Tektronix TDS 210 & TDS 220 Display Repair


Tektronix TDS 210 with faulty screen
Symptom:
During operation the image on the screen has low contrast and is hard to recognise. There is a circular discolouration / a large dark spot in the centre of the screen.

Problem: 
The polariser films have failed over the years, a growing amount of unpolarised light passes through which reduces the contrast in the damaged area.
Solution:  Replace both front and rear polarisers.

Theory:
LCD Layers
There is plenty of interesting and detailed information about LCDs and how they work but that’s another story. I keep it very basic here: LCD screens consist of a light source, a diffuser, a horizontal polariser film, a glass substrate (liquid crystal assembly), a vertical polariser film and (sometimes) a protective cover.
The essence is in the liquid crystal (LC) glass assembly (2,3+4), which consists of two glass panels and the liquid crystals sandwiched between the two. A network of translucent electrodes is printed on the glass panels and brought to the edge where they connect to a PCB via fine flat cables.
Horizontally polarised light enters the back of the glass panel. As it passes through the liquid crystals where the polarisation is rotated and becomes vertical. Vertically polarised light now enters the front polariser. If the front polarisation is vertical then all light passes through and the display appears bright.
A voltage that is applied to opposing electrodes on the glass panels causes the liquid crystals between the panels to twist the polarisation of the light as it passes through.  Therefore active electrodes appear dark while inactive electrodes and the rest of the display appears bright.
Rotating either of the polarisers results in an inversion of the display, i.e. active electrodes then appear bright while the rest of the display appears dark.
For more details read this interesting article on Wikipedia.

Repair Steps:
I studied a very good instruction by Wenting on replacement of the entire display unit, however, there is an easier and cheaper way around it. Below is a detailed step-by-step procedure of replacing only the polarisers. It requires some skills, common sense and patience.

Disassembly:
  1. Start with removing the power cord, any leads and the knobs.
  2. Fold down the handle on the top and pry out one of the hinge pins to remove the handle. Note the handle must be folded down.
  3. Remove two screws  as shown in photo.
  4. Remove the digital port from the back.
  5. Pull the front face forward from the top and unhinge at the bottom.
  6. Press down the power button and carefully pry the assembly out of the front of the casing
 LCD Removal:
  1. Remove user control PCB on the front.
  2. Locate the flat display cable on main PCB behind, push the rectangular connector lock ring up to release the cable and carefully pull the cable out of the connector.
  3. I didn’t but you may want to remove the light cable connector as well.
  4. Remove the screw on the left of the display and unclip the display from the left.
  5. Pull out the display to the left and be sure not to damage the flat cable as it passes through the casing.
  6.  Remove the metal frame from the display after straightening the clips on top and bottom
  7. Separate the LCD assembly from the back light panel.
  8. Check the diffuser sheets on the back light panel, replace if in doubt.

Polariser Removal:
  1. Place the LCD glass assembly (without the light panel) with a protective paper towel or newspaper on a flat surface.
  2. I used a sharp Stanley blade to peel the polariser from a corner and applied Isopropyl alcohol (window cleaner works too) as a solvent to soften the adhesive.
  3. Remove as much of the film as possible by pulling it until it tears off (as it most likely will). Keep in mind that the glass substrate is very thin and delicate! If it breaks, don’t waste a second thought on it, dispose of the whole unit and have a beer to ease the pain
  4. Use the above solvents to soften the adhesive and carefully scrape the remaining film off the screen.
  5. When finished clean the glass thoroughly and polish with newspaper.
  6. Repeat the above for the other side.
  7. Make sure no dust or fingerprints are left on the glass.

Polariser Replacement:
  1. Polariser film is fairly expensive and it can be tricky to find a local supplier. But an old laptop, computer monitor or broken TV screen works very well as an “organ donor”.
    LCD from an old computer monitor
    Old computer monitor back light panel
    I used an old monitor and peeled off the front polariser film which was pretty easy. With a sharp blade and some alcohol solvent the adhesive softened and quickly let go of the pieces I needed.
  2. Rather than removing the entire film and risking that the glass substrate could break, I cut out and peeled off only the pieces I needed.
  3. Using 2 small sample pieces on LC Glass assembly of display I found that the best contrast can be achieved by cutting the back polariser parallel to the existing edge and the front polariser at a 30° angle before peeling the pieces off.
  4. I then removed the adhesive from the film pieces with Shellite (the above solvents work too) and finally cleaned both sides with alcohol.
  5. While I had my LCD apart I also replaced the diffuser sheets that had become stained and spotty over the years.
  6. Display being reassembled.
    Note the effects of the front polariser film
  7. Instead of using adhesive I simply laid the clean and dust free polarisers behind and in front of the glass substrate and sandwiched everything between the diffuser and the outer protective cover. I guess I could have used 3mm double sided tape on the film edges to keep the film in place and to prevent future dust ingress. That will be something for next time.
Assembly:
  1. The assembly is straight forward provided you remember how it was taken apart; keep in mind that the flat LCD cable can only be inserted on the main PCB when the connector lock ring is pushed up. After inserting the cable secure it by pushing the lock ring back down.
  2. Before putting it all back together test the display and make sure it all works.  
  3. It may be necessary to adjust the contrast before the image becomes visible at first. Simply press the Display button once and then adjust with the two buttons on the bottom next to the display.  
  4. The final result will be slightly different when using a repurposed polariser film that comes from a colour display. Mine ended up being dark blue on yellow instead of the monochrome black on white, but hey… it’s so much clearer than before and it didn’t cost anything!!




Disclaimer: Original image titled "LCD Layers" was copied from this article:
https://en.wikipedia.org/wiki/Liquid-crystal_display.
Photos showing the faulty screen were published by Wenting. My display was worse but I failed to take photos.

2 comments:

  1. Hi Therma-Volt!!!!
    Incredible job!!!
    My name is Diego, I´m from argentina.
    I Have the same problem. Your post is perfect and confirm my theory, month ago I trying to find a info like this!
    At this moment use the TDS is difficult but not imposible.
    Your post push me to do the job!!! tomorrow I will looking for a old LCD, I think in a old table, the screen is IPS display, you think that may work?
    Thank you very very much for share an for your excellent job!!!
    Diego.

    ReplyDelete
    Replies
    1. Also from Argentina, mine looks equal!
      Sebastián
      carozoynarizota@hotmail.com

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