[Loffer]

Zenier ... Sounds logical. I have little exp. With stock and positive grd. Bikes . I am surprised no one with the knowledge has chimed in yet. Like said already the pop may have come from the short of points or ign switch or something of that nature . I would start to check everything for continuity with a meter . sounds like your ground is bad or something to that effect .
Don't start buying parts yet. I'm sure someone here can help guide you thru
Also classic bike.biz has online manuals and parts guides to help with parts and repairs. You'll find it if you take the time and just have patience .