DC restoration issues

[freakaftr8]

Hey guys, does anyone know how to eliminate dc restoration issues in these old tube color sets. I have a 1967 Zenith color set and the dc restoration is horrible. I was reading on here about how to fix restoration issues in a portacolor. Does this apply to most color sets of this era?

Thanks
Shawn

[andy]

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[blue_lateral]

A typical color set of the 60's, based on something like ctc-12-15-16, has about 90 percent DC restoration with the brightness wide open. So, the situation is best when he setup is such so that the brightness control is all the way up with the picture adjusted correctly. This is almost never the case, because as you probably know, a perfect setup involves a darkened room, and no blooming at all. Most people had a need to turn them up brighter sometimes, at the expense of picture quality, because they had too much ambient light in the room.

Even if everything is perfect, it still isnt quite enough. When the set is setup for maximum available DC restoration, and the program is a bright normal scene, and the controls are set so there is no trace of blooming, and the black level is right where it should be, and you can see everything in the shadows, and the feshtones are perfect, and your friends jaws are dropping because the picture is so good........ a dark scene will come on and all the black will be washed out to dark grey. This is normal.

This is just something we lived with back then. It annoyed me, but I dont think most people noticed. Maybe that was because most black and white sets did it even worse. A lot of them didnt have DC restoration at all. If anything these color sets would have looked like an improvement.

A weak picture tube will make it much worse, due to a lousy cutoff characteristic. Any slight mistracking (screens, drives) will cause black to be washed out with whatever color cuts off last. It doesnt take much light to wash out black. Then theres the issue i mentioned earlier on ctc15-like circuitry. The best you can get is about 90 percent DC restoration. If your screens/drives/kine-bias are set such that the brightness control runs halfway up on a perfect picture, then you have about 45 percent DC restoration.

The first set I ever saw that really did this right was a $2200 Mitsubishi a friend of mine bought in about 1990. I never saw the schematic, but I suspect some sort of active circuitry, or over 100 percent DC restoration. I saw quite a few high end sets not long after that with similar DC restoration performance. A few years later, the cheapies started getting it right.

John

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[andy]

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[zenith2134]

I would try and reduce contrast as much as possible, and maybe check for failing caps in the video and supply stages. Especially because of its age. DC resto has been an issue with a few of my sets and it definitely can be a nuisance on an otherwise nice set.

[freakaftr8]

Yeah it seems like what I get is I can achieve good brightness and contrast. As soon as a picture comes in with lets say it shows a sky and a field. The sky will be predominant and I cannot even see the field, its black. But then a dark pictre comes in and the black level is washy because the brightness is turned up. The only way to make all the scenes for me to be able to see everything is if I crank down on the contrast dramatically. Then everything looks horrible.

[ChrisW6ATV]

I had an 18" Sears tube portable color set, my first color TV, around when I first learned what DC restoration was. I found I could make that set display a pretty stable black level, but only if I turned its contrast super-low and the brightness low as well. It had OK DC restoration as long as I kept all the room lights off!

The next year, I bought my first new color TV. It was a 19" Sony.

[peverett]

I did a quick look at some of the Sams photofacts that I have. It seems that some of the early B&W TVs had circuits using diode/triode tubes for DC restoration. However, this went out the window in a very short time. I presume the TV makers decided that this could go in their cost cutting wars.

[yagosaga]

The early tv sets here in Germany did not use a D.C. amplifier due to problems with varying video drive currents. They instead had D.C. restoration, coupled with the brightness control.

This technique was also applied in early color television in 1967, see for example:

http://bs.cyty.com/menschen/e-etzold...T2000color.TIF

Saba used color-difference concept with D.C. restoration.

On the other hand, the Einheitschassis, made by Telefunken, Blaupunkt, Siemens, Nordmende, used D.C. amplifying circuits in the video unit with RGB concept, see

http://bs.cyty.com/menschen/e-etzold...hassis1967.tif

For keeping the D.C. drive or bias constant, some additional circuits were neccessary.

The early Metz color tv sets used color difference concept with D.C. amplifying:

http://bs.cyty.com/menschen/e-etzold...metz6285-1.TIF

The best picture quality I get with the Saba (color-difference concept with D.C. restoration).

Color difference concept with D.C. amplifying and D.C. amplifying with RGB concept are the best, but does only operate best with an absolutely strong crt. Saba with color-difference concept with D.C. restoration does work well even with a weaker crt.

- Eckhard

[freakaftr8]

I see,, so technically a set with great D.C restoration would need a stage tube for each grid control. Most of our vintage sets just use x and z demods which blend all RGB together, and sends out grids at the same time.

[old_tv_nut]

Originally Posted by freakaftr8

I see,, so technically a set with great D.C restoration would need a stage tube for each grid control. Most of our vintage sets just use x and z demods which blend all RGB together, and sends out grids at the same time.

The DC restoration (or lack thereof) we have been discussing is in the luminance.

The chroma circuits always must have DC coupling or DC restoration, otherwise the background color (grayscale tracking) varies with the average color of scene content. If that would happen, a shot of a performaer in a red dress would make her face go cyan. In the RCA X-Z demods, the following color difference amps matrix to R-Y, G-Y and B-Y using a common cathode resistor. Blanking is also inserted at the cathodes (as a negative pulse), which produces a DC restoring action by drawing some grid current during the blanking pulse.

The first schematic posted by Yago has DC restoration in the luminance (diode OA 151) and also in each color difference (diodes GR351, GR352, GR353), if I'm reading it correctly.