I am servicing an old NBX 100 system and trying to get the MOH to work. I found a setting that was like "disable MOH input" that was set to yes and I changed to no, and have tried both a mono and stereo plug for the input and even tried a little pre-amp to boost the volume of the input but I get nothing. I'm thinking now that the board that the MOH input is on maybe has failed and needs to be replaced?

Anybody have any thoughts on this?