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  #46  
Old 09-02-2016, 10:51 AM
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As stated above, this is due to the video output transistor in that color saturating, that is, driven to the point where the collector-emitter voltage goes to zero or minimum possible. There may be a video drive limiting adjustment somewhere or a bias or G2 adjustment that will improve it. It can occur if the particular CRT (due to either original tolerances or age) requires cutoff voltages that then make the video outputs be run at low collector voltages. Referring to service info that shows the scope traces at the cathodes can tell if the outputs are biased lower than usual even though the video B+ is normal.

[Edit: oops - I just saw your post about G1 being low. This will definitely cause this problem, because the cathodes now must be lower also so that the CRT is not cut off.]
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Last edited by old_tv_nut; 09-02-2016 at 10:55 AM.
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  #47  
Old 09-02-2016, 11:02 AM
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Quote:
Originally Posted by old_tv_nut View Post

[Edit: oops - I just saw your post about G1 being low. This will definitely cause this problem, because the cathodes now must be lower also so that the CRT is not cut off.]
Probably I should have checked this up front, chalk it up to a lack of experience. It also makes sense logically to me because the brightness control has to be in the top 1/3 of its range to produce a picture at all. I guess the low bias on G1 is requiring the video outputs to be driven into clipping to get a picture, and the effect is more noticeable with saturated colors. Will be interesting to see what happens when G1 is at the correct 94 volts instead of stuck at 0. Also not sure why the resistor went open (if it indeed is open and not a shorted G1 or shorted 10k resistor)

As an aside I find it interesting the video amplifiers are just wide open transistor stages with no negative feedback to control gain or improve linearity. This really is an early model solid state design.
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  #48  
Old 09-02-2016, 11:16 AM
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TVs ain't HiFi audio amps....Feedback is rare (ignoring AGC, and sweep oscilators).
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  #49  
Old 09-02-2016, 11:23 AM
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TVs ain't HiFi audio amps....Feedback is rare (ignoring AGC, and sweep oscilators).
How about in more modern stuff based on ICs? Any sort of op amp would
Never be run open loop. I ought to study some schematics of some newer stuff just for education purposes.

This Moto is my first foray into solid state TV repair, everything TV related
I've done prior has been tube based.
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  #50  
Old 09-02-2016, 02:56 PM
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1978 is about my cutoff on how new a set I'll fix. Can't tell ya if and how the newer SS stuff works.
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  #51  
Old 09-02-2016, 02:59 PM
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I think you are more likely to find CRT drivers with feedback in computer monitor designs. Usual TV drivers were class A resistive loads, with large power dissipation. Like tube video outputs, they were run with the maximum gain possible to reduce the need for multiple earlier stages. Transistor sets, especially once video ICs were introduced, still commonly used class A outputs, only now there were three for R,G, and B. Push-pull outputs were also developed ( I remember them in European sets)- much less power dissipation, and less hefty transistors could be used, but the CRT arc protection had to be better to avoid frying the smaller die. The class A outputs would get quite non-linear at high frequencies and large signals due to the capacitive loading of the CRT, but it didn't make much difference for a 3 or 4 MHz bandwidth video signal - just put in emitter peaking capacitance to get enough amplitude and don't worry about the shape of a 4 MHz sine wave. Computer monitors needed more bandwidth, and hence more complex design; also compromises like even more output current and dissipation, less output swing, etc., etc.

The above is my partial knowledge and memory of the status of things in the mid 80s, so I am sure there is more to learn.
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  #52  
Old 09-02-2016, 05:37 PM
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So new resistor installed and the problem is 90% fixed. g1 is at 95 volts as per spec m. Still a bit of bleeding on very saturated colours, like fine block letters on a black background, but the picture is genrally superb. Way better than my RCA CTC38.
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  #53  
Old 02-18-2019, 05:25 PM
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This TV never got a lot of use after I restored it, I couldn't find a good spot for it. Recently though, it has become useful. It has a strange characteristic, the picture begins quite dim and green, and takes about ten minutes to achieve full brightness and good colour balance. At that point though, the picture looks great, and it looks fantastic as long as you want to use it, except for red which is a bit saturated. Is this just how it goes with a weak CRT?

I, believe red is weak, but certainly not so bad I can't live with it. It's sure nice how there's no cataract on this tv!
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  #54  
Old 02-18-2019, 05:39 PM
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Quote:
Originally Posted by maxhifi View Post
This TV never got a lot of use after I restored it, I couldn't find a good spot for it. Recently though, it has become useful. It has a strange characteristic, the picture begins quite dim and green, and takes about ten minutes to achieve full brightness and good colour balance. At that point though, the picture looks great, and it looks fantastic as long as you want to use it, except for red which is a bit saturated. Is this just how it goes with a weak CRT?

I, believe red is weak, but certainly not so bad I can't live with it. It's sure nice how there's no cataract on this tv!
You may be able to improve grayscale by adjusting it well after the 10 min warmup... the rest is standard fare for a weak CRT... you may find balance warmup varies with time between last run. On a roundy I plan to replace it will take around half an hour to balance with a week off... With over 1 month off it will take 2 hours to balance, and start out barely lighting at all.

Cataracts especially the green ones are easy to fix... certainly less hastle than recapping a Dumont, or refinishing a wood cabinet.
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  #55  
Old 02-18-2019, 07:29 PM
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Originally Posted by Electronic M View Post
You may be able to improve grayscale by adjusting it well after the 10 min warmup... the rest is standard fare for a weak CRT... you may find balance warmup varies with time between last run. On a roundy I plan to replace it will take around half an hour to balance with a week off... With over 1 month off it will take 2 hours to balance, and start out barely lighting at all.

Cataracts especially the green ones are easy to fix... certainly less hastle than recapping a Dumont, or refinishing a wood cabinet.
This is the only tv I've done the cataract procedure on, and it looks fantastic. What you describe is what I saw, it hadn't been on in a year or two and it came up very, very slow.

It does balance out well when it's warm, except red smears a bit because it's pinned all the way up. The Motorola chassis makes a wonderful clean picture, very little geometric distortion and great colour. Sounds good too.

I have an RCA tv which has a nos testing 25XP22 in it, which would likely look better with the motorola chassis than it does with the CTC38 it's in. Some day maybe will make the swap - that one does have an RCA style cataract needing repair still though.
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  #56  
Old 02-19-2019, 04:00 PM
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I will update - the two colour controls (tint and hue) make it hard to get it looking right, but once it does, I can say without any hesitation that this is the best CRT TV I've ever had.. The scan lines are razor sharp, there's no noise in the picture, no buzz in the sound. The audio is powerful, the cabinet looks great. It actually has bass, and the slider controls work well. And the geometry is really nice - squares are square everywhere on the screen, and nothing bends or looks funny. I actually find it hard to believe they made such a good TV in 1968, I've seen worse pictures on sets 20 years newer.
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