not much of a handyman... 
I changed out the thermostat from analog to digital, followed the instruction and color code, 4 wires on the old one, to the same 4 pins on the new ones. Y W G R.
Now my HVAC will turn on, heat the room, cool down,turn off. BUT, there is a weak cool air coming in that won't shut off. AT FIRST I took the new stat off and the whole thing shut off. But within 2-3 cycle of testing, the same thing happened and now the only way to shut the fan off is to turn off the breaker. When I turn the power comes back on, the fan starts blowing again.
My first question is can I have short something major (eg burn the control board) by switching thermostats without first turning the power off at the breaker?
I have a 30+ yr. old Rudd combo HVAC that the ID plate had worn off, any suggestion on how to identify this model is appreciated. (ETA: don't have the privilege to post pic yet)

I changed out the thermostat from analog to digital, followed the instruction and color code, 4 wires on the old one, to the same 4 pins on the new ones. Y W G R.
Now my HVAC will turn on, heat the room, cool down,turn off. BUT, there is a weak cool air coming in that won't shut off. AT FIRST I took the new stat off and the whole thing shut off. But within 2-3 cycle of testing, the same thing happened and now the only way to shut the fan off is to turn off the breaker. When I turn the power comes back on, the fan starts blowing again.
My first question is can I have short something major (eg burn the control board) by switching thermostats without first turning the power off at the breaker?
I have a 30+ yr. old Rudd combo HVAC that the ID plate had worn off, any suggestion on how to identify this model is appreciated. (ETA: don't have the privilege to post pic yet)