> from: Steve Bell <[EMAIL PROTECTED]> > date: Sun, 15 Feb 2004 21:20:27 > to: [EMAIL PROTECTED] > subject: Re: iMac Rev A analogue board and flyback transformer > > Flyback transformers on tray loading iMacs can fail in 2 ways. The most > common is that the iMac starts up, power LED is orange. Just as the screen > display would normally appear, the iMac turns off. It will restart, but with > the same symptoms, shutting down again. In most cases you can disconnect the > EHT cap from the CRT and read less than 50K ohms using a DVM from the cap to > common. It should also read around 3.5 - 4 nF to common on a capacitance > meter if good. > > The other mode of failure is the iMac starts up, display appears, and can > run from seconds to hours. It will then chirp or crackle and shut down. It > will not restart. If unplugged from the mains supply it will restart if left > for 15 - 30 mins, longer if left plugged in. In this case the only way of > 100% proving the FBT has failed is to replace it. This is because the HV > capacitor connected internally in the FBT breaks down in use, it isn't leaky > as the other failure mode. > > Flyback transformer failure on the tray loading iMacs (they never fail on > the slot loaders) is the most common hardware reason for failure. On rev A > and B's the FBT is usually cream, the newer replacement was black. > > Pulling the internal DB15 video plug and attaching an external monitor is a > good way to check for FBT failure. By pulling the connector you disable the > H & V syncs, this puts the monitor section into power saving. Power saving > disables the B+ supply to the flyback transformer, so if faulty it doesn't > put an excessive load on the power supply shutting it down. > > It looks as if your replacement board has a defective FBT. Does the > refurbished analogue board have a clean dust free flyback transformer? > > Steve Bell >
Steve Thanks for your response. The replacement board displays exactly the same symptoms as the original. The iMac orange button lights, but as the green seems set to go on, it shuts down. Leaving it for a while will allow it to go through the process again, but it will do nothing if you attempt to start right away. In both cases, using an external monitor, the light glows orange but I suppose as there is no synch with the internal monitor it stays that say. The whole replacement board looks clean, but on closer inspection you can see that most of the parts have been cleaned, and this is apparent on the FBT. I honestly cannot say if the fault lies with the original supplier or the person I purchased it from as he claims he got the wrong one by mistake and has never tested it. The upshot is that he has agreed to get a new FBT and replace it on the board. I'm willing to accept this, but I needed to make sure it had failed. I'm grateful for your detailed instructions but I'm so damm ham-fisted when it comes to things like that, I'd better try and find someone who can do it for me (hopefully). I can change the whole board, as I've been shown how to successfully discharge the CRT, but beyond that I'm afraid my electric/electronic talents are restricted to changing fuses. Apologies for being so long winded, but I've now got two dead analogue boards. I was hoping to get this one running for my brother in Liverpool who is disabled and would benefit from access to a computer at home (I was going to get him internet access as well - but will hold off until I get this sorted). Thanks again, and as before any further thoughts suggestions will be welcome. Richard -- Mac UK is sponsored by <http://lowendmac.com/> and... 123Inkjets.com <http://lowendmac.com/ad/123inkjets.html> Support Low End Mac <http://lowendmac.com/lists/support.html> Mac UK list info: <http://lowendmac.com/lists/mac-uk.shtml> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive: <http://www.mail-archive.com/mac-uk%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
