View Full Version : High contrast issue

12-29-2015, 07:43 PM
On both my Zenith roundies it seems when there's a bright background the closer images become darker. Is there a way to solve this?

Tom S
12-29-2015, 08:28 PM
Could be an AGC issue My CTC 11 was like that

David Roper
12-29-2015, 11:20 PM
Sounds like a poor DC restoration issue to me.

12-30-2015, 06:56 AM
What's poor DC restoration???

12-30-2015, 04:47 PM
What's the chassis number?

Could be AGC or DC resto, though later sets had DC resto pretty well nailed down.

Electronic M
12-30-2015, 06:13 PM
Could be HV sag/blooming, or a tired CRT....A picture is worth 1K words.

12-30-2015, 06:39 PM
The chassis is 26kc20. I never heard the term DC rest. What is that?

David Roper
12-30-2015, 07:41 PM
When video signal is passed through a capacitor, the DC component is lost. The result is a black level that varies with the amplitude of the signal. That's why on TVs with zero DC restoration (as are the vast majority of 50s black and white sets) there is no such thing as a fade to black because black content appears gray and very dark content appears very washed out. This was an acceptable condition because content with normal lighting looked normal. But with the contrast adjusted for scenes of ordinary brightness, extraordinarily bright content (think J.G. Wentworth ads!) will result in the detail of darker objects being lost as black goes blacker. Just like any other circuit, there are conditions under which it can fail to work properly, but the question is how "properly" it worked in the first place. I have sets with DC restoration that ranges from excellent to poor. I have succeeded in restoring the restoration in sets that have a well-designed circuit that stopped working, but I've never been able to improve it in sets where it barely seems to make a difference.

12-30-2015, 09:03 PM
Early color sets (all roundies, for sure) had about 85% or so DC coupling. This resulted in the effect you see. You can turn up the "brightness" control (actually black level) somewhat and get the opposite effect, where bright scenes have correct blacks, but dark scenes are washed out and have gray shadows. This lack of 100% DC coupling (or restoration) helped prevent attempts to draw too much beam current, making the high voltage go out of regulation. The bad effects on picture contrast in the shadows were decried by Consumer Reports for years, until later solid state sets were designed with 100% DC restoration. The first of these sets had beam current limiter circuits that reduced the black level to prevent over-current (giving the same effect you see, but only if the "Contrast" was turned up too high). Later sets with integrated video circuits had DC-controlled "Contrast" or "Picture" (really video drive level) that could be limited automatically instead of "brightness" to prevent drawing too much beam current. This gave Consumer Reports one less thing to complain about.

So, people who are used to modern sets (and computer monitors) with 100% DC restoration readily see the flaw in older sets. Back when these were built, consumers didn't know better, plus the contrast range available with ordinary room lighting on a gray-faced CRT wasn't very great anyway. I would just consider this a learning moment that you can lecture about when you show your sets to non-collectors.

12-30-2015, 09:15 PM
Some more things:

The CT-100 had DC restoration near the end of the video chain, at the CRT. This compensated for the inevitable drift of DC level in the tube video circuits. When it was decided that DC restoration was too expensive for the later consumer chassis (CTC-5 and onward), DC coupling was substituted. Using 80-85% DC coupling meant that the drift was reduced by 15 or 20%, which the older engineers told me was believed worth it. DC level in early broadcasts drifted or changed from program to program too, and a similar argument was used by receiver makers to justify reducing the DC coupling to less than 100%.

Two of the greatest improvements in video pictures over the years have come from precise control of DC (black) level at both transmit and receive sides, and increased contrast ratio of displays, that is, reduced room light reflection.

12-31-2015, 04:28 AM
It's a bit of the compromise we have to make to enjoy older sets.

12-31-2015, 07:28 PM
Keep in mind these sets never looked good with the contrast cranked up all the way, they were never designed to perform like that.