Freedom 80 gas furnace,thermostat blown

I have a gas furnace freedom 80 ,heating only with two wire thermostat with mercury switch, Anyway a couple of days ago I increased the themostat setting to put the furnace on,it had been off for a few days before that as the weather was quite warm, as soon as I moved the thermostat up it began smoking and a thin wire inside ,kind of like a preset was burnt and a 5 amp fuse blew on the furnace.
model no. AUD100C936H4
Any idea why this happened .
The furnace has a 'White -Rogers'integrated furnace control circuit board model 50A50-473