Altec Lansing 1650 equalizer problem

I find old Altec Lansing 1650 Equalizer. There have been several changes over time.
The equalizer works but sometimes totally random made a very loud "boom", exactly like a shotgun fire.
I can not use in this way, the boom effectively scares the audience.
What to do ? Where can I find a schematic ? Have anyone else the same problem?

Here's the 1650 Schematic .

From; GPA's Library

Electronics of that vintage should have all the power supply capacitors replaced.

:)

Thank you very much.