Philips CM8833-II video monitor repair

[steve30]

I was wondering if someone could advise me regarding some CRT video monitor issues.

My Philips CM8833-II monitor's flyback transformer conked out a few years ago, so I replaced it with a new HR Diemen brand replacement. Fortunately the monitor worked after this replacement.

The monitor seems to have a bit of an issue though. The whole picture dims a bit if you display a bright scene (e.g. a mostly white picture). This is noticeable if you try and display a dim dialog box on top of the bright picture, as the dialog box will become even dimmer still.

This isn't too noticeable, but I'm a bit of a perfectionist. Does anyone know what causes it? I can't remember if the monitor did it with the old flyback transformer or not.

Thanks
steve30.

[johnkenyon]

RGB or composite video input?

[RobertPS]

Could be a problem with solder joints. These are getting old!
Check and re-solder.

This link is for a document in dutch with common faults and repair tips:
http://www.bas-ditta.info/doc/Philips%20CM8833-2.pdf

[stj]

replace the main capacitor on the B+ rail.

it smooths the 100-ish v line that feeds the lopt and pretty much everything else.
when it starts to fail the usual symptom is when you try to draw more current my for example, displaying a bright white screen, the image with shrink a bit or shake because of volt-drop.

[steve30]

I eventually got round to looking at this again.

The monitor has a 128V rail. There are two electrolytic caps smoothing this rail. I tested both and they were fine, both in terms of capacitance and ESR.

The rail varies by 0.5V between a completely black screen and a completely white screen. I'm not sure how acceptable this would normally be, but it is enough to cause the picture to change slightly.

[steve30]

Just wondering, would it be worth trying a bigger capacitor on the 128V rail?

[stj]

look up the spec of the original, in this application the ripple current is key.

[perieanuo]

Just wondering, would it be worth trying a bigger capacitor on the 128V rail?

A little more yes but not much.I woldn't increase more than 50%.remove the old one.and of course readjust the voltage, in theory with flyback disconnected.I was doing this only if I hadn't the original value in 'stock'.
I remember when I was repairing tv's maybe your transformer wasn't correctly adjusted (focus bias and acceleration if I remember well, I was doing it based on eye impression on max luminosity)
Of course dry capacitor can do same trouble but I doubt, you can see if it's diformed on his upper side in most cases.
Regards,pierre


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