In standby mode , look at the current on your power supply , if it is above 3 amps , usually this is an indication this is an older unit and both 2SC3133 OR 2SC1972 transistors are shot. You can tell by the current draw that they are oscillating. If it is the newer 746 , there is one SMT FET right before the driver that goes south all the time.
Service manual is free to download all over the place.
On the older units there is an issue with the (pre) driver board it have two transistors Q1 and Q2 (2SK2975) these can become to hot/ damaged and give problems with low or non tx.
The issue is related to lack of proper heat sinking. I have pulled a few of these in the past and replaced the transistors. There is a fix on the internet somewhere in a document called SEN-015-03.
There is also an unofficial extra fix putting on a heat-sink on top of Q1 and Q2.