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.