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!

Check your house wiring and see if you have some voltage on the neutral to ground path. I had a bad antenna amp mess my sets up until I found the culprit.

Dave A

The description sounds like a 60hz hum from a possible external source.How are you connected externally to your VCR? Is it cable? You might want to disconnect all of your sources and see if that eliminates the hum.I have a similar problem with my cable.Most likely a staple creased the coax or is touching the copper.My only solution was to get a 60hz filter and attach it to the coax output at the wall.

Agreeing with the earlier suggestions, eliminate external causes first. Disconnect everything from the TV and back off the AGC pot to get a blank white raster and see if the hum is still there. (I don't think a CTC-10 had a 3-position service switch, but if so, all the easier on the blank raster!!!)

Next wild guess is that I had a Magnavox roundie doing something like that (single-color hum bar, one at a time). I recall that it was a heater-to-cathode short in one of the 6GU7 tubes (might be the G-y amplifier tube in your case). Just a guess, so don't be disappointed if it's a bad guess... That type of problem can be a real head-scratcher to locate.

Charles

ok I'll check my antenna/cable connection and the antenna rf box this weekend, thanks... Frenchy

Back to the tube shielding... what tubes in general should have tube shields in a color set of this era? I'm pretty sure there are some in there that have the little grounding spring on the socket but there's no shield on them. thanks

Definitely the 3.58 oscillator, the burst amp, and all the RF-IF tubes will have shields, and sometimes others depending if the engineers put them close to sources of interference. BTW: In a CTC-4, all you have to do is turn a shield can a degree or two and the entire TV is thrown out of alignment! :thumbsdn:

But as far as a hum bar, I don't see a missing shield can being the cause. But who knows?

Charles

I eliminated the power supply (used an isolator) and antenna as culprits, but I think it was a combination of things - I had the color turned up too high, I've switched to a straight connection to the antenna terminals instead of a vhf/uhf splitter box, tweaked the horizontal hold a little better (it can start to mess with colors on this set if you get sloppy with it), and dialed in the red purity. Plain red screen just plain sucked but had looked good enough on programming and on blue so I hadn't really touched it. Now the hum is basically not noticeable and picture is a ton better to boot. It has shields on the ifs and the burst so I think I'm ok on those too. Thanks!

But as far as a hum bar, I don't see a missing shield can being the cause. But who knows?

Charles

Some demod tubes get bothered by stray magnetic fields from the power transformer. But the TV set designer/ manufacturer should have designed the set to avoid this issue.