Neutrik A1 Test System

I've encountered a problem with my Neutrik A1.
The output of the amplifier is working only on the (2) +pin of the XLR and the (3) -pin is presenting a very low voltage square-ish wave.
As a consequence of this, all the measurements are 6dB lower than expected.
I looked everywhere but couldn't find a service manual or schematics.
Is anyone able to provide any help?
Thanks in advance!!