It sounds like you got a loose filter cap or something. I’d take it back to the store and have them check for bad solder joints on the circuit board.
The instructions:
PREAMP GAIN ADJUSTMENT: The preamp gain in the Micro Synth has been set at
the factory for use with a guitar equipped with single-coil pickups. If you will be using
another instrument with higher or lower output, it may be necessary to readjust this
setting. A screwdriver or alignment tool with a tip width of no more than 1/16″ is
required. To access the preamp gain adjustment, remove the bottom plate from the
Micro Synth. Look for the small trim pot in the lower left corner of the circuit board, it
is labeled TRIM1 GAIN TRIM. Turning the trim pot clockwise will increase the preamp
gain. Turning it counter-clockwise will decrease preamp gain. Please do not touch any
other components when the bottom plate is removed or you risk damaging the circuit
board or the components.
But mine isn’t labeled.
Any help?