Tek TDS744

[cncjerry]

Today I powered on my beloved TDS744 (upgraded to 784) scope.  People either love or hate the displays, can't remember what they are called, color planes?  I always loved it, the trace is very crisp, or was until I get to the problem, almost like drawing on velvet.  Lots of great colors.  Love the purple on deep black.

Anyway, I powered it up and all was great.  I checked an oscillator output, took about 5 minutes, and then the display started rolling outages, bottom to top, etc. Lots of colors intermixed and scrambled.  I immediately shut it off, waited a few minutes and powered it up.  Same thing after 3 or 4 minutes.  I'm hoping it is power related, maybe a leaky cap drawing down a regulator until it gives out?

I'm going to open it up, do a visual and check the voltages first, of course.  Anyone run into this?  The display is sharp for 2-3 minutes so I doubt it is that, knock on wood.

Thanks

Jerry
 

[BlownUpCapacitor]

Something wrong with the CRT display section. Use a thermal camera to look at the CRT PCB and see if anything gets hot.

The failure occurring after a couple minutes indicates a thermal related issue.

[TERRA Operative]

First stop, replace capacitors. Bad caps are a known issue in these scopes and caps are cheap, so I usually just replace all of them on the CRT board to eliminate the possibility they are bad.

[cncjerry]

Well, the good news, if you can call it that, is that VGA display output remains proper when the scope display does it thing.  So that means it is in the display, maybe high voltage, if it has such a thing.  I remember hearing about these displays, whatever the are called, color plane? failing pretty regularly.  I'm going to open it up in the morning.

[cncjerry]

Of course I couldn't go to bed without at least looking.  I took the black bezel off, slid the cover off, and you can't even see a capacitor of substance.  There are two major mother boards top and bottom over each other.  The screen is fully enclosed in a metal housing.  With a flashlight, I can see in thru the fan, that alone is about 6" across, and what looks like a high voltage supply is tucked in a cage on the back of the tube.  So I have to see if there is a manual online and if not, start unscrewing things taking pictures along the way.

More to come.

Jerry

p.s. want to bet the front trim breaks: When removing the trim ring, grasp its back edge and vigorously flex it upward before pulling it forward.

[TERRA Operative]

To acess the CRT board, you need to remove the centronics/serial port PCB,  then unplug all the cables from the top processor board (Take note of the jumper PCB on the right side! Draw an arrow pointing up on it. If you plug it back in upside down the scope won't be damaged but it simply will not boot!)
Then remove the top processor board.
Then remove the top plate
Then unplug and remove the CRT
Then you can remove the CRT driver board.

While you are in there, replace the Rifa caps and C17 on the PSU PCB (if it is a revision with these parts). The need to be replaced so you don't blow up your PSU.


(Click to embiggen picture)

[PartialDischarge]

Most probably is a leaking cap in the CRT board, something similar happened to mine. Since I did not want to spend time repairing it what I did was spray the board with plenty of water/alcohol. This was in a hot summer so it self-dried pretty quickly, problem solved.

Next year the power supply board failed, and didn't want to spend time repairing it, but the previous post probably hits the nail.

[cncjerry]

I found the limited service manual but will take the approach above to get to the PCB.   The fact that it runs for about 5 minutes, is hopeful, I hope!  I'll try to get in there today and get it apart.  Every bolt on the thing is torqued to the max.

Thanks for the input.  I know people with the TDS500 series had a nightmare with caps.  Nightmare as said by Inga in "Young Frankensteeen."

Jerry

[shakalnokturn]

It's a small electrolytic towards one of the front corners of the display PCB that leaks onto some high value resistors used for the CRT bias.
Can go dark or too bright depending on the weather 
Easy fix once you've pulled the PCB out without breaking anything, I recommend lifting / replacing the contaminated resistors and cleaning the PCB in that area.

[yanhanxing]

(Take note of the jumper PCB on the right side! Draw an arrow pointing up on it. If you plug it back in upside down the scope won't be damaged but it simply will not boot!)
Thank you for your post. I did make the mistake you mentioned. I planned to remove the monitor and replace it with an LCD, but when I tried to install it again, it kept prompting ". c" or "e". This problem has been bothering me for a long time. After reading the forum, most of the descriptions about this fault point to an NVRAM fault. I have already removed the battery and measured that the voltage was actually 3V. Then I saw your post's prompt. Thank you again.
hope this find could help other friends