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Old 12th Sep 2020, 5:48 pm   #1
WDeranged
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Default Philips CM8833-II convergence issue

I've been hunting for a pristine example of this monitor and ended up with two pretty good samples but weirdly enough they both have convergence issues in the same area of the screen.

I'm seeing some fringing on the bottom of the screen, getting more severe towards the bottom right corner.

Does anyone know if this is a common issue with these screens? Anything I can do to fix it?
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Old 13th Sep 2020, 12:21 am   #2
simpsons
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Default Re: Philips CM8833-II convergence issue

The Service Manual is very helpful as it tells the user a PIL CRT is used and how to perform static and dynamic convegence. Not shown though but seen in the You Tube video are permalloy convergence strips which are used to correct slight misconvergence at the screen edges.

These are sold as arcade convergence strips or CRT yoke thin magnetic card tab

I hope using them solves the problem.

Chris
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Old 13th Sep 2020, 5:13 pm   #3
Maarten
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Default Re: Philips CM8833-II convergence issue

I've found that harvested permalloy tabs from (Sony) VGA monitors are useful for fixing convergence issues like this. While the procedure is somewhat different for inline than for trinitron, I've always found Sony manuals useful for understanding convergence issues too.

Before using such tabs, first correctly (re)position the yoke, as a fault in the entire lower half of the screen might be partially fixed this way.

P.S. While permalloy is a magnetic material, the strips aren't magnetised as the deflection field would quickly undo that. The material is used to locally distort the deflection field.
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Old 13th Sep 2020, 8:49 pm   #4
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Default Re: Philips CM8833-II convergence issue

Thanks for the replies. I'm wary of messing the yoke as I tend to go down rabbit holes with this sort of thing. If I absolutely have to then I will but I'd rather try the convergence strips first.

Only problem is getting hold of some. I've seen conflicting reports on how to make them at home, some people say just use fridge magnet and others say to use pieces of snap off stanley blade.
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Old 14th Sep 2020, 5:24 pm   #5
Maarten
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Default Re: Philips CM8833-II convergence issue

Before messing with the yoke or making your own permalloy strips, check whether the yoke has sagged a bit. It should be tight on the rubber wedges.

Fridge magnets may be used to correct severe purity errors, when placed on the bell of the tube (not underneath the yoke). Sony was the only manufacturer to mention this in the manuals,though they didn't use fridge magnets but supplied custom ones.

I'd say pieces of a stanley blade might resemble the effect of permalloy assemblies close enough. Check if they get hot from the deflection energy, too hot poses problems.
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Old 15th Sep 2020, 1:33 pm   #6
dominicbeesley
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Default Re: Philips CM8833-II convergence issue

When i got mine with my Acorn A3000 it was like that. I put up with it for years thinking that it was add good as it would get. After reading around i had a twiddle of the yoke and convergence and got it nears as dammit perfect. If only I'd had the courage all those years ago!

Not sure if they were all like that or we just got duff ones converged at factory by the same person or they were just prone to being upset in transit
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Old 15th Sep 2020, 7:49 pm   #7
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Default Re: Philips CM8833-II convergence issue

Ahh that's interesting, so yours had issues in the bottom right too? I'm thinking at some point I'll probably have to man up and have a go.
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