Beep circuit description.

Circuit sections.
The circuit can be divided into three sections:
  1. Detect and switching - detect when transmit ends, switch oscillator on, and control transmit until beep ends.
  2. Oscillator - creates beep sound.
  3. TX control - takes over transmit switching from the microphone.

Detect and switching circuit.
While the microphone PTT is pressed, TR1 emitter-base junction conducts via R2 and D1.
This causes the emitter-collector junction to conduct heavily, switching on the TX control (TR2) and supplying power to the beep circuit. At this time, C1 becomes charged via D1. The connection from D1 cathode to TR3 base switches off the oscillator while the microphone is keyed, allowing just the transmit control to work.

When the microphone is released, C1 discharges via the combination of TR1, R1, and R2.
This keeps TR1 switched on for a short time, allowing the radio to continue transmitting. At this time, TR3 base is no longer grounded, so the oscillator can now operate.
After the delay period (set by R1, R2, and C1), TR1 switches off, returning the radio to RX and disconnecting the supply from the oscillator circuit.

Oscillator.
TR3, together with the three resistors 'R' and the three capacitors 'C' form a phase shift oscillator.
The frequency of the oscillator is set at about 430Hz by the values of these resistors and capacitors.
The oscillator circuit only comes into operation when the supply is present and the base is not grounded, which is only for a short period just after the microphone is released.
The output level is set by the combination of R6, R7, and RV1 - with RV1 allowing some adjustment to suit the radio and the supply voltage used.

TX control.
The transmit control circuit must hold transmit while talking, and for a short period after you let go of the PTT - to allow for the beep. Another issue, is that the oscillator must be disabled while you speak.

As long as TR1 is conducting, current is passed through the TR2 base-emitter junction, causing the collector-emitter junction to conduct and thereby grounding the "Radio TX" output pin.
To disable the beep while you speak, the base of TR3 is grounded while the microphone is keyed.