Motorola roundie

[dtvmcdonald]

Originally Posted by old_tv_nut

I'm surprised that only one adjustment was needed.

With the fixed matrix resistors, you can adjust I/Q relative phase and the COLOR control to compensate for errors in one color matrix, say red. Then you still need I and Q gain adjustments in both green and blue to get them perfect, so that's four additional adjustments total.
If you insist on the I and Q waveforms being exactly correct, then you need I/Q ratio adjustments for errors in all three matrices, plus luma vs chroma gain adjustment in two of them, so that's five additional adjustments.
As you say, these adjustments can be avoided by the use of precision resistors in the matrices.

I'm not thinking as hard as I did when I actually did it.

I though about getting the signals right at the CRT.

First consider B&W, color turned off. Brightness sets black level for one color. the CRT brightness controls the other two black levels. Contrast and the blue and green video gains controls the peak brightness of the three CRT guns.

Thus the three B&W matrix resistors are arbitrary but now considered fixed.

There are six color matrix resistors. There are these color adjust controls:
Color saturation, user hue, I-Q phase difference, and I gain.

Hmmm ... unless I screwed up today, it looks like I needed two more adjustments rather than just one. But in any case, adjusting just one
of the matrix riesistors got it essentially correct.

In another view there really need to be two or three more adjustments, but there's no easy way to do these . Those are adjustments for the three gammas of the three CRT channels. Of course the relative settings of the three screens and the two video gains can do a bit of this, as could adjusting the fraction of DC restoration (R249/R273/R274 values, etc.) in each of the three channels (its intentionally not 100%).

Edit: There is one "gamma" tuning that RCA made, and that is the 2.7K resistor in the red CRT cathode. This may also be there
to save it from overdrive in grid conduction ... which is awfully easy to do!

On my set I have added little red and blue masking tape arrows to the brightness, contrast, hue, and color level controls.
The blue ones are my chosen "correct" settings, while the contrast, color level, and brightness have red ones that are placed
so that if I don't exceed them (with the others set at blue) there is never current through that 2.7 K resistor. If there IS current
through it, the CRT red cathode and red DC restorer clip both positive and negative red levels. These red arrows are placed
by using my scope in subtract mode with the DC level and gain of the inverted scope channel so there is zero deflection
with the two probes on the same side of the resistor.