Symptoms and tests I have done:

• When I press the power button on battery, the LED turns on for about 5 seconds and then turns off. There is no startup sound.
• The speaker does not connect to Bluetooth.
• The physical buttons do not respond.
• When connected to my PC through USB-C, the PC recognizes the speaker and I can play audio through it.
• I opened the speaker and checked the connections.
• The battery measures approximately 4.28 V.
• I also measured the battery voltage while trying to turn the speaker on. It stays around 4.28 V and does not noticeably drop.
• The battery voltage also reaches the motherboard connector.

Could this be a motherboard/power-management problem, or could the button-control circuit or another component cause these symptoms?

Is there anything else I can safely test with a multimeter before replacing the motherboard?

Also, is there any USB recovery/service mode that can be used to reset or reinstall the firmware when Bluetooth and the physical buttons are not working?

If the motherboard needs replacement, how can I identify the correct motherboard/revision for my JBL Go 4?