Tascam M-216 humming/drone problem

[m?ms]

Good day!

I got a Tascam M-216 and it works quite good.

Unfourtunatly on the channels 1-9 there is a low humming noise, when the trim goes above the 11clock position and a cable is connected to the input. (other channels 10-16 work fine!)
on the tape-ins there is no humming. xlr seems to be working, too. surprisingly my kondensator microphone works with it, besides the mixer doesn´t have phantom power (???)
ah, on channel 3 there seems to be something blockading the jack-input.

my question is: does anyone have an idea, what the problem might be? why only the first 9 channels, and why only the jack-inputs

Help and ideas would be much appreciated! txxx

 

[m?ms]

supplement:

the condensator microphone works only on channels 1-9. i get the notion that somebody managed to install phantom power to the first 9 channels. that´s odd... could that be the resolution?
i got it second hand from ebay, without further information

 

[m?ms]

supplement:

........."on channel 3 there seems to be something blockading the jack-input."

ok, there is nothing inside. i just had a bad jack-cable when i tested it. crooked!