Author Topic: Tektronix THS720P Bad Screen / LCD LM32P10  (Read 6439 times)

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Offline rvalente

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Tektronix THS720P Bad Screen / LCD LM32P10
« on: August 28, 2015, 02:52:26 pm »
Hello Mates,

I've scored this THS720P, few years ago for a bargain, at the time the LCD was just fine, but in 6 months or less the LCD went really bad, unreadable, but working.

Is it a delamination problem? I may see few cracks in the back, maybe the light diffuser?

The Display seems to be a Sharp LM32P10, but its really expensive, 170USD (and I think if I'm not gonna have the same problem in the near future). eBay auction: #http://www.ebay.com/itm/281481345695?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Anyone experienced the same problem? Suggestions? Alternative screens? Mods?

I really enjoy the fact of this scope be 600V differential and portable!

Greetings, Renan!
« Last Edit: August 28, 2015, 02:53:57 pm by rvalente »
 

Online PA0PBZ

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #1 on: August 28, 2015, 08:35:03 pm »
Take it apart and see if you can find anything weird with the different polarisation layers.
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Offline HighVoltage

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #2 on: August 28, 2015, 08:54:17 pm »
I have a few of these scopes and never had a problem with the screen at all.

Yes, I would also suggest to take it apart and look around for some problems.
They are easily taken apart and the service manual is available online:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/THS710,20,30.pdf

There are 3 kinds of people in this world, those who can count and those who can not.
 
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Offline rvalente

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #3 on: August 28, 2015, 09:45:24 pm »
I'll tear it down, but If I finde anything, what can I do them change the display?

 

Offline HighVoltage

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #4 on: August 29, 2015, 02:55:13 pm »
First lets have a look how the display is inside, probably not repairable but you never know, what you might find.

The THS710A has the same display and for some reasons they are available "broken" for very low cost from time to time on ebay. I bought a broken one once for spare parts but then it was easily repaired and I kept using it.

There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline rvalente

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #5 on: August 29, 2015, 04:08:11 pm »
Sure, I'll disassemble the scope this week and will let you guys know...

Thanks by the support until now!
 

Offline dcmxx

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #6 on: February 10, 2016, 02:42:30 pm »
I own 2 ths720 that had the same problem of yours, i got them pretty cheap with that problem.

the damaged part is the polarizer film in the LCD, might be both (front and back) or just one, you see marks like cracks because the film starts to delaminate from the screen and it seems to shrink.

It's repairable, but not the easiest thing in the world, you have to disassemble the LCD, rip off the old polarizer, clean the heavy adhesive residue, place a new polarizer film, and reassemble.

You have to be extra careful becuase you could crack the LCD in the process. If you do all the process carefully, it's 100% repairable.

I managed to replace the polarizer film in the two lcds, but i misassembled one of them, and ended with one cracked lcd after some time. It's usable but partially.
 

Offline rvalente

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #7 on: February 11, 2016, 01:03:28 am »
Very nice tip! I never tought It would be repairable, since the cost of a new lcd does not worth, the instrument was retired.

i'll try to get some polarizers and to the procedure, thanks!
 

Offline fcuzzocrea

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #8 on: November 23, 2016, 06:27:32 am »
Hi
I am facing the same problem. Assistance told me that the lcd cost 300 euro.  Too mutch.  Where did you find polarizer in order to repair ?
regards
 

Offline jal1234

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Re: Tektronix THS720P Bad Screen / LCD LM32P10
« Reply #9 on: December 16, 2017, 11:42:56 pm »
I just changed both polarizers on a THS720 I got off eBay for relatively cheap. I used one of these heated scrapers off eBay to help separate the polarizers from the glass:

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F322448570325

The adhesive is tenacious, and pretty thick. It took awhile to remove them. Then the adhesive residue was quite heavy. I used a bunch of single edged razor blades to get the remaining residue off the glass. When it was about 99% removed, I used acetone to remove what little was left. If you try to use acetone on the thick residue, it just smears it around, so scrape off the vast majority.

I bought these polarizer sheets off eBay:

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F263269289709

Putting these on it a lot like putting a screen protector on a cellphone; one bit of dust causes a bubble. Fortunately, the adhesive allows them to peel off cleanly. I ended up using 4 of the 5 sheets I got, and I cut two polarizers per sheet.

If the front polarizer is one way, you get white on a black background. Turn it 90 degrees, and you get black on white. Between dust and getting the front sheet polarity wrong caused me to use up so many sheets.

The LCD is super easy to remove. 4 screws in the back corners allows the front of the scope case to come off. Fold down the front metal touchpad plate, and the LCD is exposed. There are no screws holding it in place, just some rubber pads. With the scope face up and in the normal position, simply lift the LCD from the right and fold it to the left. Disconnect the zif ribbon cable from the back, and the connector for the backlight cable. Once you have the LCD out, turn it over, and bend back the small tabs holding it together, and the front and back of the metal case can separate. You should NOT remove the two small screws. They simply hold the plastic cover for the backlight, and don't keep the display from being disassembled. I laid the lcd panel on some microfiber cloths on a flat surface to remove the polarizers. The glass is pretty thin, so you need to be careful not to break it.

When reassembling, make sure that the glass is absolutely flat and not cocked in the metal frame, or it could crack. Note that there are some silicon strips between the glass and front frame on two sides, the sides away from the circuit boards. They keep the glass from touching the frame.

The new polarizer sheets are slightly thicker than the old, but work great. Note that there are plastic protective sheets on both sides of the new polarizer that need to be removed, one over the adhesive. Don't remove the other sheet until both polarizers are installed, and all bubbles are out between the polarizers and glass.

My scope was barely legible even in the dark, and then only portions were readable. It looked a lot like the one in this thread. After replacement of the polarizers, it's like new. A great way to save $175.

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