Mesa MK IV footswitch system

[mac1amps]

I've got a Mesa MK IV in my shop with a faulty footswitch network. The manual controls work fine for channel/effects switching, so do the ¼" external switching functions. I'm pretty experienced with the [analog] signal circuits, but I'm "in the dark" with the "logic" networks that operate the footswitch system. I know the cable and XLR aren't the culprit. The 5 V. regulator is doing what it's supposed to do as well. It's seems next to impossible to get ahold of Mesa for technical assistance these days (I've never had to troubleshoot one of these footswitch networks before). Can SOMEone shed some light on this subject??