On 15/2/04 8:30 pm, "[EMAIL PROTECTED]" <[EMAIL PROTECTED]> wrote:
> I wonder if anyone can help? > > I have a Rev A iMac with a presumed dead analogue board (fly back transformer) > as it exhibits the well known symptoms and I can run the iMac with a different > monitor. I recently obtained a supposedly refurbished analogue board but on > installing this, the problem is the same. > > Does anyone know if the symptoms are indicative of anything else or does the > replacement board have the same dead FBT? (Original symptoms were the strange > sparking noises then iMac which attempts to start up, then switches off > withing a few seconds. Immediate attempts to restart achieve no response, but > if left for some time will begin restarting then switch off again. Works with > separate monitor). > > I'd be willing to travel a bit to have this checked out if possible, I live in > NW London. > > I have been offered a fix to the analogue board by replacing the FBT but I > want to make certain it isn't some other problem. > > Any help or suggestions greatly appreciated. > > Richard > 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 -- 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
