Wont cool below 50 degrees

Question

had no problems until i adjusted the temperature to ferment a lager beer at 45 degrees. adjusted to near 50 degrees for a few weeks and now the unit will not cool less than 50. the coils were really dirty so i cleaned them and still no change. if i turn the thermostat to off the compressor turns off. the fan works fine. its an older unit, the date on the compressor is 1993. i dont usually mess with the thermostat and i wonder if this is the problem or if i need to have the freon recharged. any help would be great because there's beer at stake here. many thanks.