There is an internal f ailure and the unit will require servicing. To get by until you can get it fixed, you can get line transformers that will allow the mics to be used on the higher channels.
If you have inadvertently plugged a source into the :insert" jacks of these bad channels by mistake, that may have blown the input op-amps.
A lot of dis-assembly of the board must be done to get to the parts to repair.
Another possibility is if the sliders have gone bad... SAME prognosis for repair.
Answers & Comments
There is an internal f ailure and the unit will require servicing. To get by until you can get it fixed, you can get line transformers that will allow the mics to be used on the higher channels.
If you have inadvertently plugged a source into the :insert" jacks of these bad channels by mistake, that may have blown the input op-amps.
A lot of dis-assembly of the board must be done to get to the parts to repair.
Another possibility is if the sliders have gone bad... SAME prognosis for repair.