Another RCA CT-100 discovery B8001599

[Penthode]

Originally Posted by old_tv_nut

Ditto.

I think black level ("Brightness" control) is a tad high in these photos - an easy adjustment. Things like this can happen if you adjust the set with room lights on and then turn them off to take pics or vice versa. If the brightness control is at minimum, it would indicate that CRT bias voltages need some adjustment.

Also want to suggest taking the time to get the camera lined up dead center to avoid perspective distortion.

Blame it on an impatient photographer! I was busy with other things and did not spend sufficient time grabbing snapshots.

To digress a little bit, I am impressed with the setup procedure of the CT100 however. Because the matrixing is external from the CRT and R, G and B are DC restored immediately before the signals are applied to their respective grids, the setup was simple and effective:
1. Set Color Control to minimum. With brightness control maximum and contrast control minimum, adjust screen controls for a barely viable gray raster.
2. With a bright image, adjust blue/ green gain for white highlights.
3. Adjust blue/background controls for gray in low levels.

I found after this adjustment, the resultant brightness range was ample and correct and the grayscale very good.

Further digressing, with good DC restoration I feel I now have a better understanding why later monochrome sets had little or no DC restoration. With blacks firmly clamped, if the CRT limited beam current and/or high voltage supply is not up to reproducing high average picture luminance, picture blooming results.

This and other CT100s I have seen appear very susceptible to this. You may set to haved good overall contrast and brightness on an average picture when a bright overall scene will cause the image to bloom out of focus and disappear. Watching in a darkened room with lowered brightness and contrast is fine.

It is amusing to reflect on a an article I saw in "Wired" regarding the anniversary of the launch of the CT100. It included a republished current (1954) review of the CT100 and the reviewer complained about the soft and fuzzy picture. No doubt in comparison with a bright 21" monochrome set of the same period, the viewing of the CT100 must have been in less than ideal conditions with the brightness and contrast too far advanced!

I am glad therefore I have the opportunity here to reduce rather than increase the brightness (black level).

[dtvmcdonald]

Originally Posted by Penthode

Further digressing, with good DC restoration I feel I now have a better understanding why later monochrome sets had little or no DC restoration. With blacks firmly clamped, if the CRT limited beam current and/or high voltage supply is not up to reproducing high average picture luminance, picture blooming results.
.

I think it actually is something else. One: cheapness. Two: customer complaints about dark pictures.

Look carefully at both the CT-100 DC restorer circuit and the results on screen.
It does not do full DC restoration .. by intent! Its, if I remember rightly,
somewhere between 75 and 85%. This produces a bit extra brightness on
dark scenes, which the customers actually liked better. In fact, **I** still like it better, even in my dark room.

Of course, back in the day at least for live TV ... there were no dark pictures! Producers made sure that their shows didn't have any, and TV stations avoided black screens like the plaque. I supposed telecined movies did get dark scenes through though.

[Electronic M]

Originally Posted by dtvmcdonald

I think it actually is something else. One: cheapness. Two: customer complaints about dark pictures.

Look carefully at both the CT-100 DC restorer circuit and the results on screen.
It does not do full DC restoration .. by intent! Its, if I remember rightly,
somewhere between 75 and 85%. This produces a bit extra brightness on
dark scenes, which the customers actually liked better. In fact, **I** still like it better, even in my dark room.

Of course, back in the day at least for live TV ... there were no dark pictures! Producers made sure that their shows didn't have any, and TV stations avoided black screens like the plaque. I supposed telecined movies did get dark scenes through though.

ISTR some soap (dark shadows?) from the 50s had black backgrounds and would illuminate the characters and maybe the table they were sitting at to make it look like they were floating in the void.

[old_tv_nut]

Originally Posted by dtvmcdonald

Look carefully at both the CT-100 DC restorer circuit and the results on screen.
It does not do full DC restoration .. by intent! Its, if I remember rightly,
somewhere between 75 and 85%. This produces a bit extra brightness on
dark scenes, which the customers actually liked better. In fact, **I** still like it better, even in my dark room.

The CT100 DC restorer discharge path is 1M ohm, and the charge path looks like about 10K ohm, which appears to be 99% DC restored to me.
I stand ready to be corrected if someone can detail how this circuit could produce only 85% restoration.

Later sets with luma drive to the cathodes typically had deliberate ~85% DC coupling, which Consumer Reports complained about for years. Later solid state sets, especially those with video analog ICs and auto tracking, had 100% DC restoration, which stopped the complaints.

The CTC-5 super chassis (the cost-cutting disaster) had less than 100% DC coupling in the color difference outputs also, resulting in reduced saturation in an all-red image, for example. This was probably done to reduce tracking shift when replacing demodulator tubes.

