Denon MJ6000 MKI audio problems

Hi I Owned an Denon MJ6000 MKI and I´m having audio problems using it as an standalone mixer.

First problem begun with channel 4 when source selector stop working. No matter with position you put it (aux2, line1, line2 or midi) you can hear what is connected in line 2 (Turntable). On a strange way. At first you can not hear anything, unless you turn up level of channel 4 slowly, and when you reach almost 3/4 of level, you start hearing what´s connected in line 2. And if there´s nothing playing in Line 2 an something playing in line 1 (another turntable) you do the same in CH4 level, and when you reach 3/4 of level you begin to hear what´s playing in line 1. Annoying… Second Problem: Master output stop working. You can not hear anything unless you do the same I´ve explained before, you turn up Master level slowly and when you reach almost 3/4 of level you start hearing what´s been playing. Annoying 2… Third Problem: As Master output was not working well, I begun to use BOOTH/SEND output, but in a short time, Assign selector for BOOTH/SEND (CH1, CH2, CH3, CH4, MAS.) stop working well too, and no matter in which position it is, you hear what´s sended to Master. But you can not hear anything unless you do the same I´ve explained twice before, you turn up BOOTH/SEND Master level slowly and when you reach almost 3/4 of level, you start hearing what´s been playing. Annoying 3… In the three cases with level, sometimes you have to do it several times to make it work and hear something. Fort Problem: CH2 stop working completely. No matter in which position you put the input selector of the channel (LINE 1 - USB), you can not hear anything. And the thing of turning up slowly level reaching 3/4 doesn’t´t work in this case. All problems didn’t´t appeared at the same time, they where one at a time. Last one a week ago… I´ve updated firmware long ago and was the last one I found from the official website. Rear switch (INT MIX - PC MIX) is set on Internal Mix. Don´t know if the problems have to do with hardware or what? Anyone happening the same? How I solve it? I´ll apreciate any help… :- )