Sennheiser GSP 370 USB-C conversion/repair

My brother asked me to fix his Sennheiser GSP 370 that was no longer taking charge with the Micro-USB port. The battery was certainly not dead as it was working very well for hours (until flat).

This mod requires you to drill in your headset to get the cables out the casing. Be aware of the thin wires to your speaker, don't damage it when working on it.

Required hardware:

• HW-373 TP4056 Type-C Mini Battery Charger Board, or similar. https://www.aliexpress.com/i/1005006162326287.html
• Some small cables.
• Small screws (M1.4x5mm, M1.7x5mm), I'm actually not sure what I used here).
  • I used screws from this kit: https://www.amazon.se/gp/product/B08TWVYH8W
• Transparent filament for the charging indicator.
• Double-sided tape to re-mount battery.

The battery found inside my headset can be seen below.

The white wire is (as far as I know) for temperature, and will no longer be used.

You will need to cut the cables to your battery and re-use the connector. Don't cut all wires at the same time as it will short your battery! Cut the wires one by one!

Battery+ (red) to B+ on USB board.
Battery- (black) to B- on the USB board.

OUT+ from USB board to re-used battery connector, red.
OUT- from USB board to re-used battery connector, black (ignore the white in the picture above).

Note! Don't solder anything to the board before you route the cables through your headset casing! I had to resolder what you see above…

Place your cuved printed part where you want it on your headset backside. Make sure you can still install your ear cushions. Pre-drill small holes (not to big!) for the small screws to thread into your headset. I used thin double-sided tape to hold the mount in place while marking the drill holes with the drill bit by hand, then removed the part and drilled. I think I used a 0.8mm drill bit, not sure. 

Mount the USB board holder to the curved printed part on your headset. 

Solder your cables to the board accordingly, the cables needs to be pretty short so it's a bit fiddly…

All soldered and mounted. Connect the battery connector again and test the headset before reassembly.  Add the LED diffusor to the cover (press fit) and mount the cover with screws. Mount your battery inside the headset as it should be, with new double-sided tape. 

Done! Still working, 3 months after fix :)

Print Settings

0.15mm layer height in my case, PLA.
Orient the parts properly before printing (see picture).
Print the LED diffusor in transparent filament, only perimeters.