I got a 1993 50 watt JCM 900 SL-X.  It works great except for that fact that there is no sound when FX loop is not being used.  I currently keep a cable in the loop and it plays fine.  Some people would recommend to do that any ways to improve the tone.  However, I want my amp to be in proper working condition.  Does anybody have any advice about what could be causing this?  I am not qualified to do serious amp work, but could this be something minor enough that an unskilled person can fix it on their own?