I have a VGA tube that was pulled from an old Tektronix TDS510A. It's now being used as a little monochrome VGA monitor. Discovered a weird issue after its been running for a while. The picture slowly expands, then the screen goes blank.

I am driving it with a 640x480 VGA signal at 60Hz refresh
Never seen this before. Could it be capacitor related?

video of issue (https://youtube.com/shorts/YE-dT8oIcZs?feature=share)

Video is without crt turning off.

Hi to all,
Hi Devicemodder,

i watched your short video, operation looks OK. As you don't show the picture expansion & fade, it's hard to diagnose anything.

Questions:
does just the Windows computer content display disappear & screen stays lit or does the CRT brightness die?

In traditional TV, image expansion & decrease in brightness usually points to a dying EHT rectifier.
In the schematic you posted, EHT rectification seems to be included in the Flyback transformer (modern "split-diode" transformer technology).

To assess if the transformer is at fault (or not) additional data is needed:
- the schematic showing the CRT, its type & hopefully electrode voltage.

Something might be drifting & driving the CRT to cutoff, blanking the display.
How is the Tektronix VDU powered; own power supply, bench PSU,...?

Have test gear: multimeter, HV probe,...?

If possible, a new video showing the "expand & fade" fault,

For anyone interested, the TekTronix TDS-510A digital Scope service manual* (284 pages/4Mb) is on TekWiki:
https://w140.com/tekwiki/images/5/52/070-9704-02.pdf

*didn't see schematics.

TekTronix TDS-510A wiki main page:
https://w140.com/tekwiki/wiki/TDS510A

Best Regards
jhalphen
Paris/France

Looks like it runs on 12 VDC. See top L of manual. Be sure thats not creeping up first. After that hoz / HV may be iit. Would need that part of schematic.

73 Zeno:smoke:
LFOD !

Hi to all,
Hi Devicemodder,

i watched your short video, operation looks OK. As you don't show the picture expansion & fade, it's hard to diagnose anything.

Questions:
does just the Windows computer content display disappear & screen stays lit or does the CRT brightness die?

In traditional TV, image expansion & decrease in brightness usually points to a dying EHT rectifier.
In the schematic you posted, EHT rectification seems to be included in the Flyback transformer (modern "split-diode" transformer technology).

To assess if the transformer is at fault (or not) additional data is needed:
- the schematic showing the CRT, its type & hopefully electrode voltage.

Something might be drifting & driving the CRT to cutoff, blanking the display.
How is the Tektronix VDU powered; own power supply, bench PSU,...?

Have test gear: multimeter, HV probe,...?

If possible, a new video showing the "expand & fade" fault,

For anyone interested, the TekTronix TDS-510A digital Scope service manual* (284 pages/4Mb) is on TekWiki:
https://w140.com/tekwiki/images/5/52/070-9704-02.pdf

*didn't see schematics.

TekTronix TDS-510A wiki main page:
https://w140.com/tekwiki/wiki/TDS510A

Best Regards
jhalphen
Paris/France

The image is expanding in the video, just very slowly as it happens over the course of 5 minutes or so. The screen dims slowly while this happens and eventually shuts off.

I have the service manual for the scope that the tube came from. I'll check the schematics and report back.

The Tektronix display is powered from a 24VDC wall adapter as i don't have the rest of the scope. I just have the VDU.

And the expand is shown in the original video. watch the windows taskbar, and you'll slowly see it going offscreen.

CRT Model: 678PAMGLX

Here's the service manual i got the schematics from: https://files.catbox.moe/owboca.pdf

page 370.

I'll try to make a new video of the fadeout and expansion.

Only test equipment I habe is a multimeter and a scope (not the one this tube came from)