I have the same problem only its with the am side of the radio. The filter caps (capacitors) are bad in the Univolt. I have yet to see anyone post a schematic of a Univolt or I would have pulled and rebuilt mine already.
I really have never even seen who the manufacturer of the UNIVOLT was.
Roger & MaryLou
F350 CREWCAB SW LONG BED
7.3 liter Power Stroke Diesel
1977 27ft OVERLANDER
AIR # 22336 TAC- OH-7
May your roads be straight and smooth and may you always have a tailwind!