Happy Halloween! NTSC "Pumpkins" slide on CTC-5

[old_tv_nut]

Official NTSC test slide #18, "Pumpkins"
Displayed on RCA 21CT785 "Westcott", CTC-5 "Super" chassis.

[Yamamaya42]

Originally Posted by old_tv_nut

Official NTSC test slide #18, "Pumpkins"
Displayed on RCA 21CT785 "Westcott", CTC-5 "Super" chassis.

OK! you went and had to tempt me!

[Yamamaya42]

Be sure you are wearing your Halloween masks when watching!
https://www.youtube.com/watch?v=C6Oe...ature=youtu.be

[etype2]

Looking very good!

[Penthode]

I trust that is on thee 21AXP22A?

[old_tv_nut]

Originally Posted by Penthode

I trust that is on thee 21AXP22A?

Yes indeed. I have to adjust the camera white balance to match the TV white point, which makes the incandescent "TV lamp" on top of the set look very orangy also.

[Yamamaya42]

Originally Posted by old_tv_nut

Yes indeed. I have to adjust the camera white balance to match the TV white point, which makes the incandescent "TV lamp" on top of the set look very orangy also.

I wish I had a fancy camera like that

[old_tv_nut]

Originally Posted by Yamamaya42

I wish I had a fancy camera like that

What are you using? A phone? It is possible to get good pics with a phone. The main thing is to get the exposure right, and then you can make other adjustments in software later.

[Penthode]

I have just pulled my CTC5 apart to try and understand why I cannot get the three guns to track properly. I was going to blame the 21AXP22A but testing it the emission of all three guns is great on the B&K 467. The tracking test show the guns follow so I feel I can declare the tube healthy. (In fact it amazes me how good the CRT is. My guess is the CTC5 was a dog of a chassis - low High Voltage, rather dim phosphors and a pain to adjust tracking and convergence resulting in the set not being used much).

Anyhow I want to find out why the shunt regulator has too easy a life: you disconnect the 6BK4 and the high voltage remains at 21kV! And the tracking problem probably due to my CTC5 incompetence! Even the manual is rather arbitrary when it comes to trying to track with the screen, background and gain adjustments. It is so much easier with the Zenith color set!

[old_tv_nut]

Originally Posted by Penthode

...
Anyhow I want to find out why the shunt regulator has too easy a life: you disconnect the 6BK4 and the high voltage remains at 21kV!...

The HV circuit in the CTC-5 Super chassis is not capable of much in the first place. I suspect that 21 kV unregulated was normal. I also suspect the Deluxe chassis was capable of much more. Maybe someone who has worked on both can comment.

And yes, the tracking arrangement in the Super chassis is a pain. Part of of the problem is that offsets in the demodulators can affect it, hence the background controls on the CRT G1's.

The Deluxe chassis and all following RCAs using XZ demodulation plus following matrix/amplifier tubes for R-Y, G-Y and B-Y amplification are much more stable, because the three output stages also perform a DC-restorer function. The negative horizontal blanking pulse applied to the three interconnected cathodes causes the tubes to draw grid current, thus DC-restoring the X and Z signals at the input coupling caps and stabilizing the G1 voltages. There can still be some difference between the amplifier sections, but nowhere near as bad as in the Super chassis B-Y and G-Y demods. I have never, ever seen anywhere an explanation of the DC restorer function in the XZ demod system. I think RCA just didn't want repairmen worrying their poor little heads over understanding something that had no effect on repair diagnosis. If the tubes were bad, it would be obvious and fixed by replacing them. And if a component failed causing the blanking pulse to be lost, there would be retrace line problems, so even though it would cause loss of DC restoration, that is not the symptom per se that you would use for diagnosis.

[Electronic M]

Originally Posted by Penthode

Anyhow I want to find out why the shunt regulator has too easy a life: you disconnect the 6BK4 and the high voltage remains at 21kV! And the tracking problem probably due to my CTC5 incompetence! Even the manual is rather arbitrary when it comes to trying to track with the screen, background and gain adjustments. It is so much easier with the Zenith color set!

If you compare the HV regulator circuits in the CTC5 and CTC7 (free Sam's for both are on the ETF Color sets -> technical documentation page) the CTC5 and CTC7 circuits are identical except that a fixed resistor on the (non-adjustable) CTC5 is replaced by a potentiometer and series resistor on the CTC7...on the second CTC5 I did I upgraded the HV circuit to the adjustible CTC7 circuit.

