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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 27th Dec 2017, 7:17 pm   #1
lyndhurst25
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Default iMac G3 CRT display problem

Hi.

My dad's trusty computer, a 1999 Apple iMac G3, screen has developed a fault. The problem occurs displaying black text or lines against a white background; the left hand edge of characters or vertical black areas have a variable amount of red "bleeding" across from left to right where the black should be. Fine vertical black lines appear as red lines, but horizontal lines are unaffected. The whole screen area seems to be similarly affected. Turning the brightness down so low as to make the screen virtually unusable does cure the issue however. Unfortunately, I have no experience in repairing or adjusting CRT displays (but I am aware of the dangers involved). Does this sound like an easy repair on this machine?

Thanks in advance for any advice given.
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Old 27th Dec 2017, 9:03 pm   #2
SiriusHardware
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Default Re: iMac G3 CRT display problem

Unfortunately this sounds to me like the tube is 'Flareing' - that used to be a commonly observed effect on TVs with tired old tubes back when I worked in a TV / Radio shop many years ago, but I have rarely seen it happen on CRT computer monitors.
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Old 27th Dec 2017, 9:26 pm   #3
mole42uk
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Default Re: iMac G3 CRT display problem

Unfortunately, it’s a 19-year-old computer screen and Apple at that time were using the cheapest monitor chassis. It’s probably time to retire the G3.
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Old 27th Dec 2017, 10:06 pm   #4
cmjones01
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Default Re: iMac G3 CRT display problem

That does sound like the 'flaring' effect I've seen on CRTs with tired (or improperly driven: I transplanted a high-resolution monitor tube into a Philips K30 chassis once and got this effect) tubes in the past. Does anyone know what causes it? And why does it affect typically only the red gun?

Chris
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Old 27th Dec 2017, 10:18 pm   #5
Skywave
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Question Re: iMac G3 CRT display problem

Isn't the red gun at the 'top' of the equilateral triangle that constitutes the 3-colour guns in a 20AX CRT? Aren't the phosphors on the tube inner 'face' in a similar geometric arrangement?

Al.
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Old 27th Dec 2017, 10:46 pm   #6
Refugee
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Default Re: iMac G3 CRT display problem

You are thinking of delta gun sets.
It might be worth doing a recap on the video board that plugs on the back of the tube.
That used to be a stock fault on VGA screens.
They are an isolated chassis so if you are very careful a scope will show the problem as a change in the waveform either side of an electrolytic capacitor.
The video board is often stuck on to the tube base with glue so some care with a Stanley knife will also be required.
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Old 28th Dec 2017, 3:09 am   #7
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Default Re: iMac G3 CRT display problem

The red flare is actually clipping from the red video amplifier, not some phenomenon that is directly caused by the tube, even though it can be sign of a weak tube.

The most likely causes are either that a supply voltage to the video amplifiers is sagging (bad capacitor), the AKB feedback is bad or the red gun has gone low emission. In monitors, it can also be a coupling electrolytic in the video path itself.
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Old 28th Dec 2017, 11:17 am   #8
cmjones01
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Default Re: iMac G3 CRT display problem

Thank you, Maarten, that's interesting and makes a lot of sense given the symptoms. So replacing electrolytics on the tube base board may well fix it.

Chris
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