I assume you can force the dimmers on by using the test switches. Have you "exercised" the signal format slide
switch by moving it back and forth a few times? If the
MUX indicator does not light up when you apply a
DMX signal to the 5-pin male
XLR, the next thing to try might be replacing the
DMX receiver IC, which should be near the input
jack. Probably a SN75176 8-pin IC or similar. If you don't get any other suggestions, go to:
http://ediserviceandrepair.info/