Zenith 25CC50 Question

[sgtrob]

Hello everyone! Just picked up a really nice zenith 25CC50 (model T998DE11), see photo. Just had to change a few tubes, and the set works great. Since a new CRT was installed in 1985, and the TV was put way shortly after that. This set does have an annoying problem however. The vertical sync seems to "flicker" at the lower half of the screen. Sometimes it acts up, sometimes it does not. I replaced the vertical output tube, the vertical oscillator, sync sep, and even cleaned all of the vertical controls. It still acts up every once in a while, can be kind of irritating. Any thoughts?