I bought a MG50GFX combo and it behaves abnormally, so that on 2 channels, after selecting any effect (e.g. chorus or flanger) to a preset (STORE mode) and always when returning to the saved preset, ONLY the octaver is started - so the effect circuit is unusable (you can never play another effect after returning, only the octaver - only on 2 channels). I remind you that I did not select the octaver and I do not use it. Apparently a faulty part. Any advice?