The TX-2B Receiver Circuit
The encoded 1kHz data signal is fed into Pin3 then amplified and decoded. Once the corresponding function has been determined from the decoded signal, the appropriate output pin is enabled for either LEFT, RIGHT, FOWARD and REVERSE.
Two standard H-bridge circuits are used for the propulsion motor and the steering motor. The H-bridge allows the direction of the motor to be controlled by switching the direction of current flow through the bridge and motor. The turbo function increases the current through the propulsion motor (via Q3) but only when the forward direction is selected.
The receiver is the circuit built around Q1. This type of receiver is known as a regenerative receiver because it uses positive feedback. A tuned LC circuit consisting of L2 and C3 provides positive feedback but only at the tuned frequency (27 MHz). So this means that only the intended signal gets amplified by the positive feedback. One downside of this type of receiver is that L2 must be custom coiled in order to accurately tune the receiver.
The output signal from the receiver circuit goes to pin 14 on the RX-2B. This signal is now filtered to remove the carrier and ran through two internal inverters. By the time the signal makes it to the RX-2B signal input pin (pin 3) it has been filtered and cleaned up enough that only the 1 kHz data signal remains. As with the TX-2B, a 3V zener regulator is used to power the RX-2B. Rosc again sets the internal oscillator frequency to 128 kHz.
Thanks to Predicable Designs for the explanation.