Hi,
I can think of 3 possibilities here:
(1) The cable you are using isn't screwed/attached securely enough to the TV;
(2) The coaxial tuner input needs to be resoldered or replaced;
(3) You need a new tuner.
You are wise to avoid opening the TV, because the voltage warnings you have heard are all too true; however, try option number (1) first. If you still have to take it to a shop, then be advised that tuners for some models of TVs can be rather pricey (about $60-$100+), depending on the model (I hope this isn't the case with yours). If the shop determines that you only need a coaxial input then you may be in luck- they don't cost that much at all.
I hope this helps.
Jim Lee- technician, May 2004