Products > Test Equipment
Tektronix TDS744 Color Display issue
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james_s:
I think a good place to start would be to swap in a different shutter control board and see if the symptom changes. Or figure out how to drive the shutter segments and wire it up to an external test circuit. IIRC it is split into several bars so it looks like one of the bars is being driven into the wrong state. This is where having a second similar scope would be really handy.
H1t3ck:

[/quote]
I'm not 100% convinced. The image shows what looks like a perfectly shaped bar across the screen. If you look at the schematic, you'll see the LCD shutters are driven from a counter + EPROM to produce the patterns for the shutters. If one of the bits of the counter or one of the outputs of the EPROM is stuck, you could end up with a horizontal bar across the display. This could be an easy fix after all. Maybe all it takes is reseating the EPROM.
[/quote]

You are correct nctnico! 

I was able to look at some of the signals feeding the LCD display earlier today.  (Kind of difficult to access them while the main board is plugged it covering the HV section and the LCD shutter board. 

Bottom line is that I found a missing signal feeding the LCD display pin8 on the connector (BO4). Tracing back I found a DFF that feeds AD12 with no output, the input D3 is ok.  See attached schematic)

So, I have ordered a MM74H374 from ebay which I should get in a few days.

I will report back when I replace the chip.

Thanks everyone for the suggestions and access to the  Schematic which is always very helpful.

It's a very simple circuit, just a pain to access to make measurements.



james_s:
I'll be following this with interest. I find these displays fascinating, it was actually one of the main reasons I bought the specific scope I have.
TERRA Operative:

--- Quote from: H1t3ck on February 25, 2023, 12:06:06 am ---
--- Quote from: nctnico on February 24, 2023, 11:21:47 pm ---
--- Quote from: TERRA Operative on February 24, 2023, 05:29:10 pm ---Yup, that's a bad LCD shutter.

The shutters work as a single pixel the size of the entire screen for each of red, green and blue.
Any partial colour defect is not due to the electronics (which only drive the entire single pixel for the respective colour either on or off) rather it is due to a known problem with old shutter panels that fail as yours has.
Often it is a circular blob that will fail, but in any case it is a terminal fault.

--- End quote ---

I'm not 100% convinced. The image shows what looks like a perfectly shaped bar across the screen. If you look at the schematic, you'll see the LCD shutters are driven from a counter + EPROM to produce the patterns for the shutters. If one of the bits of the counter or one of the outputs of the EPROM is stuck, you could end up with a horizontal bar across the display. This could be an easy fix after all. Maybe all it takes is reseating the EPROM.

--- End quote ---

You are correct nctnico! 

I was able to look at some of the signals feeding the LCD display earlier today.  (Kind of difficult to access them while the main board is plugged it covering the HV section and the LCD shutter board. 

Bottom line is that I found a missing signal feeding the LCD display pin8 on the connector (BO4). Tracing back I found a DFF that feeds AD12 with no output, the input D3 is ok.  See attached schematic)

So, I have ordered a MM74H374 from ebay which I should get in a few days.

I will report back when I replace the chip.

Thanks everyone for the suggestions and access to the  Schematic which is always very helpful.

It's a very simple circuit, just a pain to access to make measurements.

--- End quote ---

Well, I stand corrected. I guess jet lag got me... (I had just traveled halfway around the planet when I wrote my reply...  :-DD )

I actually have some spare CRT PCB's so if you need one, or parts, we can figure something out once I'm back in Japan in a few days. :)
H1t3ck:
SUCCESS!!!!

The Color issue I was having is fixed and the screen is looking sharp...

It turned out to be one of the inputs to a DFF  (D3)  was low impedance and dragging the signal from the EPROM down to under 2Volts which is lower than the threshold for a CMOS High signal. So it affected both AD0 from U505 and AD12 from U510 which are both derived from D3.  See attached schematic with some notes.

This affected BD4 and BD7 driving the LCD, which affected the bottom 1/3 of the display.

Ended up replacing both U505 and U510 74C374 for less than $5 so not a bad deal.  Not counting troubleshooting labor  :)

I hope this may help others that may have a similar issue(s) in this area.

Thanks everyone for your help.

Tony
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