Lab Series L3 - popping sound

[vac4873]

I just came by a used Lab Series L3 amp. It's the first transistor amp I ever liked the sound of, and is amazingly light for its size, and potentially a very good backup.

The problem is, that every now and then, a loud "pop" sounds from the amp. This seems to increase in frequency the longer the amp is on. Could this be one of the caps in the power amp section, or does anyone have any experience with this problem?

Thanks in advance for any help/advice you can give.

I found a good schematic that was link to an earlier post on this forum.  There are two big 2200 mF electrolytics in the power amp section.  Could one of these be the culprit?  I see no physical damage.

[Enzo]

Isolate the problem.

Turn on the amp and set ALL the controls to zero.  Let it sit there running.  Does it still pop?  If so, the power amp is doing it.  Could be a filter, but I don;t usually look to filters for popping.

If no pop, then turn up the master volume only.  Does it start popping now?

What we want to do is find ANY controls that have ANY effect on the pop sound.

For example, maybe we cannot stop the pop, but we do find that the tone controls affect the sound of the pop.   That would mean that the pop was entering the system before those controls.