I've just been given a slot load G3 Imac 350 that has startup issues and after doing a bit of research I found an excellent site for gudes to repairs to some of the more common types of electronic components found in computers - more specifically those usually of the power supply kind.The original iMac g3's were particularly prone to the Power supply failure problem, and these are mostly but not always due to the Flyback transformer failing. As a lot of us seem to have or had one of these at some stage , I just thought I would post this here for interests sake. This is taken from...

http://www.repairfaq.org/sam/flytest.htm

Why do flyback Transformers Fail?
While flyback transformers can on occasion be blown due to a failure elsewhere in the TV or monitor's power supply or deflection circuits, in most cases, they simply expire on their own. Why?

Flybacks are wound with many layers of really really fine wire with really really thin insulation. This entire assembly is potted with an Epoxy resin which is poured in and allowed to cure.

In some ways, these are just short circuits waiting to happen.

Flybacks get hot during use and this leads to deterioration of the insulation. Any imperfections, nicks, or scratches in the insulation or trapped air bubbles and impurities in the Epoxy fill material contribute to failure. Temperature cycles and manufacturing defects result in fine cracks in the Epoxy potting material reducing the insulation breakdown particularly in the area of the high voltage windings, rectifiers, and focus/screen divider network. They also physically vibrate to some extent. A whole bunch of other factors are also no doubt important.

Once a breakdown - sparking or arcing - develops, it is usually terminal.

It is amazing they last as long as they do with the stresses they are under.


How Do Flyback Transformers Fail?
Flybacks fail in several ways:
Overheating leading to cracks in the plastic and external arcing. If there is no major damage to the windings, repair may be possible. However, arcing from the windings punctures their very thin insulation so that shorted windings may already have developed. Even if the windings are currently in good condition, long term reliability of any such repairs is questionable.

Nonetheless, it doesn't hurt to try cleaning and coating with multiple layers of high voltage sealer, corona dope, or even plastic electrical tape (preferably as a temporary repair though I have gotten away with leaving this in place permanently). If possible, moving the point to which the flyback is arcing further away (i.e., a piece of metal or another wire) would also help.

(The following from: Tom Riggs (thriggs@mail.netusa1.net))

For sealing flyback transformers, I have found that silicone sealer has worked very well. I used the clear variety, though others will probably work as well. I have heard of burn through with corona dope. (Author's note: make sure you allow ample time for the silicone sealer to setup completely - or else it will breakdown instantly - at least 24 hours. Also, some types (those that smell like vineger - acetic acid - as they cure may result in corroded wiring in the long term).

Cracked or otherwise damaged core will effect the flyback characteristics to the point where it may not work correctly or even blow the horizontal output transistor and other expensive parts like the low voltage regulator or switchmode power supply. If the core can be reconstructed so that no gaps (other than the required ones where the two halves join) are present and clamped and/or glued in place, it should be possible to perform testing without undue risk of circuit damage but consider a replacement flyback as a long term solution.

Internal shorts in the FOCUS/SCREEN divider network, if present. One sign of this may be arcover of the FOCUS or SCREEN spark gaps on the PCB on the neck of the CRT.

Internal short circuits in the windings.

Open windings.
More than one of these may apply in any given case. As noted, temporary repair, at least, is sometimes possible for failures (1) and (2). For failures (3) to (5) replacement is usually the only alternative.

Stewie

Some time ago now when I bought my second CRT iMac I looked into this. My research revealed that while the flyback transformer on the rev' A tray load iMacs was prone to failure (through overheating and consequent insulation failure), on the later revision slot loaders it was the analogue board (PSU?) that crapped out. The third CRT iMac I got was dead (the same symptoms you describe with yours) as Tiger had been installed via firewire target disk mode without the firmware (4.1.9f) update, the absence of which results in, ahh... problems... Neither part is cheap or easy to replace.

... the upshot is you need a working iMac carcass to repair it

I'm still fooling around with mine (iMac 500); it will run perfectly for a good while, then will shut down ... and tick.

