CRT dims gradually after power on

How old is the monitor? Is this a new problem? Or been that way since you acquired it?

When it goes dim, can you reduce the brightness? And, by that, I mean does the first step down from max reduce the brightness more? What happens if you set it to the level to which it dims and wait? Does it dim proportionally more?

May be some kind of screen protection function.

I'd also peer at the filament to see if it also dims.

It is not the CRT.

answer to on

hosphors lose their efficiency but wish to confirm my doubt.

seems to have solid brightness levels. However, displaying a full white pic ture dims the display over a period of some 30 seconds - the picture notice ably loses brightness.

a full white again, the brightness is back, but starts to fade again like d escribed in the prior paragraph.

r is this in fact an aging CR tube?

Does the image size get bigger as it dims?

G²

No, no picture deformations whatsoever.

Mike,

I tried reducing the brightness level then did the same test, the problem d oesn't occur then. Also, this monitor has a "Highlight" function where you can define either a portion of a screen or the whole screen and additionall y bump the brightness in the defined area. But regardless, if the "highligh t" is turned on, or just the standard brightness control is too high, it wi ll dim to a certain level below its standard 100% after a short period. Wit h reduced brightness setting this doesn't occur.

So, I'm guessing some circuit is at fault here rather than the tube. Now, I 'm not a professional, but I do know the principle of a CRT and all the dan gers that come with servicing it. So my question is, where do I start looki ng for the fault? I'm guessing something that feeds the flyback transformer might be giving out?

Nope, if the geometry is staying put, it is something in the video.

Check whether something overheats. See if cooling cures the brightness change.

Well, my remote diagnostic ability has been stymied by your snipping and ignoring most of my questions. Suggest you disclose the make/model and contact the vendor.

doesn't occur then. Also, this monitor has a "Highlight" function where yo u can define either a portion of a screen or the whole screen and additiona lly bump the brightness in the defined area. But regardless, if the "highli ght" is turned on, or just the standard brightness control is too high, it will dim to a certain level below its standard 100% after a short period. W ith reduced brightness setting this doesn't occur.

I'm not a professional, but I do know the principle of a CRT and all the d angers that come with servicing it. So my question is, where do I start loo king for the fault? I'm guessing something that feeds the flyback transform er might be giving out?

Just an FYI, measuring the G2 voltage on the CRT will require a high voltag e probe not because of high voltage but high impedance. The 11 megohm load of a digital meter will give inaccurate values so you need the gigohm resis tance to get valid readings. Drifting values on the CRT grids can cause all sorts of issues. Anode voltage change will cause the size changes but grid changes will not - within reason.

G²