This is a Electronic Extended Play Circuit Diagram. A single op amp-one of four contained in the popular LM324-is operating in a variable pulse width, free-running square wave oscillator circuit, with its timed output driving two transistors that control the on/ off cycle of the tape-drive motor. The Oscillator` s positive feedback path holds the secret to the successful operation of the variable on/ off timing signal.
The two diodes and pulse width potentiometer R8 allows the setting of the on and off time, without affecting the oscillator`s operating frequency. One diode allows only the discharge current to flow through it and the section of R8 that it`s connected to. The other diode, and its portion of R8, sets the charge time for the timing capacitor, C3. Since the recorder`s speed is controlled by the precise off/on timing of the oscillator, a simple voltage-regulator circuit (Ql, R3, and D4) is included.
Connecting the speed control to most cassette recorders is a simple matter of digging into the recorder and disconnecting either of theÂ· motor`s power leads, the ground or common side might be best, and connecting the recorder through a length of small, shielded ci!ble to the control circuit. In some recorders, a remote input jack is furnished to remotely tum on and off the recorder. Before going in and modifying a recorder with a remote jack, try connecting the circuit to the external remote input.