes and/or relays, from where the diodes are installed.
Input Diode protect on IC151, so far seems to be the solution. This is an ICOM update and is a pair of diodes to pin 1 of IC151, one diode cathode to pin8, the other diode anode to ground. (I will try to get a pic insirted here). This tends to lead that the actual cause of IC151 is a RF hot chassis. This can be from RF feedback, poor or no RF grounding (use strap, not WIRE) or in the case of the Pro, PA oscillations on earlier units. ICOM changed the Pro PA, somewhere in Production.
Bias Resistor, The current resistor is 10 ohms, I have been replacing this with a 22 ohm resistor. This reduces the overall stress on IC151 by about 10%. The only side effect is a slightly lower (1 bar segment, usually) ALC meter display. This helps, but is not as effective as the input diode mod.

746/746Pro Drivers

The 746 uses the 2SC1972, which sometimes go leaky, they just have to be replaced. The symptom is about 5 to 30w out with RTTY or FM. (no power out in FM/RTTY is usually IC151)

The 746Pro, use the 2SK2975, and they are not cheap. The symptom is power output jumping around, No Power, 5W or the ALC/SWR/Power meters pegging for no good reason. There is apparently an issue with the drivers oscillating, and if it goes on long enough, driver failure could happen. ICOM does have an update for the pre-driver, Driver and control unit to stop the oscillations. I will try to get a pic of the information posted soon.

This is the old information as a reference

There has been allot of questions about the apparent Tx failures on these two radio's. IC151 (RF unit), and or drivers seem to be a weak point on "production" units. After IC151 has been replaced, repeat failures are rare. I also started putting a heatsink on the IC, pictures will be posted shortly on what I do. As for the drivers. The 746 uses the 2SC1972, which sometimes go leaky, they just have to be replaced. The symptom is about 5 to 30w out with RTTY or FM. (no power out in FM/RTTY is usually IC151)

On the 746Pro, the symptom is power output jumping around or the ALC/SWR/Power meters pegging for no good reason. There is apparently an issue with the drivers oscillating, and if it goes on long enough, driver failure. The Pro uses the 2SK2975, and it is not cheap. I have also noticed there are some changes happening to the driver board when radio's come in for service. There is no official "mod" though, so I am not sure what is going on here. I have found weak solder connections to the drivers (mostly on early versions), which causes the irratic power output (not pegging). The drivers are "MosFet's"! be very careful resoldering them, a static pop will destroy the devices. They are also mounted on a very good sized heatsink, so a 30w iron will not do the job very well, it takes between 60w and 100w, flux highly recommended. Not recommended to try and remove and reseat, just resolder.