High contrast = color blowout

[lnx64]

This is a continuation of what I mentioned in my introduction thread here:
http://www.videokarma.org/showthread.php?t=253180

I thought it was just the blue colors, but it does affect red also, but not seemingly green.
(I didn't want to post technical stuff in the introduction forum, as it would be off-topic).

This is the high contrast color blowout issue I was describing there..

Low contrast:
http://img.photobucket.com/albums/v3...owcontrast.jpg

High contrast:
http://img.photobucket.com/albums/v3...ghcontrast.jpg
http://img.photobucket.com/albums/v3...lorblowout.jpg

Convergence or AGC issue? I could be simply overdriving the poor hybrid tube/SS board, but I usually like super bright whites..

[rcaman]

you have a weak crt seen it many thousands of times. it,s like me gettimg old and tired. steve

[zenith2134]

Hi.. Havent seen this posted on VK since it's a commonly diagnosable dying crt.
The infamous Bleeding.
My great grandparents had an old color set that did this. I don't know which because it got thrown away when I was very young.

I believe this happens because the cathode output is getting too weak and is down real close to the cutoff.

[jpdylon]

You see this symptom in solid state sets more than tube. The cathode gets weak, reducing brightness. So the user cranks up the contrast and brightness a little. The smear to the right I believe is actually caused by the driver transistors on the CRT board going into saturation - but I don't remember for sure. More of a tube guy than SS. Never seen this symptom on a tube chassis yet.

[old_tv_nut]

Originally Posted by jpdylon

You see this symptom in solid state sets more than tube. The cathode gets weak, reducing brightness. So the user cranks up the contrast and brightness a little. The smear to the right I believe is actually caused by the driver transistors on the CRT board going into saturation - but I don't remember for sure. More of a tube guy than SS. Never seen this symptom on a tube chassis yet.

Yes, output transistor saturated by overdriving it. When a transistor is driven into saturation, it also stores charge that has to be pulled out before it can be turned off again - hence the delayed transition.

Could be weak picture tube or just a design that doesn't limit the drive levels to prevent this. You could try adjusting the cutoff (G2) voltage, may find a compromise of G2 and brightness control. Or just turn the contrast down some.

[lnx64]

Hmm, being a hybrid design, im not too sure what drives the CRT directly.. It looked like it was tubes though, but other parts are solid state also..

I'll need to see what I can adjust or replace.

[andy]

...

[sampson159]

we called this "firing up" back in the day.usually a weak crt would cause this.also as mentioned about drivers.customers would turn the contrast up full and watch away,24-7

[lnx64]

I think it has to do with the circuitry, not the CRT, because on the original SS board, it didn't do this.

Unless the hybrid board wasn't designed to be run on this CRT? Which is a possibility actually.

[compucat]

It is a weak CRT. I had a 1989 solid state Zenith console that after about twelve years or so of use as my living room TV the picture got this red smear. The only thing I could do to compensate is turn down the brightness and contrast controls. It is a tired tube, nothing more.

[ChrisW6ATV]

Originally Posted by lnx64

I could be simply overdriving the poor hybrid tube/SS board, but I usually like super bright whites..

You may already know this, but "super bright whites" is a good way to make a CRT die relatively young.

The high-voltage power supply in that set is being overdriven in the high-contrast picture for sure; compare the "TV Guide" logo in the lower right in that picture to the same logo in the low-contrast picture. The distorted logo is the sign of the power supply driven into non-linearity. Another common sign of overdriving is channel logos (those "bugs" in the lower corners of many TV channels) that change position depending on the video content elsewhere on the screen. Crank the contrast down and that problem often goes away.

[Geoff Bourquin]

I often "fixed" that by turning the brightness down to almost minimum then running the screen control up as high as required to get acceptable brightness. Sometimes you get retrace lines before the brightness is good enough. In that case you're "circularly inclined planed". In any case, this is a bandaid approach to a dying CRT and wont last too long. I always suspected pushing the tube that hard I was accelerating the demise of the CRT, but if the alternative is taking it out back and shooting it , it was worth a try.

[radiotvnut]

Those horrible '90's Zenith CRT's would do this, among other things that are related to a bad CRT. I've done all sorts of things with the adjustments on those sets in order to squeeze a little extra life out of the tube.

[mbates14]

Ya, well in my experience on 90s zeniths, the colors would go "watercolor" and flash different color rasters as the tube warmed up.

[holmesuser01]

Has the OP tried adjusting the focus slightly? I've seen much worse than this.