1. Field of the Invention
The present invention relates to a flyback transformer having a focus pack.
2. Description of the Prior Art
A television receiver requires various voltages ranging from a low voltage to a high voltage. In order to obtain such voltages, the receiver uses a flyback transformer which is designed such that a DC voltage of 100 V produced by a power supply circuit is received at the primary winding, and a high voltage of 30 kV and a low voltage of 10 to 200 V are respectively obtained from the secondary and tertiary windings.
A high voltage (e.g., 25 kV) generated by the flyback transformer is applied to the anode of a cathode-ray tube. At the same time, this high voltage is appropriately decreased across a resistor to obtain a focus voltage (e.g., 10 kV) to be applied to an electron beam focusing electrode arranged in an electron gun, and a screen voltage (e.g., 1 kV) to be applied to a screen electrode for color adjustment.
FIG. 1 shows a typical flyback transformer. In order to obtain a focus voltage and a screen voltage from a high voltage generated by the flyback transformer, a unit called a focus pack, denoted by reference numeral 2, is used.
FIG. 1 shows a flyback transformer 1 and the focus pack 2. The flyback transformer is designed such that a core 1a is inserted in the center of a bobbin having windings wound therearound, and the bobbin is entirely housed in a transformer case 1b. The focus pack 2 is mounted on an outer surface of the flyback transformer 1. A high voltage generated by the flyback transformer 1 is applied to an anode cap (not shown) through a lead 1c and to a high-voltage application point (to be described later) in the focus pack 2.
The focus pack 2 has outer and inner cases 21 and 22, each consisting of a resin, as shown in FIG. 3. A ceramic board 20 on which a high-voltage resistor portion 20a and variable resistive elements 20b and 20c are printed as wiring patterns is arranged in the inner case 22, as shown in FIG. 2. A high voltage from the flyback transformer 1 is applied to a high-voltage application point A on the board 20, and a point B on the board 20 is grounded through a terminal 2b. As is apparent from FIG. 1, a focus shaft 3 and a screen shaft 4, both serving to perform voltage adjustment, are arranged on the front surface of the outer case 21, and sliders S1 and S2 (see FIG. 2) are respectively pivoted by the focus shaft 3 and the screen shaft 4 to slide on the variable resistive element 20b for focus voltage adjustment and on the variable resistive element 20c for screen voltage adjustment. One end of each of the sliders S1 and S2 is connected, through a corresponding one of printed wiring patterns 20d and 20e on the board 20, to a terminal (not shown) extending through the rear surface of the board 20.
As shown in FIGS. 3 and 4, the focus pack 2 of the flyback transformer 1 of this type has the outer and inner cases 21 and 22. A focus insertion hole 5 and a screen insertion hole 6 are formed in the outer case 21. A focus shaft retaining cylinder 7 and a screen shaft retaining cylinder 8 extend from the inner case 22. When the focus shaft retaining cylinder 7 and the screen shaft retaining cylinder 8 are respectively inserted in the focus insertion hole 5 and the screen insertion hole 6, the outer and inner cases 21 and 22 are fitted together.
The focus shaft 3 is inserted in the focus shaft retaining cylinder 7. The screen shaft 4 is inserted in the screen shaft retaining cylinder 8.
The outer case 21 is fitted in a transformer case 1b. In this state, an insulating casting resin 10 such as an epoxy resin is injected into the transformer case 1b, so that the interior of the transformer case 1b and the gap between the outer and inner cases 21 and 22 of the focus pack are filled with the insulating casting resin 10. As a result, the resistor circuit formed on the board 20 is insulated from the outside by the outer and inner cases 21 and 22 and the insulating casting resin 10.
According to the focus pack 21, however, since the outer and inner cases 21 and 22 are integrally assembled by fitting the retaining cylinders 7 and 8 for the focus and screen shafts 3 and 4 in the insertion holes 5 and 6, the outer and inner cases 21 and 22 must be independently manufactured. For this reason, the work efficiency in the manufacture/assembly of the apparatus is low. In addition, when the insulating casting resin 10 is injected between the outer and inner cases 21 and 22, the insulating casting resin 10 leaks from gaps 9 between the retaining cylinders 7 and 8 and the insertion holes 5 and 6.