bluetooth speaker no charge/work with dc adapter and blown diode when charged battery attached

Hello everyone. My first post here and I'm not quite good in electronics but need some help/advice. Thanks in advance.

Approx 1 year ago I got a fender newport bluetooth speaker from ebay. It was sold as "broken/for parts" and it turned to be DOA.
I've managed to disassemble it.
It has a 7.2V LiIon pack inside which was totally discharged. Charged it separately with RC charger plugged it in (with no any external power supply attached) and the speaker turned to be alive and working!
Then I've attached the AC/DC power supply (the back cover states 15v and specific polarity - and this was a correct power supply), battery still attached. Result - blowed transistor.
I'm not exactly sure for diode specs as at this point only the first letter was seen - it was R. I've got something similar from china and today had a chance to replace it and got the same blown transistor once again.

Summary:

• item: Fender Newport BT speaker PR5001
• external DC power - 15v
• internal batt lithium 7.2v
• no charge from DC power
• no power from DC power (when battery is not attached)
• item works only if battery is charged with external charger and attached to the device
• when charged battery and DC attached = blown transistor (Q6 at the photos below)

Any advice? Can check anything with a standard multimeter.