The converters of that vintage were rather poorly filtered. They depended on the battery to filter and regulate the output power. A decent battery is the first step.
Several reasons it could fail: transformer shorting, rectifier failing, capacitors getting weak.
A replacement of both converter and battery is probably the shortest and best path.
Nearly 40 years is pretty good service.
Jeff & Cindy
'09 27FB Flying Cloud
'91 350 LE MH