Horizontal Linearity - CTC38 - Videokarma.org TV - Video

maxhifi · #1 09-12-2019, 03:24 PM

I've noticed that horizontal linearity isn't perfect across the screen. An object moving across the screen, will appear slightly wider on either side, than it does in the middle. This is really obvious with something like on demand movies, where it shows a matrix of movie covers on the screen, with those in the middle appearing narrower than those on either side. With normal program material, it isn't extremely obvious.

I did some reading about it, and I'm thinking to either change the horizontal output tube and damper, or try and adjust the horizontal linearity (efficiency) control. The thing I don't understand, is why this control is adjusted for minimum cathode current, and not best linearity - how are the two related?

I won't be terribly bothered if this issue can't be corrected, because it is fairly minor - but given there is a control marked horizontal linearity, I really would like to see just how linear things can be!

trinescope · #2 09-12-2019, 03:56 PM

My own understanding of horizontal sweep circuits is incomplete, but I do know this: The horizontal output tube is used as a switch, not an amplifier. Applying a constant voltage across an inductor causes a linear current ramp to flow until saturation. My own guess is that anything that interferes with that linear current ramp will cause non linearity. It could be a tube whose voltage varies with current at device saturation (tube fully turned on), or some other less than ideal circuit condition. I'll wait to see what others have to say about this.

The linearity or efficiency coil Is part of a tuned circuit, most likely to adjust the switching point so that it occurs when there is minimum loss; at zero voltage or current, like tuning a Class E RF amplifier. Again, I'll see what others have to say here.

freakaftr8 · #3 09-12-2019, 04:39 PM

Right. I was able to achieve low current with a CTC 17 but the linearity was the same as you described. if I were to get it better to my liking, the H cathode current was not happy about this. I would look to the capacitors in the horizontal section, including filters and see if any are off spec?

Also have you checked cathode current?

Are any of the filter can electrolytic capacitors getting warm? This would have to be checked before the main transformer radiates heat into the first can capacitor.

Also do you have any crawl in the picture? I had a crawl on the CTC 17. Once I isolated the filters and replaced some high ESR caps, my crawl was gone and I was able to establish a better linearity with low cathode current from the efficiency coil.

maxhifi · #4 09-12-2019, 04:47 PM

Quote:

Originally Posted by freakaftr8

Right. I was able to achieve low current with a CTC 17 but the linearity was the same as you described. if I were to get it better to my liking, the H cathode current was not happy about this. I would look to the capacitors in the horizontal section, including filters and see if any are off spec?

Also have you checked cathode current?

So, narrower in the middle than on the sides of the screen? That's interesting, I wonder if it was intentional, or what could cause this issue. Logic tells me that as the beam scans across the screen, a small change in beam angle would make a bigger difference in the position of the spot at the edges of the screen versus in the middle just due to geometry - and there must me something in the set which compensates for this. I don't understand how TVs work well enough to be able to tell what, though.

I have not checked cathode current. What was the effect on the picture when you adjusted the linearity control on your CTC17?

Electronic M · #5 09-12-2019, 04:52 PM

I'd sub the tubes first. If you are set on adjusting the linearly control make sure to monitor the hot cathode current while doing so and avoid running it too high.

There's a possibility the set is fine, and the problem lurks in your cable box. I've seen systems that stretch only the edges of 4x3 programs to fill a 16x9 screen(resulting in a 16x9 image with good center linearity and lousy side linearity)...it is entirely possible for the cable box to be doing the opposite (non-linearly smooshing a 16x9 image into a 4x3 frame.
Do you have an old-school hardware based TV test pattern generator that predates broadcast HD?... If so hook it up to the TV and see what a grid pattern looks like...if that looks linear on the set then your cable box is probably the problem.
Another check would be for vertical lines or stripes of varied brightness that show up on both program content and blank raster....if beam velocity changes much across the screen (which is the definition of non-linear scan) then brightness will vary too...the longer the beam dwells on a spot the brighter the spot becomes (as you probably will learn if you play with a scope).

freakaftr8 · #6 09-12-2019, 04:54 PM

Sorry reread my post I added some info to it.

The efficiency coil, is also a linearity control, but adversely affects the amperage output of the flyback. if set too high causes the fly to work harder. Also islf there is ripple current in that drive it's hard to achieve a low cathode current off the h osc. If that happens, it causes the h out tube to red plate. Then all hell breaks loose.

maxhifi · #7 09-12-2019, 05:00 PM

Here is a photo of it hooked up to a test pattern DVD, to give you an idea what I'm talking about. This photo was taken several years ago when I originally got the set, but the symptom hasn't changed at all.

freakaftr8 · #8 09-12-2019, 05:06 PM

Ok thats vertical pin phasing. there are two adjustments for that. Try pin phase adjustment in a mirror or tweak the pot a little and look. It's on the back right side.. should be able to get a screwdriver and their turn it and look at the screen to see if that straightens out.

freakaftr8 · #9 09-12-2019, 05:08 PM

I'm not seeing an issue with h linearity in that picture. Can you snap a grid picture straight in front?

I have found with linearity is only one side seems to stretch out and other side compress in.

old_coot88 · #10 09-12-2019, 07:53 PM

IMHO it was very stupid for the mfr. to label the eff coil "linearity" in the first place. Its primary and vital function is to 'dip' the HO cathode current to minimum. The HO tube is run very hard by design, with very little headroom. For longevity of both the tube and flyback, the current needs to be dipped as low as possible, period.

(Yeah I know there was some obscure directive to set the current slightly to one side of the dip for some obscure reason. But we always just dipped it.)

maxhifi · #11 09-12-2019, 09:59 PM

Quote:

Originally Posted by freakaftr8

I'm not seeing an issue with h linearity in that picture. Can you snap a grid picture straight in front?

I have found with linearity is only one side seems to stretch out and other side compress in.

Have a look at the width of the square in the middle, compared with the squares on the far sides.

I will get a better photo soon, that setup with the test DVD was years ago, but the problem hasn't changed. I have never attempted to adjust horizontal current, but it sounds like a good idea.

freakaftr8 · #12 09-12-2019, 10:10 PM

I see it now. Do you have a switch on the back for norm/high? This is a voltage setting that taps off the transformer for line voltage fluctuations.

You could try switching it to high to see if there is a change.

This is kind of what I was talking about with my CTC 39 in my post. Same issue but my gripe was how stretched the horizontal width is compared to everything else I have.

maxhifi · #13 09-13-2019, 12:03 AM

I do have it on "high"

The CTC38, according to RCA literature, has a precision voltage divider rather than a high voltage adjustment pot (presumably to make sure it cannot be adjusted to produce x rays)

I wonder if we are getting excessive width due to drift in the voltage divider resistors.

freakaftr8 · #14 09-14-2019, 12:01 PM

That's a really good question. I would have to obtain a sams and check resistance and tolerance around that area. Alot of these sets had very loose tolerances but there were a few critical areas.

maxhifi · #15 09-14-2019, 12:12 PM

Quote:

Originally Posted by freakaftr8

That's a really good question. I would have to obtain a sams and check resistance and tolerance around that area. Alot of these sets had very loose tolerances but there were a few critical areas.

I have the RCA field service manual for the CTC38, it has factory schematics, parts list, alignment instructions, etc. I will scan it and post a link, if it helps. Actually I also have the training manual for the 1969 product line, which explains the features of the ctc38 in detail.