Power supply distortion?

[CTorp]

Are they powered from the same wall wart?

FREQUENCY CENTRAL Microbus (Power Supply) [BOM]

I figured it out now! The boards were working on their own, but the problem was that when I connected the two together, I mixed up the cables! I thought with AC it is impossible to do a reverse polarity error, but this is only true if you have only a single board So I did not connect the two ground planes of my boards together, but connected the ground of one to the . I am still wondering a little why I did not get a short immediately, but could measure the correct voltages of +12 and -1…