TDS Color Series Shutter problem

[mwbarth36]

This is a general question about the operation of the Tektronix 'Shutter' system for producing a color display.  I have a TDS684A that is operating as it should, but the display has an area that is not the correct color.  This area can be seen even when the scope is powered down, and is shown in the first and second photo.  It is an area in the lower left of the display that is a near perfect circle, and has a greenish halo around it.  With the scope powered up and a signal that occupies that portion of the screen, this area displays the signal perfectly, just in the wrong color, as can be seen in the third photo. 

My question is this; I have heard about the infamous 'Gel' between the CRT and the Shutter, but I do not see how the problem described here could be that kind of problem.  On the other hand, I do not see how the Shutter could come up with an almost round area of defect.  Can anyone venture a guess as to whether the problem is in the Gel, or in the Shutter?  If I were convinced it is a Gel problem, I would be tempted to try to remove the gel as described elsewhere in this blog.  If a Shutter problem, what mechanism can cause such a failure?  Any ideas will be appreciated.

[Testtech]

This looks to me like the LCD is damaged. The gel separation looks like worm tracks.

[mwbarth36]

Thanks for the comment.  Sounds reasonable, what do you think would cause such a 'perfect circle' failure of the elements in the LCD?  There is nothing basic in the layout of an LCD array that would lead to a circular failure.  Any ideas?

[David Hess]

I have seen that happen when a fungus started eating the polarizer.

[mwbarth36]

I would guess that there is no practical way to replace the Polarizer................sad, as this scope operates 100% except for the display.

[andy2000]

I have a TDS544a which had a very similar purple spot.  I suspected it may have been caused by pressure on the LCD from the optical gel, so I separated the LCD from the CRT and removed all the gel.  There was an immediate improvement, but it took several weeks for the spot to completely disappear.  It's been almost 5 years, and the CRT is still spot free. 

It's a very delicate (and messy) operation, but doable.  Note that there are two LCD panels with gel between them, and also  gel between the LCDs and the CRT.  I only removed the gel between the LCDs and CRT, but it's possible the other layer of gel could cause trouble.  Later versions of these CRTs didn't have gel, just air gaps.  Although they don't look quite as good, they are more reliable. 

[mwbarth36]

Thank you for your input!  I may just try to see if that is the case here.  It would appear that pressure on a single spot would cause the circular distortion in the color.  It sounds like a very messy job.  Any advice on how to get the Gel off, or how to separate the Shutter from the CRT?  It seems to be fastened with a fairly hard rubber.  Did you have any problem re positioning the Shutter after you removed the Gel?  I would assume the spacing between the Shutter and the CRT is important.

[Jwalling]

Thank you for your input!  I may just try to see if that is the case here.  It would appear that pressure on a single spot would cause the circular distortion in the color.  It sounds like a very messy job.  Any advice on how to get the Gel off, or how to separate the Shutter from the CRT?  It seems to be fastened with a fairly hard rubber.  Did you have any problem re positioning the Shutter after you removed the Gel?  I would assume the spacing between the Shutter and the CRT is important.

Use Goo Gone to remove the gel. Re-attach with an RTV. I use Marineland aquarium sealant.

[andy2000]

I was able to carefully slip a razor blade between the rubber and the CRT to cut the bond to the CRT.  Don't go too deep to avoid scratching the LCD.

To release the gel, I injected some air with a hypodermic needle to break the bond.  One of the corners has 3 small tubes that were used to fill it with gel.  They will be covered with silicone, but you can still tell where they are.  You want to use the one closest to the CRT.  It's a sandwich of 3 layers.  There are two LCD panels, and an outer protective glass sheet. 

[CaptDon]

Those 'round spot' failures are quite common with the older units. The spot starts out small and grows. Often if there is room on the bench folks just use external VGA monitors. They are bigger and better in most cases and a whole classroom can view your waveforms.