The On/Off switch on my HDR120 is not making a positive contact. How do I repair it?

The first thing to do is to disconnect the Sennheiser wireless unit and try another headphone, preferably a cable type and see if both channels work. If the sound is OK on both channels, the fault is within the headphones. If you are lucky it will be the wireless unit plug or the cable and can easily be tested and repaired by somebody familiar with electronics. Otherwise you will probably need to have it repaired by a service shop or a technician. I hope the guarantee is stilll valid.

Sennheiser Wireless Headphone HDR 120 Instruction Manual
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The system is working in the 860 Mhz range, so not that common. Most of the time it is cheaper to by a complete new system then to tray to buy components. Just keep looking in the thrift stores you find a transmitter for this headphone for a good price.

assuming the battery is charged, have you tried to switch to a different frequency band, etc.?
check page 28 of the user manual for possible actions when there is no sound in the headphone.

Check the batteries in the head phones, check to see if they are switched to the samechannel that your transmitter is on. In many of the wireless headphones there are 3 or more channels that you can switch to in case your neighbor has the same unit.

Hello,
Sennheiser sent me the manual.
Thank you, Eletta

Couple of things could be going on..

Try the obvious first

1. Select a different channel on the base unit: Located on the front, under the logo is a small switch. Note the position and try the other two settings
2. Make sure your base unit is not next to other items that may interfere with it.. cordless phones and wireless network routers etc.. microwaves or other RF sources .. fluorescent lamps.. etc

Lastly.. just about all wireless headphones have some noise or background hiss.. but it should be minor.. if the rechargeable batteries are not taking a full charge or cannot retain the charge properly.. the reception will also weaken.. and that noise will increase.. and the reception distance will decrease.

You can buy a new set of rechargeable batteries .. just make sure they are exactly like those in your headphones..

If I remember right.. they are Nickel Metal Hydride… (NiMH).. Radio Shack will have them.. or a store like Batteries Plus.. If you replace them.. go for a slightly beefier battery.. a higher mA-h.. may take a bit longer to charge initially.. (If you try this.. remember.. they will need to charge overnight before use) but will “play” longer in use.

It has been noted that many of these headphones shipped with inferior batteries.. that problems seems to have been addressed by the mfg.

Good Luck!

Did you try to remove the batteries and at the same time unplug the power of the base for 30 sec ?

They need a minimum level of signal to switch on the transmitter.
Verify if it works on yet another system.
Make sure you have the right output on your Samsung.
Try to increase the output level of your Samsung...(up the volume, but it may be a fixed output..)