[old_tv_nut]

In the early days, the limited dynamic range of the gamma correction in the camera, combined with some judicious black clipping, combined with the limited contrast of reflective CRT screens, could be and was used to produce the "subject in front of black void" effect. There were some puppet performances on TV (like acts on the Ed Sullivan show) where the puppeteers were in scene but covered in black, and the video engineers just clipped them out of existence.

This became much less possible with the advent of Plumbicon cameras with wide dynamic range including gamma correction down into the shadows. Current solid state cameras for HD and cinema have made the shadows even more clearly visible.

I saw a talk online by a well known cinematographer who said that with film they had figured out the lighting and exposure to give the director either visibility or invisibility in the shadows, but with current digital cinema cameras, they always see into the shadows. They tell the director what he wants to hear, and then achieve the desired effect in post production.

This was illustrated in the live NBC production of Sound of Music, where they had a transition from one location (set) to another by actually dollying the camera through a door with a dark surround; but instead of seeing a black void surround, you could make out details of the studio/set wall.

[etype2]

Originally Posted by old_tv_nut

In the early days, the limited dynamic range of the gamma correction in the camera, combined with some judicious black clipping, combined with the limited contrast of reflective CRT screens, could be and was used to produce the "subject in front of black void" effect. There were some puppet performances on TV (like acts on the Ed Sullivan show) where the puppeteers were in scene but covered in black, and the video engineers just clipped them out of existence.

This became much less possible with the advent of Plumbicon cameras with wide dynamic range including gamma correction down into the shadows. Current solid state cameras for HD and cinema have made the shadows even more clearly visible.

I saw a talk online by a well known cinematographer who said that with film they had figured out the lighting and exposure to give the director either visibility or invisibility in the shadows, but with current digital cinema cameras, they always see into the shadows. They tell the director what he wants to hear, and then achieve the desired effect in post production.

This was illustrated in the live NBC production of Sound of Music, where they had a transition from one location (set) to another by actually dollying the camera through a door with a dark surround; but instead of seeing a black void surround, you could make out details of the studio/set wall.

This subject is interesting. I don’t know if you have watched recent highly touted movies in the past few years that are of interest to this viewer, but there seems to be a trend with flat, very dark, low color content films. I read that this is the desire of the Director who wants to set a moody foreboding atmosphere. The new Bladerunner movie was particularly disappointing along with a recent chain of movies with dark, almost colorless scenes throughout. I may be wrong, but is this an excuse for Directors to use this trend as shortcuts in cinematography? I’ve spent hard earned money to purchase equipment capable of bring out the best in films. It’s not my equipment to blame, it’s important to me that all is properly set up and calibrated. A few movies come to mind, “Mama Mia Here We Go Again”, Brigerton are noteable stunning exceptions and others.

I want to see well calibrated color and excellent shadow detail in movies and content.

[old_tv_nut]

Originally Posted by etype2

This subject is interesting. I don’t know if you have watched recent highly touted movies in the past few years that are of interest to this viewer, but there seems to be a trend with flat, very dark, low color content films. ...

Preceded by the cyan/orange look, a fad and crutch for lack of real creativity in most cases. It's never a green/magenta look, because that is too unacceptable as a possible range of natural lighting color.

As far as dark scenes go, having the action mostly invisible most of the time seems stupid to me. Brief clips of unrecognizable motion to convey the frenzy of a scene are fine, but they live by contrast with the recognizable parts as far as I am concerned.

With the excellent contrast range of LCD and other current displays, the content can have dim but recognizable scenes for night, just as movies could be dark for night scenes in an unlit theater. This was not possible with a CRT in a normally lit room, so good practice was to brighten the dark scenes when transferring to video.

I recall clearly when local stations would run 16mm copies of movies that had no such adjustment. The dark scene of a train in the night was reduced to seeing only the round circle of the headlight, and multiple fading ghosts of it as well due to the lag of the vidicon film chain.

For a while, Kodak provided a special color print film that had a deliberate contrast-reducing fog in the shadows. It looked washed out to the naked eye, but reducing the black level in the telecine brought the contrast back to normal while reducing lag and loss of shadow detail.

[old_tv_nut]

Note on the TK-41 image orthicon color cameras:
The gamma correction was a three-linear-segment approximation to a power curve, so it was definitely too shallow at the dark end to compensate for the real CRT power curve, resulting in compressed shadow contrast. Also, it approximated a gamma of 1.7 instead of the nominal gamma of 2.2, resulting in increased contrast and color saturation. This was the same trick as applied in earlier color reproduction processes like color photography and printing, but resulted in exposure level being critical, with set design color selection needing a restricted contrast range for pleasant results, and the occasional appearance of bright white shirts as somewhat fluorescent and glowing. White clothing sometimes was dyed light gray to compensate, or performers would sometimes wear light blue shirts.

