I had a TR-751A for years and this is about the way they work. I suspect "nothing" at all is wrong with yours.
It does not have a mike gain control (!) and it also does not include any speech processing, which we are used to having on HF rigs. My thought is if you change the microphone it will still behave the same way.
It was "way more than enough" to drive my homebrew P-P 4CX250Bs in a kilowatt amplifier to full power, so I had to keep it on the lower power setting for that.