Predicta Contrast Lines / Streaking Issue

[Jon F]

Hello, I have a Predicta Pedestal, I replaced all electrolytic, paper, and bumble bee style caps. I have also rebuilt K4 & K5, replaced the rectifiers with 1N4007 diodes, replaced filament dropping resistors and the thermistor.

The set runs really well except for the issue shown in the attachment. On scenes with high contrast the images has something like lines on the bright areas horizontally across the screen. I've seen it on a couple different sources to varying degrees.

What do you guys think the issue could be, the CRT is a little weak(cutoff won't set stuck at 0) could that contribute?

[DaveWM]

Originally Posted by Jon F

What do you guys think the issue could be, the CRT is a little weak(cutoff won't set stuck at 0) could that contribute?

yes.

[Tom Albrecht]

It's a little hard to know what things in your picture are supposed to be there and what are not.

A darkening of the background along a horizontal band which includes very brught features is not caused by the CRT. This is caused by 1) lack of DC restoration in the set, which is a design decision made to reduce cost, and 2) possible misalignment of the video IF, resulting in insufficient low frequency response in the video. Often the setting of the fine tuning has some effect on the second item -- you can trade off high frequency (sharp features) for better low frequency response (better blacks and whites over large areas). A well aligned set won't have too much problem with this however.

Usually the lack of DC restoration in sets of this era doesn't show up so much in terms of bands within the frame, but rather a general "greying out" of scenes that are supposed to be mainly black or mainly white.

You also have a little of the "faint white line" syndrome on your Predicta. The faint light vertical line just to the left of center is caused by a particular pulse on modern video signals during the retrace that wasn't used in the 1960s. Adding about a 30 pF cap to filter the video output helps solve this problem. If you search on "faint white line Predicta" you can probably find some previous threads on this subject on how to install the cap that fixes it fairly well. Here's one thread on the subject: http://www.antiqueradios.com/forums/...72667&start=80 .

[Jon F]

Originally Posted by Tom Albrecht

It's a little hard to know what things in your picture are supposed to be there and what are not.

A darkening of the background along a horizontal band which includes very brught features is not caused by the CRT. This is caused by 1) lack of DC restoration in the set, which is a design decision made to reduce cost, and 2) possible misalignment of the video IF, resulting in insufficient low frequency response in the video. Often the setting of the fine tuning has some effect on the second item -- you can trade off high frequency (sharp features) for better low frequency response (better blacks and whites over large areas). A well aligned set won't have too much problem with this however.

Usually the lack of DC restoration in sets of this era doesn't show up so much in terms of bands within the frame, but rather a general "greying out" of scenes that are supposed to be mainly black or mainly white.

You also have a little of the "faint white line" syndrome on your Predicta. The faint light vertical line just to the left of center is caused by a particular pulse on modern video signals during the retrace that wasn't used in the 1960s. Adding about a 30 pF cap to filter the video output helps solve this problem. If you search on "faint white line Predicta" you can probably find some previous threads on this subject on how to install the cap that fixes it fairly well. Here's one thread on the subject: http://www.antiqueradios.com/forums/...72667&start=80 .

Thanks for the info, I actually already read that entire thread last night trying to search for a solution to my problem. You are right, the vertical line is there also, but it is not as distracting as my other issue. And adjusting the fine tuning will reduce the vertical line while the contrast streaking not as much. Where on the chassis would I put the 30pf cap?

As far as the screen shot, the background "space" shot should be a uniform black, but as you can see it is split into horizontal sections of varying brightness. These sections are caused by smaller bright text or objects in the scene.

I have added another picture a little washed out due to my camera.

[Tom Albrecht]

If you read starting here on page 8 of the previously cited thread: http://www.antiqueradios.com/forums/...2667&start=140 , you'll find reference to increasing the value of C19, which is under the shield of the IF section. I usually find about 30 pF is enough. Increasing it more than that starts to affect the sharpness of the picture.

If you look at the schematic here: http://www.earlytelevision.org/pdf/P...Sams-439-1.pdf , you'll see that C19 is on the anode of the video detector diode (in the 5AM8 tube).

But the above is only taking care of the faint white line, which as you point out is fairly minor compared to the horizontal bars.

If you drive the contrast and brightness too high, then you can have this effect accentuated by grid current flowing in the CRT. Backing off brightness and contrast can help. Here I should also admit that a weak CRT might make the effect worse, since you may try to drive the CRT harder by turning up the brightness, which increases the odds that the grid goes positive on the brightest parts of the picture. So DaveWM's comment may indeed be right.

Poor low frequency response in the video IF would really require an alignment to fix. If backing off the brightness and contrast makes the problem go away completely (not just lowering the contrast of the problem), then the problem is not in the video IF.

You could try increasing the value of C20 and C24. Particularly C24 you may find changes the time constant of the "recovery" from reacting to bright content in the image. Increasing it enough so that the whole screen stays uniform brightness might be more pleasing to the eye.

[Jon F]

The closest I have on hand is an 82pf cap. I jumped this parallel to C19 and while it did reduce the vertical line, the sharpness of the image was also reduced. I may order a 30pf to try and see if it is a happy medium.

Regarding the contrast issue, if I turn the contrast control all the way down, the problem goes away, however this could be just lowering the contrast of the problem like you said. Turning the brightness control down doesn't clear up the problem, the screen just goes mostly black.

Tried upping the value of C24 and C20 by 0.01uF with no change. Tried upping C24 by 0.05uF and there was a change in the image but no real improvement. Just kind of smoothed out the transitions in the contrast. Upping C20 by 0.05uF I saw no real change.

[Penthode]

Without having a schematic at hand, I assume the capacitors mentioned are interstage coupling and cathode bypass?

Is it possible to post a copy of the video amplier diagram so we could all have a look?

Thanks,

Terry

[Tom Albrecht]

For C19, 82 pF is definitely too much, and you'll see some loss of sharpness. Around 30 pF attenuates the faint white line quite a bit, and barely impacts the sharpness at all.

For C20 and C24, you'd probably need to increase to something like 0.22 of 0.47 uF to really see a big difference. Still, this is a design alteration, and while it might be a Band-Aid, it may not be really fixing the root cause of the problem. To some extent, these sets from this era simply have this kind of problem, although this one looks worse than usual.

The ETF link a couple of posts above has the full schematic for those that want to look at the video amp circuitry.

[Jon F]

Thanks Tom, maybe I will just do the 30pF mod and leave it be. The predicta still performs better than my other in process restorations so far so I can't really complain about that.