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?
Regarding your JCM 900 SLX it sounds as though the effects loop jack socket may be faulty, we would recommend that you have it looked at by a qualified engineer.