21CT55 #2628 restoration

[DaveWM]

I have used a BT agile 450 and it works great, had the same kind of white line issue on an old GE 14" set that you describe, switch to the BT got rid of it. I would have chimed in sooner I just did not make the connection (between an old BW set and early color set.

[DaveWM]

here is a pic of what I was dealing with, its not a drive bar as you could detune and it would go away, and it moved with the horz hold, the BT resolved it.

[Zenith6S321]

Originally Posted by DaveWM

here is a pic of what I was dealing with, its not a drive bar as you could detune and it would go away, and it moved with the horz hold, the BT resolved it.

That's the problem. I will see if I can get one of those. Thanks!

[DaveWM]

they pop up on ebay all the time, prices are all over the place but you should be able to get one for around 40$ if you can wait for a good deal. they use DIP switches to set the RF out channel, and just need a audio and composite video in. I think the correct name is a Blonder Tonge agile modulator, I have the 450 there are others. It can be a bit confusing so look for the correct inputs and RF outputs and the little door that hides the DIP switches in the front.

another benefit is you can adjust the audio and video modulation levels, which is good for getting rid of sync buzz on sets that do not deal with that well.

[Zenith6S321]

Just clicked BIN on EPay to get a BT AM-60-550B for $39.99. Thanks for the tip.

[DaveWM]

if you use is as intended (a direct connection from the modulator to the TV via cable), just make sure you set the RF out the minimum to avoid swamping the tuner. They are designed for cable tv networks and can put out a LOT of RF to drive a lot of cable. I don't want the funny cookie cutters to get mad so will not talk about xmitting ota .

you will need remember to connect the IF in and IF out with a jumper as well.

[kvflyer]

Originally Posted by Zenith6S321

Just clicked BIN on EPay to get a BT AM-60-550B for $39.99. Thanks for the tip.

I have several of them in various models. So, if it doesn't seem to work when you get it, you may have to replace the voltage regulator and/or the filter capacitors in the power supply. Most of the ones I have purchased were OK. But the others were easily fixed with new filter caps and the regulator, easy to do, easy to get. I even got the regulator from Radio Shack of all places.

Cheers!

[Electronic M]

With the RF out cranked feeding into rabbit ears I can just barely tune it on the other end of my house, and given that there is a good deal of space between houses out here I doubt anyone else can get it or would even care if they did....

[Penthode]

The line I believe is actually due to the other end of the sync pulse. Consider this: the other end of the sync pulse is approximately 4.7us after the horizontal sync trigger. This falls within the active picture in the middle during the horizontal retrace period and if not properly controlled, will induce a vertical white line.

The factors which induce its appearance include IF alignment or video amplifier response in conjunction with the sync generated in a DVD player perhaps having too sharp a risetime. If the overall TV's video response is not reasonably flat, this will cause an overshoot into white which shows up as a white line.

Earlier sets did not have retrace blanking which makes this more problematic. The solution is careful check of the overall video RF and more particularly the IF response.

Color chassis's and the CTC2 chassis in particular is tough to align if you do not have the correct equipment. Fortunately good equipment is available on Ebay at a reasonable price. I trust you will find better equipment.

Forty years ago when I dismissed the thought of ever acquiring Hewlett Packard test equipment to do the job. Back then, at $5K to $10K, 1970's HP equipment was certainly out of the question. In the last 10 years, (thanks to Ebay) I picked up a 1970's HP8601a and HP8600a together for less than $100. I have used these devices to align sets RF, IF and chroma sections to and often I feel more accurately to factory specifications.

Alignment can be tough and is impossible without adequate equipment. But doing it right with the right equipment always rewards your efforts.

Originally Posted by Zenith6S321

I notice that turning the contrast all the way down removes the vertical white line. I can then turn the brightness all the way up with no line.

If I connect the Heathkit vectorscope and use its switches to disable the CRT grid video I do not see the line. Just connecting the vectorscope leads to the CRT grids reduces the white line quite a bit. Maybe high frequency ringing occurring during the horizontal retrace somehow coupling into the video?

I am using three modern 120 mH peaking coils that drive the CRT grids. I tried substituting an old style Miller replacement 120 mH peaking coil, but it shows the same white line.

Maybe I can track it down by putting in caps to filter out the noise at places along the video processing...

[Zenith6S321]

Originally Posted by Penthode

The solution is careful check of the overall video RF and more particularly the IF response.

I put a scope on the video waveform from at the grid of the first video amp and I can see a positive overshoot. Do you find that you have to reduce the high frequency IF response in order to slow the rise time to limit the overshoot?

Dave

[Penthode]

Originally Posted by Zenith6S321

Do you find that you have to reduce the high frequency IF response in order to slow the rise time to limit the overshoot?
Dave

No, I believe it is essential to make the response as close as possible to the factory specification. It may be that way it is currently aligned, the carrier is too low on the "Nyquist" slope to compensate for perhaps the higher video frequencies ("lower IF") rolloff. You can check this as I would suspect the white line will disappear when fine tuning the video carrier up the slope.

I believe the CTC2B chassis with 5 IF stages and the response going down to 21.65MHz AND the sharp transition makes it even tougher to get it right. But it can be done with the right equipment and some practice.

I recall Wayne suggested that is why the later "haystack" IF response became popular: it would get rid of the "galloping ghosts" when fine tuning and perhaps help prevent the white line. But I like the extended IF response design because I imagine if aligned right the picture will be better on a 21CT55 than on the the later CTC4's CTC5's and upward.

Terry

[Zenith6S321]

Originally Posted by Penthode

I believe the CTC2B chassis with 5 IF stages and the response going down to 21.65MHz AND the sharp transition makes it even tougher to get it right. But it can be done with the right equipment and some practice.
Terry

Thanks for the information Terry. I put the VA62 back to "external modulation" and took a look at the video at the first video amp. It did not have any video sync at all, so no sync overshoot, that's why it does not have the white line. I will take another crack at the alignment but this time I will also look at the video waveform as I move the sweep and marker generators through the stages to try to understand which has the most impact on my overshoot problem.

[Zenith6S321]

Well that little white vertical line, and my replacing lots of resistors in the RF and IF convinced me I should do a complete alignment of my set. Boy did that take a long time to do. But, I think I learned a lot, and got a good result. You be the judge. Here are some pictures starting with the final video waveform at the grid of the 1st video amp using a Sencore VA62 multiburst 10 bar sweep from 0 to 4.5 MHz in steps of .5 MHz. Next is the chroma sweep with the required 1st and 3rd bars the same size. Then the multiburst waveform at the green CRT grid with no color. Then the multiburst on the CRT showing a some discernable lines at 4 MHz (second bar from the right). Then color bars.

[Zenith6S321]

Now a few WOZ pictures.

[Zenith6S321]

And a few more. Yeah, I took way too many....