The second half controls the steering. The mechanical design is a 3 wheeled caddy with the ´single wheel´ actually a closely spaced pair of wheels which are driven by the main drive motor top provide motive power (this is the motor controlled by the first circuit). However the pair are on a motorised swivel arrangement: this second motor is the steering and can rotate the pair through 180 degrees, thus providing not only the steering but also the direction control.
Tr3 and Tr4 are a current source feeding a 9v1 zener to provide the internal supply. Tr5 with the 6v2 zener supply a 5.6 volt supply to the radio control receiver. This receiver gives two outputs - one is used for motor speed control and the other is used for the steering.
Tr13 and 14 are a level shifter and pulse conditioner to shape the radio receiver´s output pulse. The shaped pulse is fed to the pulse length detector consisting of the first 555 with Tr15 and Tr16. The 555 (which provides a reference pulse) is triggered by Tr6 on the positive edge of the input pulse. Tr15´s collector is high only when the input pulse is present and the 555 if off, so it gives a pulse output only if the input pulse length is longer than the reference pulse. Similarly Tr16´s collector is high only when the refernce pulse is present but there is nothing on the input pulse. This pulse indicates reference is longer than the input.
Tr10 and Tr11 are a ´pump and hold´ circuit. Tr11 is operating at 90 microamps (9v, 100K) maximum so its base current is around 1/3 microamp (assuming a gain around 250). This base current is supplied via the 3M3 resistor (which is actually feeding 3 microamps - more than enough).
So under worst case conditions Tr10 might have an emitter current of 3 microamps. Its base current is around 10 nano-amps. Assuming Tr9 and Tr18 are not conducting, this base current can only come from leakage - or the 470n...