The Standby Indicator on the front will flash to indicate an error has been detected. The error code will be a series of flashes that repeat after 3 seconds. Of course "0" flashes indicates a totally dead set (possible power supply problems). But "4" flashes indicate a loss of vertical sweep and "5" flashes indicate a no raster condition. The No Raster condition can sometimes be overcome by slightly increasing the Screen Control. These sets also have a Self Diagnostic function that shows a list of past failures, but in order to view it the set must have a picture. That doesn't make much sense. Eric