The problem is that the IC-746 contains a fan that turns on as soon as you go to transmit, and then turns off when switching to receive. This fan is situated such that it blows cool air across much of the radio, including the 60MHz fixed local oscillator.
One solution is to shell out another $15Ø.ØØ or so for Icom's ovenised crystal oscillator option (OCXO). We have found, however, that the apparent medium-term drift problem can be all but elliminated in a much less costly manner.
Described below is a simple modification that can be performed on the stock radio, requiring nothing more elaborate (or expensive) than a phillips head screwdriver and a small handful of cotton-wool balls.
With the radio upside down and facing towards you, remove the 5" square sheet-metal shield that covers the left-rear quarter of the radio.
Take three or four cotton wool balls and carefully pack them into the rectangular screened 60MHz local oscillator section. Take care not to damage the small coil (L1901) that is in the box. Here is a jpeg image showing the LO section partially stuffed with cotton wool. Ane here's another showing the area filled with cotton wool .
Now replace the sheet-metal shield and put the outer cover back on the radio. You'll notice in this image that the LO section is only partially covered by the sheet-metal shield .
That's it! The mod took me about fifteen minutes from start to finish, and as someone else just said in an email to me, "that's the cheapest mod I've done in my thirty years as a ham!".
Any modification that you perform (including this one) are made at your own risk. If you damage your equipment you cannot blame anyone other than yourself.