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Originally Posted by bobgary
Are you just adjusting background and screen controls, combined with convergence, to produce accurate NTSC color reproduction? Or are you working farther back in the color sync and demod-phase inversion stages?
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Let me first say, there is a vintage procedure for adjusting the matrix that requires I and Q signals from a generator. I have not tried this procedure. I do not have a generator that can supply the appropriate signal, although DaveA has kindly offered to lend me his vintage generator, and so I can perform the procedure sometime in the future.
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To begin, color sync and the quadrature transformer and the color burst signal path adjustments must be successfully completed. The set must be able to decode and provide a stable chroma signal to the matrix before it can be tweaked.
Next, purity and gray-scale tracking are set up, basically as published in vintage documentation, which I’ll annotate below, or at least my version. Grey scale tracking is relatively easy to accomplish and has been stable in this CT-100 (B8000194) once set.
Purity setup is similar to early commercial service procedures: move the yoke back, B and G screens down, R screen up full and make adjustments including to the color purity and field neutralizing coils, move the yoke back to standard position, etc.
For grey-scale setup, a CT-100 has RGB screen controls, G and B background controls, and in addition, B and G gain, which consists of a pot on the appropriate 12BH7 circuit boards (not a PC board in the CT-100). These initial adjustments are accomplished with a b&w signal; I simply turn down the Color control. An over-the-air mono transmission (no burst) is hard to find!
I’m just going to copy the notes I made for myself as a guide to this gray-scale task:
CONTRAST AND BACKGROUND DOWN (CCW)
BRIGHTNESS NEAR MAXIMUM
R, G, B SCREEN (GRID 2) SET LOW-LEVEL WHITE RASTER — NO VIDEO
W/MONO SIGNAL SET BRIGHTNESS AND CONTRAST FOR NORMAL
B, G GAIN SET NORMAL-LEVEL WHITE RASTER
SET BRIGHTNESS LOW
R MASTER, G, B BACKGROUND SET TRACK WHITE RASTER LOW-TO-HIGH VIDEO
Once the tracking setup gives a b&w picture that doesn’t add a hue to any setting of the brightness and contrast controls, I crank up the color control and assess the damage.
To review, the purity is set, the grey-scale tracking is set, the color burst amp, color AFC, quadrature transformer, color subcarrier level, burst takeoff transformer, and whatever, are all adjusted BEFORE the matrix is tackled.
There are no matrix controls to speak of. Everything is fixed. During restoration, each matrix resistor was checked and found to be in tolerance. So what gets tweaked and what do you look for? I feed an NTSC color bar into THE SET on channel 4. I do not tweak the screen controls. Ditto the background controls. With a modern dual-trace scope monitoring the green and red 15GP22 control grids and a blue filter to observe the color bars, tweaking consists of a combination of the hue control, the brightness control, the contrast control, and ever so slight touch-up of blue and green gain. I recently discovered that, on this set at least, the fine tuning plays a roll in the matrix drive signals, mostly the blue. There is no red gain because in a CT-100 that gain runs wide open to compensate for the low efficiency of the red phosphor.
[You may recall the earlier scope photos and elation over the red signal being twice the amplitude of the green. That’s because to get the same brightness from the inefficient red phosphor, it is driven with about twice the voltage as the more efficient green (and blue) phosphors. Vintage documentation predicted the modern scope display.]
What I haven’t detailed is how to interpret the scope and filter data into effective tweaking. DaveA has provided a rundown in an earlier post for modern video equipment. Basically with the CT-100, the scope display is used to monitor the amplitude, or height, of the color bars for equal amplitude. Complementing that is the blue filter that’s used to visually balance the intensity of color bars. Blue filter use is a common technique, but it assumes that the matrix for the red and green is okay. I have red and green filters on order. When I get them, I’ll be able to check the matrix for all three colors and tweak as needed.
Of course, all this assumes one is not color blind, although I guess one so afflicted could adjust a matrix fairly well using the scope technique. Sorta thinking out loud here…
The fact is, once set up, any little tweak of any operator control, let alone service controls, will throw off the matrix alignment. That’s why I used the ‘…bucket full of variables’ phrase. It’s frustrating in theory, but in practice, if the ambient light changes and the picture requires more contrast, you can crank up the contrast control a little. Although you are technically no longer aligned, for practical purposes the picture still looks absolutely great. You’ll still have no problem appreciating the wide 1953 gamut.