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Old 07-16-2021, 09:36 AM   #1
jimanez11
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Default Sennheiser Momentum M2 AEBT Headphones

Have an issue with my momentum wireless headphones. Bought them on ebay to try out repairing them - have done component level repairs on laptops and other small devices, so figured I would give it a shot - but I know very little about audio circuits and troubleshooting - so please forgive my lack of basic terminology knowledge and any help is appreciated.

An overview of the symptoms is listed below with a more detailed outline after that:
- Immediately on power on - slight static in right side of headphones
- Static only lasts for 45 seconds to a minute
- After static, complete loss of audio output from right side - left side works perfectly
- On power on - visible pressure change inside of right-side speaker (i.e. the clear plastic bit that houses the cone expands slightly and there is a small pop)
- Occasionally there is a fairly loud series of bass pulse / vibrations. I can tap the side of the can and they go away prior to having complete audio loss on right side.
- Issue is only experienced when power is applied to headset. If used passively with 3.5mm jack and no power to the device (no BT or ANC), then both sides work perfectly.

I am thinking it is a problem with the ANC circuit and how the output from there is relayed to the actual speakers, but that is a completely novice opinion. I have taken the board out and have it under my microscope (there are solder balls everywhere on this thing) but I am kind of at a loss as to where to begin. I dont have a schematic, but am happy to run through any tests or any pointers anyone may have about where to begin troubleshooting.

The pads on the board are labeled as such:
- From cable connecting to left side of headphones:
- FBR
- MG
- SLP
- SLN
- FBL
- FFL
- TKL
- MG
- NFC (Near Field Communication?)
- NTC
- BN (Battery Neutral)
- BP (Battery Positive)

- From Main Board to Right Side Controls and Speaker Output:
- MG
- FFR
- SRP
- SRN
- LIR
- LIRG
- LIL
- LILG
- PD

Based on an extremely limited amount of knowledge, it sounds like there may be some kind of power bleed into the actual ANC output line that is causing an issue? I am not sure at all, but the static noise almost sounds like a capacitor either charging or discharging slowly, then fading out completely. I will check for shorts in the lines, but any pointers or places to start with the investigation for this is super helpful.
Attached Images
File Type: jpg SHM2 Top Board View.jpg (1.06 MB, 10 views)
File Type: jpg SHM2 Bottom Board View.jpg (1.36 MB, 7 views)
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Old 07-20-2021, 09:05 AM   #2
tibimakai
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Default Re: Sennheiser Momentum M2 AEBT Headphones

I don't know, if anybody has experience with this type of device, but I would start checking the capacitors for short.
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Old 07-21-2021, 10:34 AM   #3
jimanez11
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Default Re: Sennheiser Momentum M2 AEBT Headphones

Thanks tibimakai Have poked around a bit and there doesnt appear to be any shorts. Have also found the data sheets for the ANC chips and for the bluetooth chip (both of which appear to be working, but am thinking that there is some issue with the output signal from one or the other). There is an IC that is making a high-pitched buzz - I think is either a buck converter or a voltage regulator, but cant find any information from searching the IC markings, so am not sure. The IC markings are “S48” on top and “W8o” on the bottom. Think the “o” just indicates pin 1, but am not sure so any help identifying this IC would be great.

I am getting the following measurements from this IC and cant tell if they are correct:

Powered Off:
- Pin 1: 4.760v
- Pin 2: 4.254v (but jumps between ~4.1v and 4.27v
- Pin 3: 5.234v

Powered On:
- Pin 1: 4.676v
- Pin 2: 4.390v
- Pin 3: 5.220v

Pin 1 is making a squealing / high pitched buzzing noise when the device is powered on. It seems like this pin is going to the Bluetooth chip on the opposite side of the board to provide power, but think that chip should only be receiving 3.3v.

When the board is not powered on, there is a consistent clicking on the board (which I cant locate).
Attached Images
File Type: jpg SqueallyIC Voltages.jpg (231.8 KB, 7 views)

Last edited by jimanez11; 07-21-2021 at 11:59 AM..
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Old 07-22-2021, 09:26 AM   #4
tibimakai
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Default Re: Sennheiser Momentum M2 AEBT Headphones

Maybe it is a voltage regulator, but if it is does not look good with those voltages.
I would trace down the tracks, to see, which pin feeds the two channels and try looking up those ICs power requirements.
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