The present invention relates generally to horizontal deflection circuits for cathode ray tubes, and more specifically to a horizontal deflection circuit having a retrace period which is variable in response to operation of a manually controlled switch.
There is often a need to operate a cathode ray tube with a variable line frequency to make it compatible with different line scanning systems. It is also desirable that the line scan retrace period be made variable with thresone variable trace period for adjusting the horizontal deflection amplitude of a raster.
One prior horizontal deflection circuit as described in Japanese Utility Model Provisional Specification No. 60-18965 employs a variable frequency oscillator for generating a variable frequency oscillation in response to horizontal sync pulses supplied from an external source. A switching device comprising a transistor and a diode is provided for switching the frequency of a variable frequency resonant circuit that determines the retrace period depending on whether the transistor is turned on or off. The turn-on of the transistor is effected by a DC bias supplied from a voltage source to the base of that transistor when a manually controlled switch is closed. This DC bias is applied to the transistor as long as the switch is maintained in the closed condition. One disadvantage of the prior deflection circuit is that, since a large collector current flows through the switching transistor when it is turned on, a large base current needs to be supplied to that transistor to switch it into a fully conductive state. Furthermore, the switching transistor must be capable of withstanding a high voltage flyback pulse which is generated in the deflection circuit during the retrace period and the transistor of this type has a very small value of current amplification. Therefore, a substantial amount of power is needed to drive the switching transistor.