Okay, here's a new one for you guys: Imac CRT issue. - Videokarma.org TV - Video

MRX37 · #1 01-31-2009, 01:58 PM

I recently was given this Imac. the computer itself seems to work fine, but the screen has a weird issue. The right half of it is tinted green. This is not the result of a damaged shadow mask, because when I put a dark color on the screen, the tinting to the right of it goes away, or severely lessens. Take a look at the pic I snapped to see what I mean:

I assume there's some electronic component that's drifted out of tolerance somewhere inside this thing.

andy · #2 01-31-2009, 02:07 PM

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old_tv_nut · #3 01-31-2009, 02:17 PM

This looks like poor low-frequency response in one or more of the R,G,B channels (note that the color is different to the right of the black rectangle). This could be in the video adapter of the computer or in the monitor video circuits - does the imac have an output for an external monitor you could try?

MRX37 · #4 01-31-2009, 02:23 PM

Quote:

Originally Posted by old_tv_nut

- does the imac have an output for an external monitor you could try?

No it doesn't, or I'd be using a better moinitor.

Guess I'll have to start checking caps...

*zenithfan1* · #5 01-31-2009, 04:27 PM

The monitor output is in the back at the bottom under an oval cover with vents in it. You have to pry it off, there is a slot for a screwdriver. I know quite a bit about macs, I fix them all the time and use them too. I don't however, know what your issue is. My friend here at work has the same thing going on with his. But it belongs to the school so I never investigated it.

MRX37 · #6 01-31-2009, 04:31 PM

Well, I've already ripped away most of the plastic. Still need to get the see through blue cover off. It will never look the same after I put it back together, because the green plastic under the covering is ridiculously fragile. But whatever. I never did like how the Imac looked anyway. and it might not get so damn hot if the cover had a few more ventillation holes!

maxm · #7 02-01-2009, 11:40 AM

Is this a slot loading or tray loading iMac? I've never worked on a tray loader but I've disassembled several of the later slot loading iMacs, mostly to clean out the dust. Taking off the bottom to get to the motherboard is pretty easy, but taking off the top to get to the power supply and CRT is harder, there are those awful plastic tabs all over that you will end up breaking off most of them. I think the tray loading ones are more difficult to take apart. The plastic in these is notorious for degrading due to the heat, particularly the ribbed plastic that surrounds the screen, it has been cracked on severs of the ones I've worked on. The tray loading iMacs have fans, but the slot loading ones work with convection only, though I think there were some fans made that fit around the handle.

karmaman · #8 02-01-2009, 12:46 PM

I replaced the flyback on a Rev. A tray-loader. Managed to do it without cracking the plastic

but I came mighty close. I'd suspect caps because these things run HOT!

Also, I think the tray loaders are easier to take apart than the slot loaders. Getting to the PAV in my Rev. E slot-loader is a pain.

MRX37 · #9 02-01-2009, 01:11 PM

It's a slot loading imac.

I already KNOW I'm going to have to break that plastic cover to get it off. I'm fine with that, as I'll probably put on a cover with more ventillation anyway.

Okay, after breaking a few tabs, I have that damn blue cover off! After uncovering the CRT neck board, which is surrounded by metal and plastic, I've found that it has a LOT of Teapo caps on it, and I know Teapo is a bad cap brand...

I'm probably going to change out every Teapo cap I see, then put it back together well enough to power it up and see if the new caps help.

andy · #10 02-01-2009, 02:58 PM

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MRX37 · #11 02-01-2009, 03:00 PM

Well screw the cover for now. I've been testing caps on the CRT neck board. So far I've found at least two that measured low. One, a 1uf@ 160 volt measured with no significant capacitance whatsoever. So I think I've found the problem area, or at least one of the problem areas.

Now I just need to hunt down a 1uf @160 volt cap...

MRX37 · #12 02-03-2009, 10:28 PM

Okay, just about every capacitor on this CRT neck board has tested at well below its rated capacitance. I'm surprised the damn thing worked at all with all these bad caps in it.

mbates14 · #13 02-04-2009, 10:37 AM

you cant go by that half the time. you have to ESR test them.

it looks like the CRT may be burned in. its weird. shine a bright light at it and see.

I have the same exact imac, and the analog board is bad on it also. it works ok, but the picture starts to shrink as it runs for a bit. if you tap the board, itll pop back out. but if not, itll continue to shrink until it shuts down. it has to be a hairline invisible crack in the power supply or something, i hit up every single solder joint in the sucker.

MRX37 · #14 02-04-2009, 11:27 AM

Quote:

Originally Posted by mbates14

you cant go by that half the time. you have to ESR test them.

it looks like the CRT may be burned in. its weird. shine a bright light at it and see.

Um, you DID see my image about where I showed that the green goes away whenever there's a dark color on the screen, right?

No, there is no burn in.

Also, they're Teapo caps, I already know they're bad. When a 100uf capacitor tests at only 35uf, I call it bad.

mbates14 · #15 02-04-2009, 12:02 PM

maybe so, but like i said, i would never go by that. unless you pull them out of circuit, then i would go by that. as theres other things in the circuit that will affect capacitance readings. everyone would tell you that. However. it wouldnt surprise me if it was the capacitors though, certainly looks like it might be the issue. hehe.