On switching on the picture is normal. After a
few seconds, however, the screen loses contrast, leaving a picture which is almost negative. This
fault will sometimes correct itself, very often
remaining normal for hours. This trouble first
presented itself after the picture positioning
adjustment had been altered.-P. B. Fincher
(Brighton, Sussex). If the brilliance (as opposed to the contrast or signal strength) remains normal, check the crystal diode detector and the video amplifier circuit com- ponents. If the brilliance is also lacking and the whole effect is “washed out,” check the tube heater voltage across pins 1 and 12. Tap the neck of the
tube sharply to clear any particles or embers which may be partially shorting the heater element. The tube may have to be changed.