Yamaha YH-E700B Wireless Noise-Cancelling Over-Ear Bluetooth Headphones (Black) - Manual (Page 40)

Sound can only be heard from one headphone
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The product is temporarily not functioning in a stable way.
Turn the power off and then on again.
If this does not solve the problem, initialize (factory reset) the product.
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“Initializing (performing a factory reset)” (p.43)
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The signal may not be coming in clearly, making the connection unstable.
Move away from wireless devices such as access points, or appliances that produce electromagnetic
waves such as microwave ovens. Turn the power off and then on again.
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You may be using a smartphone app that generates sound from only one channel.
Check whether you can hear sound coming from both left and right channels by playing back sound from
a different app.
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The connection information for the registered Bluetooth device may not be working properly.
Delete the Bluetooth device’s connection information, and then pair this product with the device.
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“Pairing with a Different Device” (p.19)
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The cable might not have been correctly connected (when using the 3.5 mm stereo mini-plug
cable).
Connect the 3.5 mm stereo mini-plug cable jack of this product correctly to your external device.
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Troubleshooting > Sound can only be heard from one headphone