Troubleshooting occasional popping from solid state amp?

Hello all,I have this problem with amp making single pops occasionally, around once every 5-20 minutes. It could be due to cold solder joint so I would like to check it myself since I have some experience with working on guitar amps, before taking it to a pro for repair. The amp is 20 years old.

Any advice on how to check this in the quickest way possible?