Try plugging the RUN jack to the output of a VCA that is turned all the way down. This should give a ground level equal to the ground level of the modular case. Then while the VCA is still turned down, you can plug in your device. Turn up the volume (half way?) and then play the sound.
What the 3N light really means is that a signal was received that is not a correctly encoded signal. Typically it’s caused by different ground potential, and depending on the Just Friends bootloader version, it can be pretty touchy about subtly different ground potentials.