On/off button fell off Sennheiser wireless mic

Search on internet for owners manual. There is usually a small set screw within mic to help with the gain/sensitivity.

This will never work together. They are on different frequencies

20mv

Contact your local service center directly or online here:
https://en-us.sennheiser.com/service-support-contact-center

This isn't a solution. But after I've gotten into my microphone with the same issue using a torx screwdriver I found that I (Electrical Engineer) don't have the proper equipment to fix the issue. Roughly 100 dollars (Flat rate) and a send off to Sennheiser is your best bet.

To Pair the Sennheiser VMX 100 headset with your cell phone:
At the same time as you open the mic, you also need to push down on the bottom/volume-down button on the side of the earpiece and hold this down (at the same time as opening the mic) until the light flashes red and blue alternating. (Just to clarify, it is the button that has an arrow pointing toward the Sennheiser logo). If asked for a code, enter "0000" (four zeros).
This video shows the bluetooth earpiece connecting to an iPhone 4S running iOS 5.0 (not jailbroken). This is the standard way to pair these devices (and it should work for Android phones as well), but as a disclaimer:
If anything should happen to your phone or your Bluetooth earpiece, it is not my fault.

Audio Technica and Sennheiser each have proprietary connectors for their mics. A lot of times the mics require Phantom power which is drawn from the battery of the pack.This is important as the mic will need to be wired correctly to function.
The first thing you will need is the correct plug from the company. Check with Sennheiser customer Support for the correct part. If you aren't extremely good at soldering, do not attempt this. You can change the plug but the wires are extremely delicate and can short easily. Moreover because of the phantom power, the plug must be wired correctly for the mic to function, or you may end up damaging the mic by sending phantom power up the wrong line. I would advise having a music store or pro audio service that handles these products do the wiring for you. If you feel you are up to it, you can do it yourself, but be careful.

To Pair the Sennheiser VMX 100 headset with your iPhone or other cell phone:
At the same time as you open the mic, you also need to push down on the bottom/volume-down button on the side of the earpiece and hold this down (at the same time as opening the mic) until the light flashes red and blue alternating. (Just to clarify, it is the button that has an arrow pointing toward the Sennheiser logo). If asked for a code, enter "0000" (four zeros).
This video shows the bluetooth earpiece connecting to an iPhone 4S running iOS 5.0 (not jailbroken). This is the standard way to pair these devices, but as a disclaimer:
If anything should happen to your phone or your Bluetooth earpiece, it is not my fault.

This is probably interference in the wireless connection for the lavalier mic.
Try switching channels to find a clear one!
Or switch to a wired lav mic if that's practical.

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You should'nt need a scematic, this happens all the time with the ew100 handhelds. you will need to remove the three star bit screws between the screen and the batteries, pull off the end cover and with a small screwdriver or some long needle nose pliers- put the red button back into the hole from the inside of the cover. Just take care when putting the end cover back on- sometimes its a bit difficult.
Goodluck
PTB.