Author Topic: Tek 1502B TDR restoration project and more...  (Read 2746 times)

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Online sorenkir

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Tek 1502B TDR restoration project and more...
« on: September 19, 2017, 12:45:07 pm »
Hi,

I have acquired this obviously non properly functioning TDR with the intention of fixing the display.


The LCD screen is a 256x128 pixels panel from the beginning of LCDs with greenish high voltage backlight and heating system for cold temperatures (as the instrument is supposed to be used in the field).

According to the service manual (easily found on the Internet; the one for the 1502C includes the full schematics, while the 1502B's does not), the 'image bitmap' to be displayed is simply written regularly by the main CPU into a static 4096x8 bit RAM located on the display board. The display processor on its side reads this memory to drive the LCD panel.

My plan was to sample the write operations (Address + Data) to this RAM with a micro-controller and then to drive a simple small color LCD display I have (used in black & white). Then, I could find a bigger display to fit into the instrument.

The first part of the project was OK, and I managed to have a functioning display with some very simple glue logic and a few program lines. The TDR was functioning OK, with a clean trace and the ability to measure cable length was OK (picture below is after the incident):


Then, while probing some connector to search for power supply to power the micro-controller, I think (but I'm not sure) that I accidentally shorted two power pins together.
As a result the TDR then powered-up with an error message:


Also the displayed trace was flat with a lot of noise.


The error message is undocumented, but by using the troubleshooting charts and a USB logic analyser, I identified a defective 74ALS161 counter that I replaced (with a 74LS161, not ALS).
This brought back some life to the instrument: the pulse can be seen, but the trace is very noisy.


When the trace is moved vertically, at some positions, the line is less noisy.


The trace is digitized internally by sampling a 'video signal' with an ADC (top right):


The video signal at the input of the ADC seems OK, so I think the ADC is damaged  :(
The part seems to be available, but quite expensive (around $20), so I want to be sure that my diagnosis is OK...

Now the restoration project has switched to a repair project!
Michel.
 
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Offline texaspyro

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Re: Tek 1502B TDR restoration project and more...
« Reply #1 on: September 19, 2017, 02:32:51 pm »
Tek eventually replaced that first gen LCD with a newer one that did not have the issues of the original.  The newer LCD backlight is whiter than the green one. 

The backlights are an electro-luminescent panel.  I have replaced MANY of them.  I found some EL panels that were longer than the ones Tek used.  You can trim them to fit with scissors (seal the cut edge with kapton tape.

Also,  Tek has (had?) a surplus components store on Ebay that sold off old inventory.   I bought around 100 of the newer LCD assemblies... all they had...  but have used them all.

The 1502B/C sell for stupid prices.  Apparently there are some aviation repair applications that spec their use and are required for compliance reasons.   Hence the demand and people willing to pay big bucks for them.

The 1503B/C uses the same LCD as the 1502...
 

Online sorenkir

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Re: Tek 1502B TDR restoration project and more...
« Reply #2 on: November 26, 2017, 03:13:38 pm »
Hi,

I have replace the ADC (ordered from eBay seller hifiic, $15.50 shipped). And now, the trace is clean:


Step 2 (unexpected fix) completed, now, back to Step 1 (screen replacement).

I have orderer and receive from HAOYU Electronics a 480*272 LCD display with SPI interface and FT800 Embedded Video Engine which might physically fit in the instrument.
The screen is smaller, the frame is a bit too large and the resolution will require some software adjustments, but that's all I could find:


It seems that the frame can be cut/resized:


Now, back to software development...

Michel.
 

Offline BFX

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Re: Tek 1502B TDR restoration project and more...
« Reply #3 on: November 27, 2017, 03:05:15 pm »
It seems to be great project  :popcorn:
I'm waiting for next part ;)
 

Offline tecman

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Re: Tek 1502B TDR restoration project and more...
« Reply #4 on: November 27, 2017, 06:49:17 pm »
First get the 1502C service manual.  It is essentially the same, except for the battery setup.  Next, the display in the unit may be good.  There is a  pot for LCD Contrast (I believe) that often will cure the "blown out" screen look.  I have a 1502B and 1503B and both had "bad" screens.  A simple adjust of the pot brought them back to perfect condition.

paul
 

Online sorenkir

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Re: Tek 1502B TDR restoration project and more...
« Reply #5 on: November 28, 2017, 09:57:55 am »
Hi Paul,
Thanks for the suggestion. I have the service manual and I can tell you that the contrast pot. does nothing on this permanent discoloration (visible even when the instrument is off, see first picture) !

Next part is to make sure that I can write an efficient enough software to drive this new display, which is quite different from the small one I have used for prototyping.

