Nicely done! I have re-soldered and repaired amplifiers with similar results because I was too cheap to spring for a new board. Intermittent problems are often the most difficult to diagnose but the easiest to repair. Nearly always a loose wire/loose connection.

It's almost certainly multi-cored solder if you bought it from an electronics store, 'coz only plumbers/metalworkers use "real" solder with separate flux - and there would just no point for electronics work, unless you were soldering honking great heat sinks...

Does give a little puff of smoke when you tin the iron with it? If "yes", there you go; that's flux burning off; solder doesn't smoke when it melts :)

Solder on that board looks a little light/scanty - but pretty good compared to most of the 80's bosch electronics I've seen - amazed most of that stuff ever worked, much less worked for 25 years before it stopped working and needed to be re-flowed...