I replaced Q25 and D22, but that didn't solve the problem. I also saw that my radio had the factory mod for the PIN diodes around IC151. They were found to be ok, but replaced anyway.
I found a page on the same problem on a IC-756PRO, that looked like it would solve my problem, but it needed adaptation to the 746.
http://www.hampedia.net/icom/ic-756proii-oscillation-problem.php
The concept of the "crowbar" relay to ground the RX line during transmit seemed the right thing to do. There was also plenty of space to install the relay. I chose a very small 12v relay that could mount right above the L28 inductor. I removed some of the green surface of the ground plane of the board with a diamond bit engraving tool to expose pure copper of the board. This allowed me to solder the relay's contact leads directly to the surface of the board. The other side of the NO contact went to the trace that joined L28.
The coil wires of the Crowbar relay were run external to the board over the edge and to the same points as RL4.
The installation is clean and the radio works great now! Full power restored to 6 meter band and the autotuner now works again.
Control Board from the bottom showing the Crowbar relay coil wires.
Postlogue:
The crowbar modification was working well for almost 10 months. Then something in the setup changed which stopped it from working. The Receive Antenna Relay Box modification plugged into the ACC2 port seems to draw enough current from the trigger line that it stops the crowbar relay from working. When the plug was removed from the ACC2 port, the crowbar relay started working again. Make sure the main power supply voltage is at specification as not to produce a brown-out condition on these modifications.
