Model 39A gets too hot too fast
That model has a thermostat (also called a high limit) that is set at 450F. it should produce a popping cycle of 3 mins, and yes will burn popcorn if you don't dump it fairly quickly.
The speed it heats cannot be too fast, there is only so many watts in the heat element. it may go to hot, but never to fast.
The best way to test temp is time the 2nd popping cycle with a stop watch. the second batch should be 3 mins +/- 30 seconds. if it is 4 mins it is too cold. a 2 min popping cycle is to hot.
A popper that is too cold is also popping poor quality popcorn, higher amount of unpopped kernels, and small chewy popcorn.
Measure carfully when doing time test popping. That model is 2oz oil, 6oz popcorn, and 1 tsp salt. Start timing when you put ingredients in, and stop when you dump it. The kettle must be hot, that is why you time the 2nd batch.