I was just wondering because my Zenith has such a good level of contrast with the controls set at a very moderate level. Many tube color TVs I have encountered years ago always had a soft picture compared to the contrast of a tube type black and white. I guess I'm surprised these sets perform so well considering they are the early stages of color TV development. I know the Portacolor has no DC restoration but I wonder if many other color tube sets lacked it as well.

CT-100 had 100% DC restoration. Every (or nearly every) following set had reduced DC coupling, maybe between 60% and 85%.

Excuses:
1) cheaper to partially DC couple than to restore at the video output, especially with cathode drive
2) if DC coupled, more components can get into the act and introduce large manufacturing variations
3) partial DC coupling means the set draws less beam current on mostly-light pictures, so less HV supply capacity is needed
4) broadcasts (and later, cable channels) varied greatly in DC set-up (black level), so partial DC restoration decreased these errors (provided the image was average in brightness).

Consumer's Digest complained about this for years and years, until finally the manufacturers caved in (long after tube sets were gone) and put in 100% DC restoration (or close to that), which is now the common design.

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Here's what I'm watching on my '65 Zenith roundie like compucat's
http://i262.photobucket.com/albums/ii90/zenithfan1/000_0221.jpg
Has pretty good low light detail while maintaing the bright areas.

chroma looks a little high there

It was the scene, it looked the same on a BPC Panasonic, his face was all red there too. We watch 2 tvs in this house:)

I wonder if the problem I have with my Emerson 1224 (circa 1958, 24" B&W, Sams 410) is related to DC restoration, and how can I fix it.

Even though the set has keyed AGC, the content of any video it receives greatly affects the gain.

If any significant part of the transmitted picture is white, the contrast goes through the roof, anything that is not white goes to a deep black with detail lost, and people appear to be in silhouette. If much of the transmitted picture is white, there will be overload and the picture will roll.
Likewise, a mostly black scene will appear dull gray.

Any coments (posted or PM) will be highly appreciated.

Sounds like one of the vast majority of 50s black & white sets which have *no* DC restoration.

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It's not that hard to add DC restoration. It generally requires a diode, a cap, and one or two resistors to bring the brightness control's range back to normal. I don't normally like to modify TVs, but I can't stand to watch a TV without DC restoration.

If DC restoration is so simple, it should be on every TV. I'm pretty sure my 8" RCA metal portable doesn't have it. My 1978 GE solid state black & white set definitely has it.

This is one of those things that,at least in the vacuum tube days of TV, manufacturers thought they could bypass and save a few cents on. Once TVs became solid state, adding DC restoration became much cheaper.

Another area that was scrimped on in Vacuum tube TVs was the sound system. The use of ratio detectors/quadrature grid circuits without adding a true limiter stage is one of the reason tube type tvs have so much trouble with buzz in the sound.

For example, I have two Hoffman Easy Vision TVs, one a 1953 Mark V chassis and one a 1954 chassis. The 1953 Mark V taps the sound off of the video detector and has two sound if stages, one being a true limiter. Sound is great. The 1954 model is cheapened with the sound being taken off of the video output and only one non-limiter IF stage used. So far, I have not been able to completely eliminate the buzz from the sound. A complete re-alignment might help(but also might not). The 1954 is also cheapened in the IF circuit(no issue using a DTV converter to drive it) and in the sync circuit(possible issue with strong signal). Both use ratio detectors for the sound.

Moral of the story-manufacturers are always trying to reduce cost to make more money. Make the product just good enough to match competitors.

I agree with Andy that the Trinitron was a huge improvement back then due to the DC resto. Not many color sets had the ability to display a high contrast picture without losing the black level then. Seems like the resto circuit became commonplace in the early to mid 70s. IIRC, The early colortraks were among the first solid state RCAs to have a really good dc restorer .

It is interesting that some very early sets had DC restoration. I have a 1952 Truetone with DC Restoration and keyed AGC. It has one of the most bizarre tuners I have ever worked on though! has anyone come up with a DC Resto ad on to late 50's early 60's colors? My Magnavox t933 could use it as it is very dependent on program material.

Would there be any benefit to adding DC restoration to an electrostatic Motorola?

DC on a TS-4 would be interesting. I restored one of those about 20 years ago and still use it occaisionally. They are fun sets, especially when the cable repairman comes to the house and all other sets are hidden except the TS-4. I like the picture on them!

DC on a TS-4 would be interesting. I restored one of those about 20 years ago and still use it occaisionally. They are fun sets, especially when the cable repairman comes to the house and all other sets are hidden except the TS-4. I like the picture on them!

I have the 8" 9VT1 with the TS-18 chassis. I plan on getting it going again (needs ballast) and adding the retrace line suppression mod and though DC restoration might be worthwhile while I'm in there. I used mine regularly for five years after I restored it and always thought it had a good picture. Plus it's a black and white roundie.