Ct-100

[Tom Albrecht]

I'd like to improve the color tracking a little more if I can. What I've noticed is that the color is quite good if the overall scene is dark, but on brighter scenes, the red becomes weak. What is peculiar to me is that the red becomes weaker in a bright scene than it is in a dark scene. For example, if there is some red lettering or a red object on the screen, and the rest of the scene shifts from darker to lighter, the red stuff actually gets dimmer. Likewise, a face that looks normal if the background is dark loses its red content if the background gets lighter, even though the brightness of the face is unchanged.

I'm wondering if there is a problem in my DC restoration circuitry (particularly for red), or if the DC restoration circuitry is having trouble dealing with the difference between the guns in my CRT?

I've got the red screen at max, blue and green screens much lower, and both the blue and green drive below their original minimum settings (resistors modified). The red background with low brightness is too high, even though green and blue background are set at max. In general, the brightness control does not have the correct range -- it basically always needs to be fully clockwise, and still the image is a little dark, although not bad.

Here is the data on the CRT (see pictures below: #1 red, #2 green, #3 blue). All three guns test with very high emission. Key difference is the cutoff bias:

Red cutoff bias: -51.5 V
Green cutoff bias: -66 V
Blue cutoff bias: -67 V

I also checked the screen voltage during the emission test on my CR-70:

Red screen: 259 V
Green screen: 188 V
Blue screen: 225 V

So although all three guns are capable of high emission, there are significant differences between them in terms of bias and screen current/voltage.

Should the set be easily able to handle these differences within normal control ranges?

Any indication of something not quite right in the video amplifiers or DC restoration circuits?

If it's merely a matter of the set not being able to handle the gun differences within normal design range, I'd like to modify things to work better with the CRT in its current state, if I can figure out what would work.

Since many of you have far more experience with color sets than I do, I thought one of you might have some ideas.

[rcaman]

i have a cr70 also and with that much emission your crt is NOT the problem. the sencore testers are spot on as far as im concerned. steve

[miniman82]

Also agree that the CRT is not the problem. I don't recall, have you replaced all the original white inductors in the chassis? Every single one of those was bad in mine, even one that tested good at the beginning of the resto went bad after the electrons started whizzing around. Don't trust them, they are always bad!

Have you tested the 6BC7 resto tube, or done any subbing yet? The schematic doesn't have 'drive' controls listed, so I assume you mean the video gain pots on the little phenolic boards. What are your actual G1 voltages doing with the chassis driving the CRT? If the voltages are wildly different, I'd expect to find trouble in the output circuits of the individual matrix channels. Not saying they are going to be exactly the same voltage, but the fact that you're having to mod things points to circuit troubles. Sams has the inductors on each matrix output at around 5 ohms (L54/55/56 respectively), but they are typically wound on high value resistors because they make a quick easy former. If a coil has opened up in the matrix output, there could still be some voltage getting through on just the resistor. Food for thought, ohm them out to check.

This is very likely a DC voltage issue at the G1 terminals and not a video issue, the fact that the brightness isn't working correctly is a clue. All it does is set the DC offset on the G1's of the CRT, thereby varying their relative intensity. You could look at it as an adjustable cutoff control, if you like. Find your missing DC voltage, you'll likely find the problem.

[Pete Deksnis]

Originally Posted by Tom Albrecht

I also checked the screen voltage during the emission test on my CR-70:

Red screen: 259 V
Green screen: 188 V
Blue screen: 225 V

Tom,

Checked the screens of my 'benched' CTC2 and came up with:

B8000194
G2
R 389
G 344
B 345

As you can see the levels are quite a bit higher. Although there are six feet of extra wire on the 15G, the 'bench' operation is normal and did not require a basic set up. Also, I dug up an unfinished CTC2 setup procedure from 2009 that never got put on my CT-100 site before it went down. Added some stuff and posted it here on AK: http://www.videokarma.org/showthread...98#post3093798

Pete