I had a problem like this and it turned out to be a bad ground on the
ptt.

On Jun 29, 2:34*pm, wrote:
Take the alternator off and have your local alternator shop check the
"diodes" in it. *They are not certified to repair it but they can sure
check it out for you. *Bad diodes will cause a hum that changes in
pitch with rpm. *They can also cause the alternator output to be
unstable as seen in a quivering amp meter.