Green Screen and Freezing on G3 iMac CRT

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Green Screen and Freezing on G3 iMac CRT

Hi - My son's Snow Slot Load 500mxh G3 iMac has recently developed some problems. It happened not long after updating to Tiger but seems not to be related to that. I installed the firmware upgrade before updating and the install went fine. It was working fine for a few weeks but started freezing up at random times requiring the power button to be held to restart. It would then restart and work fine but over the last couple weeks, the freeze ups have become so frequent that it's basically not worth bothering with. The freeze ups seem to have some sort of mechanical aspect to them - there is an actual flash or flicker or surge type thing as if the power was interupted. I have plugged the iMac into different power sources, etc. to be sure that it isn't somehow something in the houses wiring but that has not effect. In addition, in the last week or so, the screen has taken on a pronounced green cast. At first this problem was intermitent but now is pretty much permanent (it did come on once properly) It still works fine in terms of resolution, positioning, etc., no weird artifacts, just green. Both of the problems persist even if the iMac is started in OS9 or from a CD. I ran the Apple Hardware Diag CD and it gave a clean bill of health on everything....hmmm!

My guess is that it is a hardware problem and the obvious choice would be the video board. I don't want to put it out to pasture since it works great for my son but I also don't want to put bunches of time and $$$ into it swapping out different boards to try to figure the problem. Does anyone know any fairly definitive tests to determine if it is the Video board or, if not, what it is?

Thanks in advace for any help.

iMac G3 Mac OS X (10.4.9) Slot Load CRT 500mzh Snow

iMac G3, Mac OS X (10.4.9), Slot Load CRT 500mzh Snow

Posted on Apr 8, 2007 10:22 AM

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Answer 1

Apr 8, 2007 4:55 PM in response to Webvinyl

If you have a seperate monitor with a VGA plug, you can plug the monitor into the back of the iMac (little door underneath). If the remote monitor has the same symptoms it's likely the video card part of the PAV board.

If the remote monitor is fine, the iMac's monitor is shot. Use the remote if you can spare it and don't mind the extra footprint.

Neither thing is good. At this stage I just get another unit off eBay. That's usually cheaper and a whole lot less fussing around!

David

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Answer 2

spudnuty

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Apr 8, 2007 5:08 PM in response to David Jenson

Yup this sounds like the classic PAV going out. The PAV (Power-Analog-Video) board generates all the power for the CRT and video drive. Basicly the upper part of the computer. The logic board gets it's power from the "Down Converter" which sits underneath the iMac next to the logic board. It receives high voltage from the PAV board and converts it to logic board power. If the PAV is going bad it will put transients into the logic board which causes the freezes. The green color will also be a symptom of HV problems.

You could get a PAV board off eBay for cheap and swap it in. Don't attempt this unless you are very comfortable with working with the deadly voltages involved.
Richard

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Answer 3

Webvinyl Author

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Apr 9, 2007 4:26 PM in response to Webvinyl

Thanks for the help!

I plugged the monitor from my G4 into the VGA port on the back of the iMac and was a little surprized that the video worked fine through there! So, if I understand correctly, that should mean that the problem is the iMac's CRT rather than the PAV board, right? Would the CRT going bad be likely to be causing the freezing too?

If it is in fact the CRT that is bad, I assume that there are probably different models. Could I use one from any Slot Load iMac or do I need to be more specific than that?

Again, my sincere thanks for any and all help.

Kevin King

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Answer 4

spudnuty

Level 5

7,128 points

Apr 9, 2007 6:32 PM in response to Webvinyl

Well the PAV generates all the power for the computer. So sometimes the high voltage section can fail(esp the flyback) and the CRT will die. However the section supplied to the logic board will be ok. Often when the HV is failing transients in the supply will cause the logic board to freeze. Many times the CRT will die but the power to the logic board will be ok and thus the output of the VGA video out will also be ok. Many people have used their iMacs with a external monitor for years.

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Answer 5

Webvinyl Author

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Apr 10, 2007 6:33 AM in response to Webvinyl

Okay so...

Then it sounds from the evidence that both the CRT and at least the High Voltage section of the PAV are likely bad. Right?

If that is correct then my options (aside from getting a new computer) would be:

1. Let the HV on the PAV die and use an external Monitor.

2. Replace the PAV, use an external Monitor.

3. Replace the PAV and replace the internal CRT.

If I go with one of the options using an external Monitor, do I need to disconnect the internal CRT? I mean, I know that I don't have to disconnect the internal CRT to use an external Monitor but would it be better to?

If I decide to replace the internal CRT, do I need some specific model or will any Slot Load iMac CRT work?

Thanks again to everyone,

Kevin King

iMac G3 Mac OS X (10.4.9) Slot Load CRT 500mzh Snow

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Answer 6

spudnuty

Level 5

7,128 points

Apr 10, 2007 7:28 AM in response to Webvinyl

both the CRT and at least the High Voltage section of the PAV are likely bad. Right?<</div>
No only the PAV sounds like it's going bad. The fact that the CRT lights up means that it's good. The silver box on the back of the CRT that Apple refers to as the Video Board is part of the PAV. In fact when you replace the PAV you want to keep that same Video Board with the CRT so it involves cutting and splicing of 2 wires.

Let the HV on the PAV die and use an external Monitor<</div>
Most likely it's the flyback transformer on the PAV that is going and when that goes it's usually it's the Anode section that dies.

If I go with one of the options using an external Monitor, do I need to disconnect the internal CRT? I mean, I know that I don't have to disconnect the internal CRT to use an external Monitor but would it be better to?<</div>
Well if the Anode section dies then mute point because the HV will be gone anyway.

Replace the PAV, use an external Monitor<</div>
If you replace the PAV (deadly voltages involved! Also working PAVs are hard to come by) you wouldn't need an external monitor because the internal one would then work. My son has a 700 Snow that I dropped a DVD-ROM from a 500 Graphite into. Had to use Pacifist to get it to work. Hooked up an external LCD. Now he's watching Pirates II in stereo. He also uses it for Garage Band.

R

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Answer 7

Webvinyl Author

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Apr 10, 2007 10:04 AM in response to Webvinyl

Alrighty then,

So, since the HV portion of the PAV is seemingly still currently at least partially functional, if I decide to use an external monitor, should I disconnect / disable either or both the internal CRT or HV to avoid having spikes (or whatever) that seem to be causing the freezing problem? If so, how?

Re: replacing PAV / Deadly voltages - I'm not an expert but I have worked on an older (SE) all-in-one Mac CRT and some tube audio projects. I can probably avoid killing myself if I have explicit instructions. Are there good instructions on doing the PAV replacement available somewhere online? I've not been able to find any so far although I've not looked that hard yet. If anyone knows, please clue me in.

Thanks again to anyone who has taken the time to help,

Kevin King

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Answer 8

spudnuty

Level 5

7,128 points

Apr 10, 2007 10:58 AM in response to Webvinyl

I've got it. eMail me.
R

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