Sennheiser Set SET 820 Instruction Manual page 13

Adjusting the receiver channel
First turn on the transmitter (see section
"Where to place the transmitter"). Then put
on the stethoset receiver - as soon as the
receiver is put on it automatically tunes
itself to the first strong signal found
(normally the transmitter frequency). If, for
some reason, the selected channel is not the
correct one (e.g. because of interference or
another nearby system), press the T
UNE
button at the rear of the receiver until you
can hear the audio signal loud and clear. If
you decide to change the transmitter
channel, the frequency search tuning
must be re-started by pressing the T
UNE
button on the receiver again (see section
"Changing the transmission channel").
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