While the CRT itself could be a factor, this is usually an easy fix as long as the CRT isn't completely shot. Open up the Mac, turn the power on with the rear cover off, turn the external brightness knob (technically a contrast knob) completely up, and adjust the internal brightness knob. This is located on the analog board. You'll need a plastic TV adjustment tool to turn this control. Turn it up high, then gradually bring it down until the scan lines disappear. Repeat this on all your compact Macs for a uniform screen brightness. You can also adjust focus, height, and width on this board. All compact Macs from the 128K through the SE/30 use the same controls. Replace the Mac's case and you're in business.
Note the procedure is different on a Classic or Classic II--the control panel is on the back of the case and does not require a rear bucket removal. Pop off the cover above the power supply (where the battery is on a Plus) to access the controls.