Tektronix 2445A Display jitter and offset

[theworldbuilder]

I was wondering if anyone could help me out with an issue I'm having.
Perhaps it has been seen before??

My 2445A starts running fine. The display and everything is okay.

After a short period of running between 10-20 minutes the screen will start to become jittery and then suddenly the display will disappear with a faint glow on the left side of the screen. If you adjust the horizontal position of the waveform to be as far right as possible sometimes the end of the waveform will just appear on the left hand side of the display.

Sometimes this issue can be resolved by pressing the "beam find" which will display a perfectly centered small version of the image. The image will then return to a normal or jittery state in the center of the screen.
More often though, "beam find" will display a perfectly centered small version of the image but after releasing the beam find button the image will return to the left side of the screen.

Update:
I have checked all the voltages on J119 and all of them check out as okay both before and while the error is occuring.


I have done some reading through the service manual found here:https://www.tek.com/manual/2445a-and-2455a-service-manual
I've been trying to figure out what the beam find button does in the circuits operation but I can't find anything to explain it. I'm doing this because beam find seems to be able to recenter the image without issue and I'm hoping that it might point me in the right direction for a fix.

I'm afraid that this may be an issue with U800 which is a custom chip or an issue with the high voltage supply for the CRT.

Has anyone any experience with such issues or could point me in the direction of where to check?

I have fairly good experience with electronics so I believe I can complete a repair I just need help trying to track down the source of the problem.

[pbarton]

Do the problem symptoms disappear for a while, if you squirt U800, with an aerosol freezer?

[David Hess]

Sometimes this issue can be resolved by pressing the "beam find" which will display a perfectly centered small version of the image. The image will then return to a normal or jittery state in the center of the screen.
More often though, "beam find" will display a perfectly centered small version of the image but after releasing the beam find button the image will return to the left side of the screen.

...

I'm afraid that this may be an issue with U800 which is a custom chip or an issue with the high voltage supply for the CRT.

Has anyone any experience with such issues or could point me in the direction of where to check?

The good news is that I know of at least one other case where exactly this happened with beam find restoring proper horizontal operation for a limited time.  The bad news is that the problem was the horizontal amplifier hybrid which I think is U800.

[bob91343]

My fear is that it's the CRT.

[theworldbuilder]

Do the problem symptoms disappear for a while, if you squirt U800, with an aerosol freezer?

I'll actually do a quick setup today to test if it's U800 temperature related.

I'm going to measure the time it takes for the glitching to occur and the temperature of U800

Then I'll do the same but with a PC fan blowing directly onto U800 to hopefully slow its temperature climb.

I'll update this post once I have some numbers and we can see if we can spot any trends from there.

[theworldbuilder]

The good news is that I know of at least one other case where exactly this happened with beam find restoring proper horizontal operation for a limited time.  The bad news is that the problem was the horizontal amplifier hybrid which I think is U800.

I have found another post on the tektronix forums describing quiet literally the exact issue I'm having. Unfortunately there were no fixes posted, just a suggestion of checking J119 for the voltage levels.

I do hope it's not U800 as they're quiet a stinger to replace (About €150 - €200) if my research is correct. Hopefully if it is U800 I can find a way to get it to work as is, perhaps by sticking a heat sink on it??


It seems like I really need a second scope to check the outputs of U800 as I can exactly use the 2445 to operate on itself.

[theworldbuilder]

My fear is that it's the CRT.

Is there any good way I could test to see if it is/isn't the CRT causing issues?

[bob91343]

If all the potentials on the CRT are correct but the display isn't, that would indicate a bad tube.  Otherwise you'd have to try a different one.

[David Hess]

If the CRT was the problem then the issue would be a mechanical connection.  Tapping around with an insulated stick around the horizontal signal path to the CRT would reveal something.

I will not say that this is impossible but it is unlikely that the beam finder would affect it as described.