@M0YNG Perhaps check with a scope that the microcontroller of that device is actually sending and the level shifter circuit is working. Or perhaps you can do some sort of loopback tests at various locations inside the circuit to encircle the defect. and be sure there is no stupid misconfiguration :)
@M0YNG ok, perhaps high-impedance headphones will do the job as the waveform of the signal is not important but just whether there is something moving at the tx pin. rs232 should be in the audible spectrum at lower baud rates. with a bit of luck even a multimeter (AC-setting) might work
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