DAB radio restarting / speaker issue?

I have a DAB radio (Bush Horizon) that I acquired second hand. When I plug in the power supply, the LED screen lights up and goes into standby and shows the date/time correctly. When I then press the standby button in order to turn the radio on, the speaker crackles momentarily and then the LED goes black and then the unit starts back up again and goes back to standby mode. However IF I have headphones plugged in, the radio turns on without any problem. I thought it was a speaker problem BUT if I unplug the headphones while the radio is on, the sound then comes out of the speaker with no problem. Any idea what I should be looking for to rectify.