Solution to increase the voltage of cathode ray tube filament

The filaments of 9-inch and 12-inch picture tubes are generally powered by a 12V regulated power supply. When the picture tube ages, it is often necessary to increase the filament voltage to increase the brightness of the picture tube. However, there is a limit to the increase in the voltage of the regulated power supply. If it is increased too much, in addition to causing the grating amplitude to increase and the power consumption of the whole machine to increase, it is also easy to cause the power stage circuit to overload and cause component damage. In addition, if the voltage is low, the power supply ripple will increase, causing image distortion.

If the power supply is provided by the secondary AC voltage of the power transformer (16~18V) or the rectified DC voltage (18~25V) which is stepped down, the output voltage will fluctuate due to changes in the grid voltage, which will have an adverse effect on the life of the cathode of the CRT.

If the voltage of the output stage voltage booster is used to increase the voltage of the cathode ray tube filament, the effect is better and it is simple and easy. The circuit is shown below. At the positive electrode of the boost capacitor C, there is a stable voltage of 25.5~27V (depending on the model, there may be slight differences) to the ground. The 13.5~15V DC voltage across the capacitor C is used to supply the voltage of the filament. The voltage can be selected between 5~30 ohms according to the degree of aging of the cathode of the cathode ray tube. For some models, the voltage across C may be slightly lower, and the resistor R can be omitted.