The datasheet for the old BD140 shows that with a collector current of 0.5A then the base current must be at least 50mA for it to have a maximum saturation voltage loss of 0.5V.
But R4 in the circuit limits its base current to only 40mA. Then some BD140 transistors will have a max output of only about 400mA and others will be destroyed with a max current higher than 1.5A.
The BD140 is in a very old case that is difficult to cool. So it might melt if its current is too high.