IMPROVEMENT OF RELIABILITY OF LARGE-SIZED CERAMIC CAPACITORS AND DUMMY RESISTORS FOR THE HIGH-POWER TRANSMITTER

This paper describes the effective countermeasures for exfoliation of large-sized ceramic capacitors, deterioration of dummy resistors and d evelopement of a spark sensor with UVtrons at 300-500 kW transmitting stations. Cracks and exfoliation were found at the electrode of large- sized ceramic capacitors in the output circuit of the 500 kW transmitt er. The exfoliation was caused by the temperature rise and the thermal fatigues at the electrode with the Nickel plating including Irons. A pure Nickel-plated electrode including no Irons and a new soldering me thod using disk-typed solder with a large adhesive area are employed i n order to reduce the temperature rise. The temperature rise of the im proved capacitor was 18 degrees C lower than the conventional one. Det erioration of ELEMA resistors of the 300 kW dummy antenna was discover ed. The damage of the resistor was caused by the concentration of the electric current followed by the thermal stress cycle which made mecha nical exhaustion at the electrode. Therefore, oval-shaped type resisto rs with much longer electric current path (20% up) to suppress the con centration of current flow and much slower temperature rise are newly developed. In case that sparks occurred at DC or RF high voltage impre ssed sections of the high power transmitting equipment, the discharged points could be seriously damaged by the transmitter energy itself. I n order to prevent this, a spark detector using UV (Ultra violet) tron s is developed and installed at the matching circuit of the 500 kW tra nsmitter. Conventional UV sensors with only one UVtron could not detec t feeble discharges and sparks with a duration time of less than 150 m s because of false outputs by the back ground noise. Since choosing th ree out of four UV trons system is employed, possibility producing a f alse output will be just one to 445 years theoretically. This means ex tremely reliable and sensitive spark detection system are constructed. These countermeasures have improved reliability of the transmitting e quipment greatly. No damages have been found in the transmitters ever since.