[G6Tanuki]

I'd suggest:

Tune upwards in decades from a known frequency, and see how the "supposed" tuning follows the actual.

Map this out into the bit-patterns that are being fed to the synth.

0000
0001
0010
0011
0100
0101
0111

and such.

Part of me thinks that you'll find one of the lines is 'stuck' high or low.