Sounds like another instance of the infamous Wima et al brand paper cap situation, discussed here often. HP used them a lot in that era. They look like rectangular amber clear epoxy bodies - when they're not all burned up. Replace them whenever encountered, with modern "X" caps, which means they are safety rated for across the line use, usually up to 250 VAC. Lots of vendors make them, so there are lots of choices. Just be sure to get the "X" kind - it's marked right on the part.
Ed
