How i can fix my goodmans ghc 50 subwoofer?

for last few days my sound system giving an unusual sound, when i switch it on the sound start. the sound is like 'booooooo' separated all the speakers from the sub, but the sound didn't stop, then i open it and from the mainboard i unplug the woofer connection but then i found all the speakers including the woofer are working well but the problem is in main board. what can i do now?