That's correct noisy, even if you had the correct circuit board generally you could not repair that radio. Internal unit rectification is for a workshop approved by the manufacturers, usually themselves. It could be you of course, but that is unlikely if you were also a licenced engineer.
What we on this forum, licenced engineers, the only people able to certify repairs, scheduled maintenance etc on aircraft, are are systems engineers. This sounds rather grand for much of what we do which may consist of little more than switching on and seeing if it works. True nevertheless. However even 'mere' wiring , circuit breakers and so on can be surprisingly absorbing when working to aviation standards.
If you repaired radios etc in an approved workshop that of course might be lucrative, satisfying and certainly necessary. However it would not be working on aircraft but at a bench. Most of us prefer working on the aircraft because strangely we like aircraft and keep our own bench time to a minimum.
Good luck with whatever you go with. I am sure there is money to be made in providing decent avionic services in a number of ways to the light and heavy aircraft maintenance industry.
All aircraft are negative earth by the way!