Tek TDS540 CRT to LCD conversion experience

[TimInCanada]

I couldn't fix the display on a TDS540, so converted it to an LCD.  This scope has a monochrome display, but the approach might be workable on the color models as well.

Converting to an LCD turned out to be pretty easy.

The panel I got off ebay is a 7 inch, 800x600 resolution, model A070SN02.  There are also kits for CLAA070MA0ACW panels, which look to have the same specs.  I believe the chip on the LCD controller board needs to have registers set for the particular panel, so I got one of the kits with panel and controller.  Prices are all over the place for the same hardware.  My kit was $32 from a seller with lots of feedback, and it works fine.

There is a metal frame on the scope in front of the CRT that holds what I am going to call the lens holder, a black plastic piece that snaps out.  The back has a foam gasket that was removed.  Here's the front of it with the LCD panel in place:



Black electrical tape is covering the edges of the metal frame of the panel.  The panel is aligned with the hole on the front bezel of the scope, which doesn't match the hole in the lens holder.



I stuck the panel in with what-might-be kapton tape to mock up the mounting, but the tape turned out to hold the panel just fine by itself.  That saved having to design and build a mounting bracket.  The controller board is mounted to the back of the panel with some double-sided foam adhesive tape.

The orientation of this panel is such that it seems to give a better display when looking at the scope from slightly above.  The control buttons that came with the controller (not shown) accesses a menu and the display was set to upside down mode for the image to be right side up on the scope.

This TDS540 (no suffix) doesn't have a VGA connector on the rear panel as later models did, but does use VGA signals for the display.  There is a VGA RAMDAC on the logic board (above the front of the CRT) that has outputs for red, green and blue channels.  Only the red channel is used on the monochrome display.  It's terminated to ground with 75 ohms at the RAMDAC and again on the display board.  There are also HSync and VSync signals sent to the display.

The LCD controller board takes VGA input, and can be connected directly if you want to display in one color (red, green or blue).  Connecting the one video channel to three controller input channels drops the signal voltage too much to display properly.  I tried removing the termination resistors for two channels, but that didn't work.  There is some kind of resistor-capacitor network the signals pass through before reaching the controller chip.  I couldn't find a DC path from the signal inputs to the chip.

So, on to plan B.  The RAMDAC on at least this monochrome scope puts out an identical signal on red, green and blue channels, with green and blue not being terminated or connected to anything else.  The RAMDAC is in the foreground here:



Pin 28 is the right most.  Pin 25 is the red channel, which is terminated then sent to the ribbon cable connector on the left to connect to the display board.  Pins 26 and 27 are green and blue channels, here terminated with 51 and 22 ohm resistors (closest I had to 75), and two wires added to bring these channels to the display bay.

This is the pinout of the display connector:



I made a little adapter board that connects to the original display ribbon cable, my two new channel wires, and the VGA and power cables on the LCD controller.  The LCD panel draws about 330 mA at 12V.  I put in a buck converter module to drop the 25V CRT supply to 12V for the panel.  A linear regulator would work fine, too.

And with some adjustments to the display brightness, contrast, horizontal and vertical positions, here's the result:




Not too bad at all.

Now that the display works, I can go on to fix the problems causing the error messages I can now see... 

[DC1MC]

If somebody ever looks here, the mechanical part is identical for the Tek AWG520, I wish I would have known the LCD model before spending money on 2-3 panels that didn't fit  , I ended as well with a  CLAA070MA0ACW kit that is slowly crawling it's way to me.

 Thanks for sharing this, I'll put a link to it in the AWG520 topic.

 Cheers,
 DC1MC

[eb4eqa]

Very nice, good job!

Roberto

[keystone]

Can I ask you who the seller was on ebay that you bought your panel kit from?  I have a TDS 644A color scope with the blotchy screen that I would like to fix with an LCD panel.  Thank you for the nice post.

[THDplusN_bad]

Fantastic work, Tim - my congratulations! 

It is nice to see another Tektronix scope was saved from getting scrapped.

And thanks for taking your time for compiling your write-up. I am usually feeling too busy for that after I have finished a project - then I find myself looking through my notes and getting lost too often! 

I have taken the liberty and provided a link to the well-known Tektronix group TekScopes (@) groups.io

Good Luck for your ongoing repair. You might have seen this older, related conversation already:
https://www.eevblog.com/forum/repair/tektronix-tds-540-console-port/

Cheers,

THD+N_bad

[TimInCanada]

Thanks for the kind words everyone.  Fortunately this project was simple enough that I could write it up. 

This is the ebay listing for the panel kit I bought.  Looks like the price has gone up a couple bucks.  (Just wait till it's seen on EEVBLOG!)

The outside dimensions on the A070SN02 panel are 154(W)×119.2(H) ×5.08(D) mm.  It fits with room to spare in my monochrome scope that doesn't have a floppy drive, but would be worth checking the space available in the color models and the later ones with the floppy.  I think the -A models and later had VGA ports, so it's just a matter of attaching the LCD controller to that on those models.  A color scope without a VGA port would need some digging in the schematics to see if VGA signals are available.

THD - Thanks for the link.  In fact I've already got a Digikey order queued up with connectors for a console port adapter.  A while back someone on ebay was selling an adapter board that just had two connectors.  It sat on the console port finger connector and the ribbon cable from the scope serial port (if you have that option) plugged into the other connector.  This saved building another UART board.  If it works (or even if it doesn't...) I'll post the results.

Cheers,

Tim

[Tantratron]

Many thanks @TimInCanada for this thread and technical details.

Anybody did have success doing the same with color displays for TDSxxx/C/D oscilloscope or does this only works for mono-CRT replacement ?

Albert

[nctnico]

For the version which have a VGA connector at the back the easiest way is to use a TFT screen and controllerboard with a VGA input. IIRC the versions without VGA connector have these signals available on an internal connector anyway.

[james_s]

Can I ask you who the seller was on ebay that you bought your panel kit from?  I have a TDS 644A color scope with the blotchy screen that I would like to fix with an LCD panel.  Thank you for the nice post.

If the tube itself is ok, you can remove the optical gel to fix the blotchy issue. If you do swap in LCD I'd be interested in purchasing the CRT display as a spare, I think the NuColor displays are really cool, it was one of the big draws when I purchased my TDS700.

As a general rule I don't like LCD conversions but this one does look pretty clean.

[hvontres]

Can I ask you who the seller was on ebay that you bought your panel kit from?  I have a TDS 644A color scope with the blotchy screen that I would like to fix with an LCD panel.  Thank you for the nice post.

If the tube itself is ok, you can remove the optical gel to fix the blotchy issue. If you do swap in LCD I'd be interested in purchasing the CRT display as a spare, I think the NuColor displays are really cool, it was one of the big draws when I purchased my TDS700.

As a general rule I don't like LCD conversions but this one does look pretty clean.

I have a TDS 744a that has blotchy gels, but I thought that those gels are what make the color work in the first place, so will removing them just turn it into a monchrome display then?

[james_s]

I have a TDS 744a that has blotchy gels, but I thought that those gels are what make the color work in the first place, so will removing them just turn it into a monchrome display then?

The gel improves the optical coupling, but it is not necessary for the display to work. I have only seen pictures, but apparently removing the gel has very little effect on the picture quality and certainly it does not result in monochrome. There is a studio monitor using the same technology that does not have gel. The color is created by the LCD shutter, the gel plays no part in that.