iMac G3: Bizarre startup problem

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Baphijmm
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iMac G3: Bizarre startup problem

Unread postby Baphijmm » Sat Apr 07, 2018 12:51 pm



This is not my video; however, it describes exactly the issue I'm now facing with my old iMac G3, the second issue in the video. The uploader apparently got a comment that solved the problem; however, that comment has since disappeared, as far as I can tell. Does anyone have any ideas?

Things this isn't:
A firmware or software issue. The thing worked perfectly fine for years, until it sat in storage for a few more years.
Likely a PAV board issue. I just replaced that with a supposedly-NOS unit, and that's how I got it to power on in the first place.

Things I suspect:
The CRT. I do know I bumped the little rings with handles on them near the very end of the tube when I was putting the new PAV board on, and I've been told this can cause issues. Further, the symptom I've noticed with this is that the CRT is absurdly bright and continues to get brighter to the point of almost incandescence before the power gets cut to everything but the logic board.
Miiiight actually be a PAV board issue, even though this unit is supposedly new. In addition to the CRT getting brighter, the HDD seems to spin up faster and faster, but that might just be my imagination.

The original poster of that video seems to indicate that something simple like a loose plug or something fixed the issue, that he didn't have to replace anything, but then, again, the comment he thanks for that information seems lost to time. I've never seen this issue before, and this is the only other instance I can find anywhere online, so it can't be terribly common. Thoughts?
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Baphijmm
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Re: iMac G3: Bizarre startup problem

Unread postby Baphijmm » Sun Apr 08, 2018 12:25 am

Image

We get signal! Well, kinda.

So, it works! To a degree. The screen is *really* flaky and finicky. I found a pot on the side of a construct on the PAV board labeled “SCREEN”; turning this adjusts the brightness of the CRT, which allowed the system to get past the previous roadblock. In other words, I was kinda right: It was shutting down because the image was WAY too bright, and it was partly the CRT, partly the PAV board's fault.

That having been said, the knob is incredibly sensitive, and having the screen juuust a little too bright causes the CRT to start increasing in brightness to a point close to where it would’ve shut off before. Naturally, this is bad.

Having too much stuff on the screen also causes issue; in particular, having pure green on screen causes the brightness to plummet, and continue to do so each time it appears. (It doesn’t seem to be tied to frames, rather each spontaneous appearance of the color.) It’s also pretty badly out of focus; there is a knob to adjust the focus on the same assembly, but this also causes the brightness to drop dramatically, making it impossible to focus properly. This was as close as I could get it.

I do think I’m going to replace the CRT, if only to rule it out as a problem (and also get rid of a major gash in the very center of the screen, which isn’t too obvious here, but trust me, it’s obvious). If nothing else, it should come pre-calibrated with those rings in the exact correct positions. Should.
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robespierre
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Re: iMac G3: Bizarre startup problem

Unread postby robespierre » Sun Apr 08, 2018 12:38 am

The "SCREEN" potentiometer controls the voltage of the G2 anode. Effectively, it repels electrons in the beam to decrease the current hitting the phosphor screen. The potentiometer interacts with a focus/screen voltage network which may be broken if you're seeing such wide swings in brightness: see https://www.repairfaq.org/REPAIR/F_monf ... MONFAQ_016

The purity magnets (the rings you referred to) have nothing to do with the focus or correct operation of the CRT and are only there to correct color purity errors (electrons that exit through the wrong holes in the shadow mask and so strike the wrong phosphor dots). If the tube is correctly degaussed, they are usually pretty easy to fix when you display a test pattern (fine red, green, or blue grid on a black background) and minimize the appearance of the wrong colors. The point is to minimize the lateral magnetic fields which cause the electrons to spiral as they fly down the tube. A good test pattern program is absolutely essential to correct any picture problems.
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Re: iMac G3: Bizarre startup problem

Unread postby Baphijmm » Sun Apr 08, 2018 1:07 am

I don't think the issue is with the pot or that network, as this PAV board was absolutely NOS; this is the first time it's ever had power applied to it, and it had been sealed in a static bag otherwise, so it's unlikely dirt got in either. All of this just leads me to believe more strongly that the CRT might have something wrong with it.

