Bias Oscillator

Introduction:

In this Instructables I want to show you how you can make a Bias Oscillator circuit for erasing magnetic tape. Bias oscillator is used in tape recorders to erase the magnetic tape and to add ac bias to record head. The ac bias signal is added to the audio signal that you want to record on the magnetic tape. This ac bias added to the recording reduces the noise during the playback of recorded audio. So Let's Get Started!

Demagnetization of Magnetic Tape:

A magnetized tape can be demagnetized with a magnetic Erase Head and a high frequency A.C. signal. This high frequency A.C. signal is generated by an oscillator circuit. This oscillator circuit is called bias oscillator circuit. Frequency of bias oscillator is kept higher than audio frequency range(20Hz to 20KHz). Practically the frequency of the bias oscillator circuit is in the range of 30KHz to 100Khz. This high frequency signal is then given to an erase head. when the magnetic tape passes through its recorded information get lost.

Bias Oscillator:

Bias oscillator is an electronic circuit that generates an A.C. signal which has frequency range between 30KHz to 100KHz. This bias oscillator circuit is used in tape recorder to reduce the noise during the reproduction of the recorded signal.

List of Electronic Components:

• Resistors

1. 10 ohm - 2
2. 22 k-ohm -1
3. 1 K-ohm -1

• Capacitors

Disc capacitors

1. 0.022 UF - 1
2. 0.068 UF -1
3. 0.01 UF - 2

Electrolytic Capacitors

1. 47 UF/40V -1

• Double sided PCB (Printed Circuit Board)
• Bias Oscillator Coil
• Two Pin screw terminal - 2
• Erase Head
• LED (Red) -1
• BC547B BJT (Bipolar Junction Transistor)

Erase Head:

A tape head is a type of transducer used in tape recorders to convert electrical signals to magnetic fluctuations and vice versa. They can also be used to read credit/debit/gift cards because the strip of magnetic tape on the back of a credit card stores data the same way that other magnetic tapes do.

An erase head is constructed in a similar manner to a record or replay head, but has a much larger gap, or more frequently, two large gaps. The erase head is powered during recording from a high frequency source (usually the same oscillator that provides the AC bias). In some inexpensive cassette recorder designs, the erase head is a permanent magnet that is mechanically moved into contact with the moving tape only during recording. Permanent magnet erase heads are also sometimes used in machines that are equipped with DC bias.

The Electronic symbol of Erase Head and its real image are given below.