Swart Space Tone Atomic Jr Reverb/"Boost" problem

[Steve Fundy]

Hello All,
I have been brought a Swart Space Tone Atomic Jr that has a problem. When the "boost" (FB) switch is on, and the reverb return is turned past 4 the volume drops, a high pitch oscillation occurs and the tone gets rather ragged. I have zero experience with these amps. I have noticed that if the FB switch is off and reverb return is turned up beyond 7, it starts to crackle. Also when the tank is disconnected, the original mentioned problem vanishes. So I am thinking the problem exists in the reverb return circuit. Has anyone experienced anything similar? Any insight appreciated!
Thanks, Steve