Tektronix TDS744 Color Display issue - Page 1
Forum section: electronics / testgear

author: H1t3ck
author_rank: Newbie
timestamp: February 22, 2023, 03:06:03 pm
content: Hi,I just picked up a Tek TDS744 Scope from ebay and it has an issue with the color display. It has a bluish "tint" near the bottom of the display. It is always there. I believe the issue is related to the nucolor display since it is not present in the VGA output.See attached.Wondering if anyone else has had this or a similar problem?Anyone know where I can pick up a schematic for the TDS744?I have a TDS540 and had no problems finding a schematic, but have had no luck with the TDS744.Any help would be appreciated.TIATony

author: AVGresponding
author_rank: Super Contributor
timestamp: February 22, 2023, 04:45:30 pm
content: It'll be an issue with the LCD shutters. If you're really really lucky, reseating a few cables might fix it. Otherwise the best route seems to be to retrofit a VGA LCD screen.

author: nctnico
author_rank: Super Contributor
timestamp: February 22, 2023, 05:03:51 pm
content: That blue bar looks weird indeed. My guess would be a problem with the shutters or the CRT itself got burned in badly.The schematics found in the TDS540 component service manual will also apply to the TDS744 to a large extend. Or put differently: they helped me to work on these kind of oscilloscopes just fine.

author: H1t3ck
author_rank: Newbie
timestamp: February 22, 2023, 07:13:32 pm
content: Quote from: AVGresponding on February 22, 2023, 04:45:30 pmYes, agreed that is what I am thinking. i.e. hoping it's a connection issue in the LCD shutter assembly. I plan to take it apart when I get home tonight.Getting a color display with a single gun CRT display is pretty clever.. Looking forward to checking it out.Thanks!

author: H1t3ck
author_rank: Newbie
timestamp: February 22, 2023, 07:18:41 pm
content: Quote from: nctnico on February 22, 2023, 05:03:51 pmI don't believe it's a CRT burn issue given where it's located and the size of it, also, it would not be a normal area where you would have a constant beam exposure for a long time to cause a crt burn.The TDS540 is monochrome so no schematics for the color display section.Thanks!

author: AVGresponding
author_rank: Super Contributor
timestamp: February 23, 2023, 06:19:45 am
content: Quote from: H1t3ck on February 22, 2023, 07:13:32 pmIt also allows for a simpler, clearer CRT and better resolution, as you don't need a phosphor mask, and you don't have the colour "pixels".

author: nctnico
author_rank: Super Contributor
timestamp: February 23, 2023, 06:26:04 am
content: Quote from: H1t3ck on February 22, 2023, 07:18:41 pmIt is possible that it is caused by a previous defect. Another possibility is damage to the shutter due to sunlight. In some of the scopes I have owned the color display had discoloured spots. Either way, I strongly doubt this is an electronic problem. The shutter drive electronics is just a small extra board in the CRT drive unit. You could retrace and check whether each shutter is driven with the same period / frequency. You likely see a 60Hz signal with a 33% duty cycle on each shutter drive signal as the display runs at 180Hz which divided by 3 colors gives a 60Hz framerate.

author: H1t3ck
author_rank: Newbie
timestamp: February 23, 2023, 03:27:43 pm
content: Quote from: nctnico on February 23, 2023, 06:26:04 amAfter further investigation I ruled out crt burn which I felt was unlikely given the area where the issue is. I also feel pretty confident that it's not the LCD itself.The reason I ruled out the above two scenarios is when I disconnect the shutter driver I was hoping I would get a clean black and white display which I do. See attached pics.The investigation will now focus on the shutter driver board and the signals between the Shutter driver and LCD.It will be a challenge of sorts since the driver board is buried in the HV section and hard to access to measure signals. Thinking of adding a extension and bring the board out so I can make measurements.I am more optimistic than yesterday that it can be fixed. Of course, If it turns out to be the LCD itself then all bets are off since the CRT and LCD are bonded together.Thanks,Tony

author: seronday
author_rank: Regular Contributor
timestamp: February 24, 2023, 04:05:48 am
content: Circuit for colour display and shutter driver if you dont have it already.Regards.

author: argile_tile
author_rank: Contributor
timestamp: February 24, 2023, 10:02:30 am
content: Since white appears in pictures I will disreguard "blue". You don't have a blue screen (crt rgb issue).It appears to be a test pattern. Does it occur during boot? If so then you are "locking up" during boot.Is this a spam post? Posting some weird pattern to stir everyone's heads? You posted a blue pic then later a pic clearly of another color. Wasting readers time?

author: AVGresponding
author_rank: Super Contributor
timestamp: February 24, 2023, 10:42:13 am
content: Quote from: argile_tile on February 24, 2023, 10:02:30 amPlease don't post criticisms here if you have no knowledge of the subject. These scopes don't use an RGB tube; they use a white mono tube with coloured LCD shutters synced to give the illusion of a colour screen. The problem the OP is having is a known issue.

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 24, 2023, 05:29:10 pm
content: 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.Either find another CRT assembly (or cut a good shutter assembly from a bad CRT and replace your shutter) ‐ hardest to source and do.Or replace the whole CRT assembly and PCB with an LCD - easier but expensive depending on if you DIY or buy a kit.Or just leave it and live with it - easiest and cheapest.I keep an eye out for busted up and damaged scopes to strip for spares, _occasionallyyou get one with a good CRT assembly, but it depends on how lucky you are.

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 06:05:37 pm
content: Quote from: H1t3ck on February 23, 2023, 03:27:43 pmThey are bonded but they can be separated if necessary. It's somewhat common for the optical gel between the shutter and CRT to shrink slightly creating bubbles and in that case it can be removed, apparently it makes little difference in the appearance of the display. Hopefully in this case it's just a bad connection somewhere. These NuColor displays are really cool, it's a shame they didn't see more widespread use.

author: jayk
author_rank: Regular Contributor
timestamp: February 24, 2023, 10:19:42 pm
content: I have a listing on ebay for a replacement that might work for you:https://www.ebay.com/itm/165936361403.Not sure how to confirm if it would work with your scope though.

author: nctnico
author_rank: Super Contributor
timestamp: February 24, 2023, 11:21:47 pm
content: Quote from: TERRA Operative on February 24, 2023, 05:29:10 pmI'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.

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 11:24:21 pm
content: 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.

author: H1t3ck
author_rank: Newbie
timestamp: February 25, 2023, 12:06:06 am
content: 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.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.

author: james_s
author_rank: Super Contributor
timestamp: February 25, 2023, 12:53:17 am
content: 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.

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 25, 2023, 06:12:50 pm
content: Quote from: H1t3ck on February 25, 2023, 12:06:06 amWell, I stand corrected. I guess jet lag got me... (I had just traveled halfway around the planet when I wrote my reply...)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.

author: H1t3ck
author_rank: Newbie
timestamp: March 03, 2023, 04:35:27 pm
content: 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 laborI hope this may help others that may have a similar issue(s) in this area.Thanks everyone for your help.Tony

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2023, 06:28:12 pm
content: Nice job, it looks great.