| 摘要 |
PURPOSE:To simplify a circuit generating a negative voltage by rectifying an oscillated voltage of a local oscillation circuit constituting a part of a tuner to obtain a negative voltage for band switching. CONSTITUTION:When local oscillation is caused by operating a transistor (TR)Q as positive feedback, and local oscillating voltage is a 6V peak-to-peak, and a part of the voltage is utilized to generate a negative voltage. That is, an electric charge is stored at each negative half cycle in a capacitor 12 and the voltage is nearly -1V. In closing a switch SW, band switching diodes D3, D6-D8 are conductive by a positive voltage applied to a band switching terminal 4, a part of the tuning coil is short-circuited and the high band of the VHF is selected. In opening the switch SW conversely, the diodes D3, D6-D8 for band switching are reversely biased by the negative voltage obtained by rectifying and smoothing the oscillating voltage of the local oscillating circuit and they are cut off, the short-circuit part of the coil as above is released and the part is added to the circuit and then the low band is selected. |
