Panasonic WX-ST400P Operating Instructions Manual page 24

Pairing
5
Unplug the microphone from the wireless receiver
Unplug the cables connecting the microphone to the wireless receiver.
If pairing has been completed successfully, the audio from the microphone is
transmitted.
6
Place the battery cover back on
Place the battery cover back onto the product.
The [REG] button is invalid if the channel to which microphone is paired and
if that microphone is in use. Turn the currently paired microphone power
OFF, then you can pair a different microphone to the channel that is already
in use.
When a channel on the wireless receiver is in pairing mode, all other
channels can still be used as normal.
Setting the microphone selection switch is optional.
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