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TOSHIBA 28W8DBA green discoloration    
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madaload



Joined: 14 Oct 2008
Posts: 4
Location: London

PostPosted: Tue Oct 14, 2008 6:52 am    Post subject: TOSHIBA 28W8DBA green discoloration Reply with quote

Hi,

I have a Toshiba 28w8dba. Recently I noticed some green discoloration of the opicture. I have tried to bring it down via the Service Menu but it didn’t help. I can make the green stronger but it wouldn’t come down all the way. I tried both GCUT and GDRV.

I am suspecting a dried out electrolytic capacitor that has something to do with the green channel. Can you guys please tell me which capacitor on which board could that be?
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Jesse Mack



Joined: 22 May 2008
Posts: 600
Location: The Last Frontier

PostPosted: Tue Oct 14, 2008 4:16 pm    Post subject: Reply with quote

You need to concentrate on the red and blue if the green is predominate.
Look at the BDRV (Blue Drive) and the (Red Drive) and set the GDRV (Green Drive) at the same setting. The Drive setting is to compensate for a bad Picture Tube. Use the RCUT (Red Cuttoff) and the BCUT (Blue Cuttoff and see if they will come UP to make a gray scale.

Generic procedure for doing this is:
1: Go to a Aux input such as Vid 1 or Vid 2 with no actual picture or input.
2: The picture should be VERY Dark.
3: If it is not VERY Dark turn down the Brightness and make it dark.
4: Adjust the RCUT and the BCUT Down to make the red and blue disappear.
5: Adjust the GCUT so that the Green Screen is BARELY VISABLE.
6: Adjust the RCUT so that when the red comes up you back it back down so that the GREEN is predominate.
7: Adjust the BCUT so that the Screen turns grey.
8: Exit the Service Menu and turn the picture up to normal.
9: If the Low light (Brightness down low) conditions are Grey and white ( with the color turned completely off) and when you turn the brightness up and any color becomes predominate THEN you adjust the DRV control for that Predominate color up or down to make the picture GREY and WHITE.

This just a generic procedure and the exact procedure is much more involved.

Good Luck

Jesse
madaload



Joined: 14 Oct 2008
Posts: 4
Location: London

PostPosted: Tue Oct 14, 2008 4:44 pm    Post subject: Reply with quote

Thank you for your reply, but trust me when I say my problem is at a component level. I’ve been through all the adjustment procedures. The lowest GREEN I can get is still too green. Adjusting RED and BLUE to generate a shade of grey is no good because I only end up with grey not black. Even with brightness all the way down to zero not to mention the colour “mixup” resulting in predominantly RED and BLUE picture under certain conditions. On older TVs (pre 16:9 era) this was easily sorted by replacing one or two capacitors.

Because GREEN can still be adjusted, although not all the way down, I am assuming the IC responsible for colours is working. It must be something in the surrounding circuitry that keeps GREEN above zero level. If I had the schematics I would just replace all the GREEN channel components between the colour chip and the tube board.
Jesse Mack



Joined: 22 May 2008
Posts: 600
Location: The Last Frontier

PostPosted: Tue Oct 14, 2008 7:37 pm    Post subject: Reply with quote

The colors are not channel's. The grey scale is determined by the bias placed on each of the CRT's gun's. The Grey Scale has nothing to do with the video information coming from the broadcast or video source.
The bias is a preset DC level on the cathodes, screens and g2 of each gun.
If you are adjusting each of the guns from within the service menu, and the adjustments are working, (having an effect) you may easily have a CRT that is going bad. The separation of the video information for each color is done at the jungle ic and can be monitored with an O'scope at the CRT PCB assy. The best way to tell if there is any kind of video problem is to put a color bar generator on the set or use color bars being produce by a VCR or DVD. You can then tell if there is any kind of missing information going to the CRT. You could also have the G2 adjustment much too high causing overdrive on the CRT. The G2 adjustment is not in the on screen service menu. It is labeled on the flyback transformer and is usually directly above or below the focus adjustment on the flyback.
You did not say if you did or did not have any retrace lines in the picture. Any excessive drive or shorted components on any of the guns will produce retrace lines if their is any kind of a problem that should be addressed at the component level.
You also did not say how old this tv is. I looked on the toshiba website and it has dropped off of their radar. I also have service literature going back almost 30 years and this model does not show up. Do you have a chassis number to work with??

Jesse
madaload



Joined: 14 Oct 2008
Posts: 4
Location: London

PostPosted: Wed Oct 15, 2008 4:20 am    Post subject: Reply with quote

Jesse Mack wrote:

You also did not say how old this tv is. I looked on the toshiba website and it has dropped off of their radar. I also have service literature going back almost 30 years and this model does not show up. Do you have a chassis number to work with??


The TV was purchased in 2004. I could look for the chassis number if you tell me where to look for it, but appart from the serial number and model number I only found this "28W7" and that the tv was made in the UK. I believe it is the C7SS chassis though(see http://tinyurl.com/4nwatj ). The user manual and the remote got lost while moving from Kent to London.

This is what the TV looks like: http://tinyurl.com/5y9tgp
It is the third TV from the bottom.
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Jesse Mack



Joined: 22 May 2008
Posts: 600
Location: The Last Frontier

PostPosted: Wed Oct 15, 2008 10:49 am    Post subject: Reply with quote

I keep forgetting that the web is worldwide. I did not notice that you are in London. Hows the weather over there. We have about 4 inches of snow and it is here to stay for the rest of the winter.
I am in Fairbanks, Alaska
Now, for the task at hand. I do not have access to the Service literature unless I subscribed to the website that you have given. I do have access to the toshiba website but they list only the sets sold stateside.
However, All of what I have said before still holds true except for the fact that you are on the PAL system of broadcast. We are on the NTSC system and they are not compatable. The differences are for how the color and luminance is processed and the syncronization. The Grey Scale system still holds true. The Grey Scale is what you are adjusting when you adjust the RCUT etc. It is the reference point for the mix of the primary colors that the crt uses to produce a Raster. The raster term is not the same as the picture that is produced. It is the reference point that creats the ability of the tv set to produce a Black and White picture. (funny, isnt it) Without a good Black and White a good Color picture is simply not possible.
That is why you need to follow the procedure in the service manual (which you would have to purchase) when you adjust the screens. (RCUT etc). remember, it is very possible to have a good raster and still have a tinted color picture. (confused yet?) On this set the procedure would most likely have you measuring the DC levels under controlled conditions for each color when you are adjusting the RCUT etc. If you cannot achieve the DC levels then you have to troubleshoot the DC biases for the CRT or the CRT is bad. That being said, it would be prudent to check the crt before proceding with any kind of adjustments.

Hope this helps.

Jesse
madaload



Joined: 14 Oct 2008
Posts: 4
Location: London

PostPosted: Fri Oct 17, 2008 8:16 am    Post subject: Reply with quote

Thank you Jesse. I was hoping this would be a straightforward fix but I'm afraid my TV set needs replacing. I have tried to adjust every color element but for some reason, even if I manage to get a greyscale, under certain circumstances the picture shows either green or red tint.

I've just noticed the green LED that is supposed to be lit when the TV is on only blinks when I turn the tv off completely. Otherwise the red LED is on during STAND BY mode and also when the TV is on. Looks like the TV is telling me it's going to retire soon.

Thank you for taking the time to reply to my questions.
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