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Red Blue And Green CRT Driver And Cut Off Control Question?

 
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AshTray900



Joined: 03 Sep 2008
Posts: 6

PostPosted: Mon May 18, 2009 7:30 pm    Post subject: Red Blue And Green CRT Driver And Cut Off Control Question? Reply with quote

I have a crt television set that i have been calibrating and have turned up the red green and blue drivers, I am now getting an absolutely great picture when the screen is full of color, I am having a problem with the black level though, in dark areas of the picture there seems to be a greenish color in the picture.

What it looks like is that the green driver is emitting a slight bit more than the other two in the shaded or dark parts of the picture. I should be able to obtain a perfect picture if i can whip that green driver in to place, I want to be able to bottom the lows in the green driver circutry without effecting the highs, is this what the cut off controls are for??

If i drop the driver setting the low light greens fall into place but the intense colors then do not have enough green in them (faces look purpleish)

Guess I could use some advice on these controls. If I had to guess (and hopefully this is the case) I can set max voltage to a gun with its driver control and set the min voltage (or resting state) with the cut off control, is this the case?
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AshTray900



Joined: 03 Sep 2008
Posts: 6

PostPosted: Mon May 18, 2009 7:41 pm    Post subject: Reply with quote

I also notice that if i tone down the brightness the greenish effect is less, but then the picture is not adequately brite.

I know that being a set that is 27 years old the picture tube has a little wear on it which is why i had to crank up the drivers a tad, but now im getting a great picture with exception of the green in the picture.

if you crank up the brightness slightly you see areas of the black level in more detail but the picture looks a little more washed out, but i can get good black levels (with the excepton of a little green) and still have plenty of brightness which would suggest to me that the picture tube while being old is still in decent shape.

also this low level green problem varies sometimes its completely gone and sometimes its there. ive even seen it blink in just a little in the middle of a program. (not a huge problem but somewhat annoying when playing video games that have low light levels) the back is dusty and ive cleaned the controls on the neck board but sometimes thats not enough and a control likes to wander a tad due to dirt and a worn brush, would i be correct to assume that if a control is faulty that it would be the green cut-off?
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minnie



Joined: 18 Aug 2005
Posts: 2877
Location: Hell

PostPosted: Mon May 18, 2009 8:20 pm    Post subject: Reply with quote

Sounds like you turned green too high. Drop the color all the way down and turn the green down till you get a good black and white picture. Red and blue also if need be.
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AshTray900



Joined: 03 Sep 2008
Posts: 6

PostPosted: Mon May 18, 2009 8:45 pm    Post subject: Reply with quote

are you talking driver controls or cut off controls? these arent the same as green red blue settings on a monitor menu, a black and white picture is made out of an exact combination of all three drivers, so if i turn all three driver controls down i still get a color picture just with no contrast whatsoever, and when i crank them all up i get a good color picture with alot of contrast.

i used my dvd player to display a color bar graphic to the screen and all of the color bars couldnt be more perfect with the full spectrum of colors at full intensity, and the grey/black cubes look excellent in comparison.

what happens is when the entire picture is dim, the green takes over just a tad.

im thinking the cut off controls have something to do with this but i dont understand how they work, i can pretty much figure the driver ajustments control the overall voltage to the gun, and when its being drivin hard the color is great. any slack off on the green driver will produce negative results at high intensity

im pretty sure ive got a bad pot control back there thats jumping around kind of like an old radio with a scratchy staticy volume control. I just need a description of how the cotoff controls work to properly diagnose the problem.

I have set the brightness control all the way right and just down a tad, then set the screen voltage to exactly how bright it needs to be, making my optimum brightness about 3/4 into the brightness control.

I moved the keys on the neck of the crt to make each driver by itself true all over the screen.

i then turned the color completely off and set the drivers to give me a nice bright white color bar.

turned up the color to the correct level and set the tint to render the bars the same color they showed up on my computer flatscreen monitor.

overall good picture, acceptable with my cable box that really cranks out a high intensity signal, i made no modifications to the cut off controls because i dont understand them, i basically taught myself how to calibrate this set with some tweaking to the controls i could figure out.

i think im missing a step in setting the set because the cut off controls are responsible for something i just dont know what.

I need less green but only at the low end, so if the cutoff controls could help with this, would i move the green down a notch or move the red and blue up a notch, would i need to start over and set the cutoffs before i can accurately set the drivers?
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minnie



Joined: 18 Aug 2005
Posts: 2877
Location: Hell

PostPosted: Mon May 18, 2009 9:14 pm    Post subject: Reply with quote

Does the set have a menu?
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Tue May 19, 2009 8:12 am    Post subject: Reply with quote

Let us assume that this is a fully conventional set without automatic cut-off control.

Then, if you don't have any service manual telling an exact procedure, do as follows:

Set all the gain (drive) controls equal. On some models, the standard setting is mid-position, on others it is maximum gain.


Set the brightness control to mid (or standard) position and set the contrast fairly low.
Connect a grey scale pattern (no color) to the set.

Adjust the cut-off (bias) for each color so that the black level is about 10 V from black clipping in the output stages. Check with a scope at the CRT cathodes.

Now, with contrast still low, adjust the G2 (screen grid) voltage to the CRT (usually the lower of the two controls on the flyback transformer) so that you just get some weak light of one color in the black bar of the greyscale.
Then, turn the cut-off controls for the two other colors so that those also give weak light in the black bar.

Now, with a higher contrast setting, adjust two of the gain controls so that you get a proper grey scale all the way from black to white.
Check that there is no evidence of white clipping at maximum contrast, in this case you should back off all gain controls somewhat.

If necessary, repeat the cut-off adjustment to get a good greyscale in the darker regions. Remember to always use a low contrast setting when adjusting cut-off and when you are done, check another time with a scope that none of the video outputs is near black clipping. You should also have fairly dim lighting in the room when adjusting cut-off.
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