The CTC-5 Contrast Control Modification

[Penthode]

Originally Posted by NewVista

Or does it push operating range into non-linear, compressing or expanding blacks?

If the RCA notes on setup are followed, the video amplifier will always operate in the linear region. The screen adjustments affect the operating point of the CRT and the setup procedure is well defined in the CTC5 notes for an unmodified set.

[oldtvman]

Keep in mind these early sets were never known for a bright picture, in fact people who had early color sets had to close curtains or blinds in the day in order to see the picture. I never run my oldies at max contrast, you'll shorten the life of your crt and your missing the great color these sets were capable of producing. It still amazes me that some of the younger guys try to get these oldies to run with the digital sets as far as picture quality when you talking about apples vs oranges as far as the technology used back then

[Penthode]

Originally Posted by oldtvman

Keep in mind these early sets were never known for a bright picture, in fact people who had early color sets had to close curtains or blinds in the day in order to see the picture. I never run my oldies at max contrast, you'll shorten the life of your crt and your missing the great color these sets were capable of producing. It still amazes me that some of the younger guys try to get these oldies to run with the digital sets as far as picture quality when you talking about apples vs oranges as far as the technology used back then

I agree that you have to watch the set in diminished light. The same is with my older black and white sets. The picture is great with the contrast and brightness not too far advanced.

I think I am correct however in asserting that the CTC5 dc level passage is not bad if the contrast control mod is undone. I feel the mod makes setting the correct brightness and contrast easier (no interaction of controls) but at the expense of the video dc component.

I would like anyone to dispute this.

[NewVista]

Looking at the design of other brands in the sixties, I see they avoided the
RCA bias-changing contrast control opting for variable AC-bypass as in Mod.
But brightness method was still essentially RCA-inspired with either variable
cathode or grid bias from control.

[NewVista]

Originally Posted by DaveWM

as long as we are talking about the mod, what the 22pf removed from the plate to screen and the change in the 330 to 390pf all about? c404 and c410

The 22pf would help avoid RF oscillation & 390 would provide same level of video peaking as before Mod?

[frenchy]

My CTC-5 is unmodified and I agree it's harder to avoid blooming, but then it is a very early color set where they hadn't worried about keeping that in check as much as later on. Plus at least I've never had retrace lines showing up at really high levels like my later sets can tend to do. Hah maybe that's because the blooming will kick in before that point anyway. : ]

I usually seem to end up with the brightness cranked pretty high, then keep tweaking the contrast as high as I can get it without getting bad blooming on a white screen. That will usually nix any isolated blooming in a band like if there is a lot of pure red on the screen. I'm smart enough to watch this set either in the dark or very low light level so not really a problem for me.

I don't think I've yet seen the complete schematics for the mod but perhaps I did and lost them, can't remember! Does anybody have a copy of them? Was this a mod specified by RCA themselves or just some outside sources?

[bgadow]

This is from a magazine article:

[frenchy]

Thanks, I don't believe I've seen this before.

Oh, and...

1. The 390 cap and ground are what got chopped off at the right of the diagram, correct?
2. Text says 1000ohm and contrast are to be grounded but I don't see that ground connection in the diagram, I guess that is to be assumed from the text (?)

[Penthode]

As I said, I undid the modification to see how the CTC5 would perform.

The downside of the original design is that adjustment of the contrast and brightness controls is a juggle.

The upside is that the dc component performance is better.

The degenerative capacitive cathode bypass means that when the contrast control is advanced, the dc component is reduced in ratio. Hence with average picture level changes, the absolute black level is lost.

I am content leaving the set as originally designed. However, the background and screen level adjustment should rigorously follow the RCA notes for the unmodified set.