21-CT-55 vertical folding

[miniman82]

My 21-CT-55 is exhibiting a vertical fold at the top of the screen, and adjusting the lin and height controls has no effect on it. Is vertical folding a symptom of vertical oscillator issues, or incoming sync? It's not much of a fold, perhaps an inch or so. Should I be looking for bad caps?

[jr_tech]

Assuming vertical tubes are ok, the first thing I would check would be the 60uf cathode bypass cap on the 6BL7 vertical output tube.

jr

[miniman82]

Yep, tubes are fine so I'll have a look at caps.

[consoleguy67]

Is it possible that the vertical output transformer is going bad? That was the issue with my Zenith.

[miniman82]

Hope not, but anything is possible.

[W3XWT]

What kind of source(s) do you have going into it? Is the folding consistent from one source to another? Or, the same source subsequently looking OK on another set? I've seen this on more than a few sets of varying vintage with the AFC circuit being a culprit. Easiest fix is to turn the AFC off, if switchable...

[Electronic M]

Don't think the CT55 had AFC as it was made in the mid 50's, and AFC was not popular on TVs until the mid 60's.

[miniman82]

No AFC (at least not on vertical), it's a '55 set. I know it's something in the vertical circuit, because as I adjust the V-hold control the folding almost disappears right before it looses sync. Adjusting the linearity control will also make it go away, but then there's not nearly enough height to fill the screen. Think I'll poke around with the meter tonight and see what I can find, this thing has been dormant far too long.

[bgadow]

I wouldn't worry at all about the transformer; got to be a bad cap or resistor somewhere in the circuit. I usually just shotgun the caps in that area when I have a problem like this, to keep from having to lug that chassis in and out too many times. I have seen a bad pot do things like this but I think you'd pick up on that.

[miniman82]

Yeah, the pots seem to be fine. This chassis is a spotless time capsule, so I have a hard time believing they went South on me.

[miniman82]

I replaced a cap at the vertical output cathode, and the folding seems much better. Not gone, but I seem to be fighting an issue in some other area. There's this strange vertical fuzzy line in the left side of the picture I can't seem to get rid of... and still some noise in the signal. Something is still wrong in the tuner/IF, dammit!