JB

Sadly , I think given their popularity ,there must be squillions of G3 iMacs out there tucked away in basements never to be used again because their FBT's have failed ( seems to be a highly common problem ) and the cost to repair these small but relatively expensive parts , far outweighs the actual worth of the computer itself as Kim Jong Il has said. What a waste of a Mac Classic.
Stewie

I'd roughly estimate that hundreds of thousands of iMacs were sold in Australia, and nowadays it's quite a rarity to find one that doesn't have PAV problems The flipside is that they are quite small, and cute, so I'd imagine there are quite a few still in use. The fact that they were designed poorly, hard to disassemble, and not built to last (unlike many other Mac models), doesn't define it as being a "classic" in my mind.

You can adapt all iMac models to run off an ATX power supply, of course

JB

While I agree on the negative aspect of their build quality, I think you will find that they are classified as a Classic Mac because of their revolutionary design even though it took me awhile to warm to them. Even the PC industry recognises this. I still shudder at those lurid colours though. "Tangerine , Lime , Strawberry" etc. Bleeech. I wouldn't be seen dead toting around in public one of those candy coloured iBooks.The iMacs were after all a consumer oriented computer and not aimed at the Pro user so I guess some compromises in quality both in design and build were made.

I've never heard of the iMac G3's being hacked to use an ATX PSU though. Got any links ?
Stewie

Yep, here you go:

http://members.fortunecity.com/tinux.../imac-atx.html

http://www.cryogenius.com/hardware/imac/

http://www.macmod.com/content/view/179/

... since most iMac have an external monitor connector, "all" you need to do it match up the power pinouts, and hack this into a standard ATX PSU.

A company called Marathon used to make harnesses that did all this itself, but the company is now long gone.

JB

i had a rev b model which i picked up cheap a while back, used it for surfing, basic photoshop, until the fly back transformer went kaput.. had it running in a corner and i think it got to hot against the wall.. i was going to try to fix it, but it seemed too hard in the end.. i think my dad threw it out last hard rubbish.. i forgot to tell him i was storing it at their house for a reason..

i wanted to get one for my son to play with when he gets older, but i guess there'll be a lot of old intel imacs or minis around by that stage.. or at least G4s..

Thanks for the links but my mistake. I thought you meant you could hack an ATX PSU to work inside the iMac case . I actually like the iMac AIO form factor and was hoping there was some way to get an alternative PSU to power all the iMac components so that the iMac remained pretty much that , except for the new PSU. I must admit that I like the rack idea and not just for an iMac either. All sorts of Macs could be hacked to work in one of those cases. Sort of a 0.5 Xserve without the server OS. Hmmmm...
Stewie

You probably could shoehorn an ATX PSU in an iMac case stewie, and a replacement CRT or LCD

Mmm.

JB

This late in the production-cycle (ie, well after the end) of slot-loading CRT iMacs this is, effectively, a pointless post, but here goes anyway.

Even given the component-related susceptibility of the model to failure, the defence was always the same: adequate ventilation, just as was the case with the first three AIO Classic Macs. The subject of CRT iMac failure rears its head at intervals, usually follows the same sterile lines of retrocriticism of Apple, and is almost never contaminated by any reading of past discussions or appreciation that the risk of failure could/can be ameliorated.

There. I have ventilated.

de

equill - it's nice to see you around these forums, alongside 68KMLA - visit again soon!

JB

Byrd

I shifts me seegar to t'other side, and dips me lid to 'ee.

de

Given that most G3 iMacs sit up on a desk with usually nothing behind and only a keyboard in front , I doubt that inadequate ventilation on the outside causes these problems. Let us also remember that these cases are roundish and very hard to store anything on top of them so I doubt blocking the vents on top of the case would likewise cause these problems. Inadequate cooling of the components inside and lack of flow through ventilation through bad design is entirely different however.
Stewie

How does one ventilate an exterior? If you had but glanced at the link provided you might have gleaned a few nuggets from which to start. Apart from not having coats thrown over them, the slot-loading iMacs also profit from the improved internal (which is where it counts) ventilation that is enabled by elevation of their rear feet, and by not placing them immediately under wide flat surfaces. When such simple precautions could have prolonged their useful lives, it is pointless to rabbit on about poor design. If doubt rather than experience is to be the yardstick, I record my doubt that the slot-loading iMacs were poorly designed.

As convectors the iMacs were at least as effective as your average domestic convective heater. Perhaps there were inadequate components—given the level of forethought that would be used by their owners—and inadequate instruction from the makers in how to preserve them, but the time for handwringing and recrimination about those factors is now long past.

de