Check a couple of these before you disassemble-- go to the Mighty ETX site and download the ETX tuneup article-

http://www.weasner.com/etx/techtips/etxtu.htm

http://www.weasner.com/etx/techtips/etx_tuneup4.html

http://www.weasner.com/etx/techtips.html

Look around inside the base for loose wires --- or any other obvious damage-- if nothing -- these articles may lead you to the issue.