This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
If the monitor exhibits major flickering, periodic collapse of the display to a line, etc., then it may help to know that a common source of such problems is one or more bad connections where the High Voltage module is joined to the main circuit board. (This module is the black thing with a HV lead running to the CRT-- it's near the left, back. The slotted nub controls in its case set Focus and base Intensity.) Often these connections look okay because it is hard to see the small fractures in the solder surrounding the pins. The cure is to resolder all of the pins coming from the module (on the under-side of the circuit board). Before doing the soldering, clip a wire to the metal chassis and touch the other end to each HV module pin and other points in the area. While soldering, avoid touching anything conductive on the monitor with anything but the iron and solder. = = = = = = = = = = = = = =