Michel.
« Last Edit: November 28, 2017, 10:00:42 am by sorenkir »
 

Online sorenkir

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Re: Tek 1502B TDR restoration project and more...
« Reply #6 on: December 02, 2017, 07:41:03 pm »
Some progress:
By chance, the 1502B video memory is organized as a L1 bitmap, and the FT800 graphic controller of the LCD screen can display directly such a bitmap.
I just had to double each pixel in each direction to map the 256x128 bitmap to a 480x256 area in the FT800 graphics memory and voilĂ :


Of course, the image is truncated on the right: 480 is not 256x2, so 16 pixels of the original image are lost. But, except on these help screens it is not very annoying (in general, the most right of the trace is not very useful):


Now, I have to work on mechanical integration of the LCD into the instrument.
And a few software optimizations: there are some visual glitches sometimes..
Michel.
 

Online capt bullshot

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Re: Tek 1502B TDR restoration project and more...
« Reply #7 on: May 02, 2018, 06:48:15 am »
Thanks for that idea!

I've replaced the fubar display of a 1502C with an STM 32F469 Discovery board:
http://www.st.com/content/st_com/en/products/evaluation-tools/product-evaluation-tools/mcu-eval-tools/stm32-mcu-eval-tools/stm32-mcu-discovery-kits/32f469idiscovery.html

It has a nice TFT display and was an easy fit into the 1502C front panel (mechanically). The 1502C display / front panel apparently is somewhat different from your 1502B, the LCD controller chip (an OKI MSM6255) is located on the main board and a film cable is used to connect from the main board to the display board (having a lot of column / row drivers on board).

So I've connected the MSM6255 LCD driver outputs to the extension connector of the F469 discovery (only a few signals required, in particular: FRP, CLP, LD0, LD1, LD2, LD3, UD0, UD1, UD2, UD3 and /CS) and wrote some kind of frame grabber SW to capture the display output, upscale and display it.
The CLP frequency is 625kHz, for a modern MCU like the STM32F496 at 180 (or whatever) MHz clock frequency not a big deal, even using pure bit banging techniques.

There are some blank or partially blank frames output by the display controller, causing nasty flickering. I've used the /CS signal to detect the partially blank ones, and a simple SW check to detect completely blank frames, resulting in a clean and stable image.

Didn't take any photos of the mod, and the SW is a quick and dirty hack (re-using a STM provided example).
Safety devices hinder evolution
 
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Offline coromonadalix

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Re: Tek 1502B TDR restoration project and more...
« Reply #8 on: May 03, 2018, 09:57:10 am »
congrats nice job   :-+ :-+
 
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Offline texaspyro

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Re: Tek 1502B TDR restoration project and more...
« Reply #9 on: May 03, 2018, 04:39:31 pm »
If you have the thermal printer for the 1502/1503A/B/C, the drive roller surface like to turn to black goo and run all over the insides and the print head.    I have repaired the rollers by cleaning up all the goo (lots of fun),  putting a couple of pieces of thin heat shrink over the platen rod to build up the diameter a bit, and then using some 3M cold shrink to replace the drive roller surface.   Sorry, I don't remember what part number of cold shrink that I used...
 

Offline her50

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Re: Tek 1502B TDR restoration project and more...
« Reply #10 on: November 05, 2018, 04:41:50 am »
texaspyro

I have a print that the platen has melted also.  Do you know the diameter of the platen before it melted?  Why did you use the cold tubing instead of heat stink?

Harold
 

Offline texaspyro

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Re: Tek 1502B TDR restoration project and more...
« Reply #11 on: November 05, 2018, 06:24:02 pm »
Unfortunately I don't have the measurement anymore.

The cold shrink is rather thick (maybe 1/8 inch) and gives a nice, smooth, level, grippy surface.   Heat shrink tends to be a bit slick.  Also the shrunk surface can be a bit lumpy.

You could try heat shrink.  Add or subtract layers until the paper feeds at the correct rate.  Once that happens you could leave it as-is or measure the "roller" thickness and then re-cover the roller with heat shrink (if needed) and then a cold shrink outer layer.
 

Offline her50

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Re: Tek 1502B TDR restoration project and more...
« Reply #12 on: November 06, 2018, 01:52:41 am »
Does anyone have a reliable source for the roll paper for the printer?

Harold
 

Offline texaspyro

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Re: Tek 1502B TDR restoration project and more...
« Reply #13 on: November 06, 2018, 01:59:44 am »
Any office supply store should have calculator rolls that can be made to fit.   Also check Ebay for rolls for medical EEG/ECG machines.   Make sure they have the "sensitive" side in the correct orientation (in or out).
 


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