Tektronix TDS 2002B repair

[CharlieD]

Hello everyone, I buy this Tek scope locally for about 50 USD, it was working but have 2 main problems, besides some minor mechanicals issues with the case and 1 missing knob wich I'll fix easily I guess.

+ The first obvius problem is the display, looks way discolored and weird, I took several pictures of it in diferents contrast settings.

+ The second issue is with ch1 input, when I checked probe compensation, it measures a 1 Khz signal, but with the waveform showed in the pic below, I suspect some kind of open component in the signal path, since the waveform look like a capacitive coupling and measures 1 kHz with precision, I took pictures with AC and DC coupling setting and the waveform looks the same, in GND coupling the waveform goes to 0 as expected, the problem is with CH1 since CH2  works without a problem showing a nice and sharp 5V@1Khz waveform. The self test pass with no errors.

Now, regarding the first problem, I took a picture from above since a screw is missing and you can see partially inside, and the CCFL is emitting an orange light.



I was wondering :

a - if the weird look on the display can be fixed changing the CCFL backlight or replacing it with one of those LED kits
b - If it's due some degradation of the LCD itself and has nothing to do with the backlight, because I noticed the LCD modules are quite pricey (and I can't afford those right now, and I guess I can live with that weird look) if this is the case, if anyone can recomend a hack (Display addapt of some sort) or a vendor with better prices, that would be awesome.
c - Has something to do with some fault at  the CCFL driver in the PSU board.

I was also wondering if I can put a CCFL backlight I have from a TDS 220, I haven't checked, but I guess I can directly replace them?


In case no one noticed, I have 0 experience with displays.

Here are some more pics at different settings:

0% contrast



50% contrast



100 % contrast



I must add, the weird orange light on the screen corners that the pictures shows, are somehow magnified, because I can barely see them outside the camera.


For the second issue, I want to open the scope this weekend and troubleshoot the front end, I  believe It could be an easy fix, but I wanted to post it in case somebody has a similar issue and can provide assitance, here are the pics regarding this issue:

Weird waveform measuring 1 kHz ~ 400 mV spike , AC coupling




Same with CC coupling



GND coupling



I'll post some updates when I open and see the guts of this scope, I intend it to use as my main scope since in my country is prohibitive to buy a  decent test equipment, (I am building a workshop step by step, but is really hard, no ebay, no spare parts, no nothing, everything local ) and with all the issues, I consider it a win.

 Excuse my weird spelling, I am probably one of the few (if not the only) Cuban viewer of EEVblog and EEVblogForum users. Thanks in advance !

[BlownUpCapacitor]

The discolouration may be from a discolored back light. Looking at the first image the back light seems to be emitting orangey warm light?

Perhaps it was replaced with some incandescent lamps. 

The discolouration can also be from the mechanical issues putting pressure on the LCD and thus discolouring it.

And yes, the waveforms look like something is open in the attenuates.

Best of luck repairing this scope!

[CharlieD]

Looking at the first image the back light seems to be emitting orangey warm light?

Yes, I haven't opened yet, but I will anytime soon, and post the results.

The discolouration can also be from the mechanical issues putting pressure on the LCD and thus discolouring it.
Best of luck repairing this scope!

I'll check that too.

Best of luck repairing this scope!

Thanks!

[Vovk_Z]

AC coupling looks like 1M/50 R switch/relay is stuck in the 50 R position, so input impedance is too low. Or AC coupling capacitor is broken.  I'm not good at it so it's just a guess.

[晓看风云]

The discoloration of LCD screens is a common aging problem. You need to replace the front and rear polarizing films of the screen, which requires you to watch some tutorials. The waveform distortion of CH1 is relatively simple, you can compare it with a normal CH2 to find out where the problem lies!

[CharlieD]

I hadn't had a chance to update:

-The problem with channel 1 was the BNC that did not make good contact, a little cleaning and a pointed metal element to straighten the contact of the center conductor, which was somewhat open, were enough.

As for the display, I replaced the rear and front polarizing filter, and it improved, however, the result pleases me but it is not exactly what I expected, what should be black, turned out to be purple. (In the photo it doesn't look as good as it really is, the color is more uniform in reality)
I tried different types of polarizing filters and even orientations, but the result is similar.

If anyone has experience or knows what can be done or what kind of filters to use to restore the screen to the most original shape possible, I would appreciate it. Greetings to all.

[BlownUpCapacitor]

Have you checked the backlight?

[CharlieD]

Yes. It is ok. Check this thread, an user gave me another explanation about the cause of that color.

https://www.eevblog.com/forum/repair/transflective-lcd-polarizer-orientation/?topicseen