CESSNA PLT FLIES INTO RAIN; TURB AND TSTM ON VFR FLT. EMER DECLARED AND FLT SVC GIVES ASSIST TO SUCCESSFUL OUTCOME.

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I WAS SE OF RHINELANDER; WI AT 7500 FT VFR AND WAS AWARE OF A LINE OF WX RPTED FROM MASON CITY; IA TO MARQUETTE; MI. APPROX 70 MI N OF GREEN BAY; WI; I CALLED FSS ON 122.55 AND REQUESTED WX UPDATE. I WAS ADVISED THAT VFR CONDITIONS SHOULD PREVAIL ON INTO GREEN BAY ARPT. APPROX 60 MI N OF GREEN BAY I ENCOUNTERED LIGHT RAIN WHICH TURNED INTO STRONG TURB. I CALLED FSS AND RPTED DISORIENTATION AND DIFFICULTY IN HOLDING COURSE AND ALT. FSS WAS EXTREMELY HELPFUL AND REQUESTED I SQUAWK 7700. THEN MY RADIO WENT OUT AND MY VOR; WHICH WAS CAUSED BY MOISTURE COMING IN FROM WING VENTS. I HAD GREAT DIFFICULTY REDUCING ALT DUE TO RAPIDLY RISING AIR. GRADUALLY I WAS ABLE TO GET DOWN TO 5000 FT WHICH PLACED ME BACK UNDER THE WX AND INTO VFR CONDITIONS AND OUT OF THE DEVELOPING TSTM. PORTABLE GPS WAS ONLY NAVAID WHICH INDICATED CLOSEST ARPT TO BE MARINETTE/MENOMINEE MI WHICH WAS APPROX 15 MI. I LANDED WITHOUT RADIO ASSISTANCE. I PHONED GREEN BAY FSS AT ONCE AND RPTED CONCLUSION OF EMER FLT. GREEN BAY FSS WAS COURTEOUS; HELPFUL AND MADE A MAJOR EFFORT TO DETERMINE OUR CONDITION AND SAFETY. THEY SHOULD BE PRAISED FOR THEIR EFFORTS. RADIO HAS BEEN REPAIRED -- ALSO I HAVE ENROLLED IN A PROGRAM FOR ADDITIONAL INST TRAINING.