Dmitri, in my case the problem appears ONLY when the trimpot VR1 is turned to decrease the signal from box piezo, if i turn the vr1 pot at the minium resistance ( near the + ) the problem disappear, by the way a strong signal from piezo can may cause a lot problems regarding the dynamics of cymbals and crosstalk with other channels, so for megadrum is appropriate to attenuate signals strong.
I believe it's a problem of management/recognizing from Megadrum firmware, my kit is the 3.2v by synthex and i don't have shortcuts/bad soldering etc...
For last do you try with pcy135 ? The 135 and 150 have two different schematics for the edge switch, the 150s only one switch, the pcy135 have 2 edge switch , one for edge triggering and another for choke.
Der_wolff do you have the same problem?