Partybox 710 runs fine but no sound

I ve owned the partybox 710 for 2 years and start to experience an issue. At startup, everything is working fine (blutooth, lights beating to the music, coms with the jbl app ...) but no sound at all. Instead of the check startup sound I get a faint cranky noise from the tweeters. Restoring to factory settings did nothing. It does appear, however, I can get the partybox back to normal if I empty the condensators by unplugging the AC cord for 5 min and pressing the start button while it’s unplugged. Has anybody experienced that issue and how was it solved ?