Color hum bar - ctc-10

[frenchy]

What are the likely causes of a single color hum bar that travels slowly upwards (cycles about every 10 seconds)? Not really noticeable on programming, but if I put up the blue vcr set up screen there is a faint, indistinct fat band of purple-ish-ness that travels up across the screen.
I'm wondering if some of the tubes that aren't shielded maybe should be and some repairman just left them off at one time or another. If a socket has a grounding spring, does that necessarily mean it is supposed to have a shield even if the schems don't seem to indicate it? I'll try swapping tubes around tonight in the color section. All the electrolytics have already been replaced with new units. thanks!