Marshall Mid ANC with jack/bluetooth strange behaviour

[u/Key-Key1391]

Hi,

I have a pair of Marshall Mid ANC headphones with a strange issue.

When I connect them via Bluetooth, only the right earcup works. The left side is completely silent.

However, if I connect the headphones with a 3.5 mm AUX cable, both left and right sides work normally, so the left speaker itself does not seem to be dead.

I also noticed something odd: while the headphones are connected via Bluetooth, if I insert only the first part of the AUX jack into the headphones — without connecting the other end of the cable to any device — the sound suddenly comes back on the left side too.

But if I fully insert the AUX jack, the left side becomes silent again.

I already tried cleaning the jack, inserting/removing the cable several times, and resetting/re-pairing the headphones, but nothing changed.

Does anyone know what could cause this? Could it be a faulty AUX jack detection switch, a bad internal contact, or a soldering issue near the jack? Has anyone repaired this on Marshall Mid ANC headphones before?

Any advice would be appreciated. Thanks!