A significant number of hams have reported failures of IC-746Pro Transmit functions within a few hours to a few months of otherwise satisfactory operation. The remaining receive and menu functions are normal, but TX is dead on all bands, HF, 6M and 2M. Several hams(see eham.net product reviews) report multiple TX failures, with the rigs going back to Icom in Washington state more than once.
Indications are that:
IC151 on the RF Unit; and
D24 and Q25 on the CTRL unit are all bad.
These three components are all tied to the ANT jack(s) via the HRX line, between the RF Unit and the CTRL Unit. The problem is ESD (Electro-Static Discharge) related, and results from a discharge to the ANT connector that will damage these components even when the radio is powered OFF. The source of the ESD could be applied when you change antennas, and might take a step with the cable in hand to connect to a bandswitch or the amplifier, etc...or picked up on the antenna as an environmental electro-static charge, common especially during the dry winter months (especially the winter of 2002-2003). NOTE: THERE IS NO ESD PROTECTION on the HRX line.
I wrote to Icom Japan regarding this problem, and it is my understanding that there has since been an "internal" memo referred to as Internal Service Note #920 to addresses this issue.
In the memo, a pair of 1SV252 PIN Attenuator Switch Diodes are placed back-to-back on the HRX line to prevent ESD from damaging the aforementioned components. The diodes are mounted on the RF Unit at the junction of R1 and J1 (HRX) to GND.
These are very small, surface mounted components, so be forewarned that unless you are trained and equipped to mount these in an ESD safe environment, with proper surface mounting equipment, eye magnification, etc (i.e., no coffee w/in 24-hrs), then you are better off sending the rig to Icom and waiting for them to install the ESD fix.
The parts are available from Icom Parts for $1.34ea, Part No. 91213606, PIN Diode 1SV252, Qty 2 required.
The diodes are packaged as a 3-terminal device, two diodes in series, with the common junction (cathode-anode) not used. One package is connected with Cathode to HRX, Anode to GND; and the other Anode to HRX, Cathode to GND.
While you're attending to this fix, see my IC-746Pro SSB Monitor Level fix, also found on this site.