[etype2]

I hope we will see additional screenshots from this set. Particularly shot in a darkened room, with the image filling in the entire field of view.

[dtvmcdonald]

This thread about brightness got me off my duff (combined, I might add, with a
fake bomb scare that kept me away from the office) to check if something
in my set's setup had changed to make for dimmer pictures. And since checking
requires a scope, I checked the DC restoration, at least on the red.

It turns out that my occasional tweeks of the gray scale had got my marks
for the brightness control a bit off, but every other adjustment
was spot on. Fixing the "normal" settings gives a picture that is 100% watchable
on a day like today ... overcast but with the window shades open.

And yes, the DC restoration is in the 85 to 90% range. That's measuring how much the sync tip level
at the red grid changes divided by difference in grid voltage between full white screen and
deepest black blanking.

When my set was first working it did bloom, but now it never ever does. I
don't know why. It could be the parts replacements in the HV section, including
regulating the focus voltage with a string of Zeners. Or it could have been
the setting of the range of the brightness control. I'll never know.

[etype2]

Originally Posted by dtvmcdonald

This thread about brightness got me off my duff (combined, I might add, with a
fake bomb scare that kept me away from the office) to check if something
in my set's setup had changed to make for dimmer pictures. And since checking
requires a scope, I checked the DC restoration, at least on the red.

It turns out that my occasional tweeks of the gray scale had got my marks
for the brightness control a bit off, but every other adjustment
was spot on. Fixing the "normal" settings gives a picture that is 100% watchable
on a day like today ... overcast but with the window shades open.

And yes, the DC restoration is in the 85 to 90% range. That's measuring how much the sync tip level
at the red grid changes divided by difference in grid voltage between full white screen and
deepest black blanking.

When my set was first working it did bloom, but now it never ever does. I
don't know why. It could be the parts replacements in the HV section, including
regulating the focus voltage with a string of Zeners. Or it could have been
the setting of the range of the brightness control. I'll never know.

Sounds like you have it nailed, post a few current screenshots.

[dtvmcdonald]

Originally Posted by etype2

Sounds like you have it nailed, post a few current screenshots.

No need to post more screen shots. I've posted plenty. The current ones
would be identical to the last ones.

That includes my avatar, which was taken on an earlier CRT. The original
file is bluish.

[Penthode]

Sorry for the silence. Other projects have been going and I wanted to think through some of the issues. New photos will follow soon.

If you recall the speaker was badly rusted. I took it apart cleaned it and reassembled it. It works fine except that it appears to have lost some of it's sensitiveness. I tested by substitution another speaker and suspect the problem is the pemanent magnet is not as strong as it was. I am looking for a local substitute.

The AGC overload latching problem: Recall with the RF modulator fed straight to the CT100, the RF/IF amplifiers would tend to latch in overload. The CT100 performed better following the suggestion to attenuate the RF signal. Studying the circuit, I have pondered the value and placement of the 4.7Mohm resistor. With six stages of ampligication, the video level at the 2nd detector is approximately 7 volts p-p. There is a lot of gain and the 4.7Mohm resistor was intended to positively add to the AGC line to no doubt improve S/N performance. My contention is that the major noise contributor would be the RF stage. So I wonder why the resistor is situated prior to 1R132 (560kohm)? Certainly moving it to the point indicated by the arrow will delay application of the AGC voltage until after the the IF stages have been applied sufficient negative bias to prevent overload.

2R176 (4.7Mohm) should be raised to at least 10Mohm or more if it is moved to prevent front end overload. Alternatively, removing the 4.7Mohm resistor altogether demonstrated that with full signal, the agc cicuit appeared to work better with no overload latching. I want to compare S/N in each situation.

I suppose this is less of an issue now that the analog broadcasts are no longer around. It will be a purely academic investigation.

The HV power supply wilting: when attempting to advance the brightness control results in peak white pictures blooming and going out of focus. I tried monitoring both Brightness and Focus voltages as the average brightness level rises. Both the nominal 5000volts focus and 19000volts second anode take a dive. This suggests the flyback supply is not up to the job of maintaing a constant voltage as the beam current demand increases. Is this normal or is there a fault which affects efficiency?

Other than that, I see quite a bit of pincushioning of the image.I would attribute this tithe geometry of the 15GP22: The display panel inside the tube is very flat hence the scanning radius will induce pincushioning. Not a fault. I only mention this as an observation.

[etype2]

Couple of thoughts.
I think modern signal feeds into your CT100 might cause overload compared to the old analog days of 1954 and perhaps the set was designed in this way.

Would like to see a photo of the pincushingion effect for comparison purposes.

[Tim]

The blooming and out of focus with high brightness happens on my set as well. I have heard it mentioned before by others and I noticed it on one of the sets at the ETF museum also, so it is not unique to your set.