I wish I could remember a few months ago when I did my second CTC5 I had a couple of issues setting grayscale tracking. One was related to there being 2 versions of the super chassis the one I had having a electromagnetic blue lateral and no blue gun level pot and the Sam's set having a gun level pot and Permanent magnet blue lateral and writing their proceedure assuming you had a blue gun level pot (I forget if the gun level was screen or drive).
The other issue I had was the proceedure called out a voltage for IIRC the red gun and I had some circuit fault I had to fix to get there. I have some vague recall of AGC issues that may have been related.

All the 21AXP22 RCAs had circuit designs that drifted grayscale some with extreme adjustment of brightness and contrast controls. The key is to get it reasonably good in the range you are going to watch it at.

[Penthode]

I went thru the color tracking following the service notes a and my intuition. Grey scale tracking looks better now.

However I am not fully happy with the picture. When the video exhibits a wide contrast ratio, the blacks are crushed. It is as if there is no DC restoration.

Looking at the CTC 5 schematic, in both the Y and the color difference paths feeding the 21AXP22A, there appears to be coupling capacitors which suggest AC coupling only. I compared with a Zenith 21MC32Z, and both the Y and the chroma gated beam color difference paths between detector and CRT grids appear to be DC coupled.

Am I correct in suggesting the RCA CTC5 does not include DC restoration and the Zenith does?

[old_tv_nut]

All sets for a long time from this time onward had partial DC coupling in the luminance. This results in a partially floating black level depending on average picture luma. Consumer Reports complained about this for years and years. The manufacturers' reasoning was that it partially compensated for black level errors in broadcasts (and in analog cable systems, where consistency was much worse). It was finally resolved with solid state studio gear and solid state TV sets whose circuitry was stable enough to keep the proper black level once set. Consumer Reports was very pleased.

The CTC-5 also has partial DC coupling (not zero) in the color difference paths. Again, this counteracts circuit drift or changes when tubes are replaced, to some extent. It is also a technique for automatic color balance, called "integrate to gray," because many scenes are close to gray on average. This technique was even used in automatic color photo printing, where it affected the white balance, not the black balance as in the CTC-5. It is prone to "subject failure," that is, if a large part of the scene is a dominant color, the rest of the scene will be pushed in the complementary direction.

In color prints made on these early machines, I have seen pics of someone wearing a red sweater where the rest of the picture including their face was pushed toward cyan.

In the CTC-5, you can see the effect of this partial coupling if you play a DVD of a test screen that is all one color, like a complete red field for checking purity. The result is that the red is desaturated and foggy tomato-soup looking, even at a correct color level setting that shows small red areas in program material correctly. I have had thoughts about removing the caps that pass more AC than DC in the color difference channels, as the color difference outputs seem to have more than enough gain for dynamic scenes. The drawback might be that tracking would be more susceptible to tube differences and drift.

The CTC-7 and such later RCAs with X-Z demods have a DC restoration effect built into the color-difference amplifiers by drawing grid current when the negative horizontal blanking pulse is applied to the cathodes. But the luma channel is still only partially DC coupled.

[old_tv_nut]

Typical luma DC coupling was about 85%, but could vary with contrast setting depending on the circuit design. So, with correct brightness control (black level cutoff) setting for an average picture, a nearly all white picture would clip shadows at 7.5% of the video range, and a nearly all black picture would add 7.5% fog to the blacks.

[Penthode]

Originally Posted by old_tv_nut

Typical luma DC coupling was about 85%, but could vary with contrast setting depending on the circuit design. So, with correct brightness control (black level cutoff) setting for an average picture, a nearly all white picture would clip shadows at 7.5% of the video range, and a nearly all black picture would add 7.5% fog to the blacks.

I went off to study later chassis implementation. The ctc7 thru ctc11 bridged thru the brightness control some of the DC across the 0.1uF coupling capacitor. The ctc20 seemed to go a bit further and the brightness control top leg preceded the 1st video stage cathode follower. That means the black level is being sourced at the detector to provide a higher percentage of DC level with less likelihood of voltage drift.

The CTC4, on the other hand looks to have the highest percentage of DC coupling straight thru.

So it looks as if the CTC5 of all the RCA color roundies and perhaps of any RCA color set has the worst DC restoration. But it looks like can be relatively easily remedied.