Would such an issue cause the wild variations I'm seeing when different things are on screen? I would imagine they'd only cause such when those knobs are being adjusted, but then I honestly don't know.
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robespierre
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Re: iMac G3: Bizarre startup problem

Unread postby robespierre » Sun Apr 08, 2018 1:15 am

Does the picture grow or shrink when you display different images? That points to a power supply or flyback problem.
https://www.repairfaq.org/REPAIR/F_monf ... MONFAQ_015
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Baphijmm
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Re: iMac G3: Bizarre startup problem

Unread postby Baphijmm » Sun Apr 08, 2018 1:18 am

No, just the brightness sometimes goes wonky, typically dimmer; it seems more related to instantaneous change, as opposed to any one element being on screen. Also, if the brightness is even slightly too high, this will cause the brightness to continue to increase to a point of washing out, almost to the previous point of cutting power to all but the logic board like it was doing before.
Systems: :O200: :O200: :O200: :Onyx: Image :Indigo: :Octane: :O2: :Indigo2IMP: :Indy: :Indy: +
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Re: iMac G3: Bizarre startup problem

Unread postby Baphijmm » Sun Apr 08, 2018 3:56 pm

A couple thoughts on possible culprits as I sit at work and ponder:

The inner bezel is really screwed up; it's not entirely obvious in that photo, but you can see where pieces of the thing are missing, and in fact neither of the two top screws that hold the CRT to it are actually attached any more. (Superglue can only do so much.) It might be possible that the CRT's weight is pulling it back away from that bezel at the top, causing undue strain on its neck and the video board. Maybe? Unfortunately, it seems this particular component is made of Unobtainium, as literally nobody seems to have it; the only two folks who claim to, their websites don't work (and seem kinda skeezy anyway). Anyone have one they're looking to sell? :D

It's possible the CRT is gassy. I'm not a huge proponent of this, but given its propensity to steadily increase brightness when there's a lot of white on screen, and suddenly cut brightness when something sudden happens, that might be what we're seeing. In which case, replacing the CRT is about all I can do. These *do* seem to exist, and in fact I was planning on obtaining one once I can verify that it is, in fact, an LG unit. This is further possible because the unit sat in a non-climate-controlled storage unit for several years, baking in the New Mexico highlands sun during the summer and enjoying extended below-freezing temperatures in winter. As stated before, I had to replace the PAV board to even get to this point; previously, when plugged in, you could hear a regular fast-rhythm clicking (as though something were arcing slightly, maybe?), and it wouldn't even try to power on. Possible the same conditions that cooked the PAV killed the CRT too.

It's also possible there's a loose connection on the video board. I noted that the ferrite beads on the cables running to it from the main PAV board seemed to be awful close to the back of the tube and went to adjust them; when I did this, the picture suddenly dimmed significantly. I'd initially chalked it up to the aforementioned ferrite, but thinking more on it, a loose connection or something to do with those cables would make more sense, maybe.

I don't know; I'm kinda spitballing. I've never seen this sort of CRT problem before. Generally I only see issues coming even close to these sorts of symptoms when the CRT itself is dying.
Systems: :O200: :O200: :O200: :Onyx: Image :Indigo: :Octane: :O2: :Indigo2IMP: :Indy: :Indy: +
- AdamMalkovich: AMD FX8350, LinuxMint 17 Fluxbox
- Morningstar: Sun SS20, Solaris 9
- Guru: IBM RT/135, AIX 2.2.1
- Concord: Sun SPARCclassic, Solaris 2.6
- Lying Bastard: Apple iMac G3, Mac OS X 10.3.9
- Chunderspunk: NeXTstation, NeXTSTEP 3.3


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