TDS-784D Display Failure - Page 1
Forum section: electronics / repair

author: TinkerGDS
author_rank: Contributor
timestamp: July 17, 2018, 09:33:17 pm
content: I just picked up a very nice looking TDS-784D scope. Unfortunately the display does not work. I did a little testing and found something is actually wrong with the A11 DRAM/Display board.I first connected a monitor to the VGA output on the back. Video is good. It passes all of the self-tests and SPC. Next I thought the display was bad. I have an extra, working, CRT and display driver board. I connected that and it still does not work. I have a TDS-784C which I added an LCD screen to. The LCD screen came from NewScope. It's very nice, clean, and crisp. That LCD screen is connected to the A11 board's Analog Devices ADV471KP35 chip via a strange looking socket that snaps to the top of the Analog Devices chip. Another board is then attached to that socket. That becomes the driver for the LCD display. I connected the LCD screen with adapter and cables to the 784D. The LCD screen works.I'm lead to believe now that the CRT display problem lies in the Ti Video Interface chip TLC34075-110AFN or surrounding components. I was hoping someone on the forum has some experience in trouble-shooting the video processor section of the A11 board. I do not have a schematic for the A11. I do have the service manual and followed the procedures for testing the video circuit power. That's all good. Just no signal. Before I start replacing a 100 pin chip I'd like to do some more testing to make sure it's the chip and not a part feeding the chip.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 17, 2018, 10:26:29 pm
content: maybe finding an donnor board in the same series ?? tds754D has a thread here :https://www.eevblog.com/forum/testgear/hackingupgrading-old-scope-(tds754d)/who's been hacked to the 784D? maybe they share the same A11 boardmaybe a 744a tootds 784 wiki :http://w140.com/tekwiki/wiki/TDS784

author: TinkerGDS
author_rank: Contributor
timestamp: July 17, 2018, 11:33:58 pm
content: I'm not sure about using a donor board.Are the A11 Dram/Display boards and Acquisition boards calibrated to each other?Where are the options installed?I don't want to "lose" anything by replacing the entire board.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 18, 2018, 01:47:35 am
content: But if you say the vga output is goodThe A11 board should be good tooThe A30 A31 and A32 boards are the ones who's driving the crt ...Unless some stuff is going thru the A14 bus board and the A27 connector board tooBut they are before the A11 in the signal path.

author: TinkerGDS
author_rank: Contributor
timestamp: July 18, 2018, 12:54:50 pm
content: My initial thought was that the A11 board was good too. But I connected a known working A30 and CRT to the bad TDS784D and it still did not work. Just to make sure, I connected the A30 and CRT from the bad 784D to the good 784D and it worked. Now I know the bad 784D A30 and CRT are good.There are at least two chips providing the video output. The Ti TLC34075 and the Analog Devices next to it. The NewScope LCD circuit is connected directly to the Analog Devices chip, bypassing the Ti altogether. When I connect the LCD to the bad 784D, the output on the LCD is good. There are several circuits involved in the different video outputs. I see that in the TDS784 service manual in the block diagram. I think the Ti chip is only used in the output to the A30 display driver. I could be wrong, but that's why I posed the question here.

author: Johnny10
author_rank: Frequent Contributor
timestamp: July 19, 2018, 01:33:23 am
content: I repaired a non-functioning CRT display on a 784D by changing out CPU board. Similar to your problem the VGA port display worked fine.While researching the problem I believe one of the youtube posters said that the video chip was very static sensitive. And he blew out one by probing the pins??tekhobbycap youtubeNot sure if that helps.I didn't chase the problem board any further after selling the repaired TDS784D.

author: eKretz
author_rank: Frequent Contributor
timestamp: July 19, 2018, 01:58:24 am
content: Probably won't be a big deal if you do lose any options anyway. As I recall they can be easily enabled over GPIB.