At 17:43 +0100 7/4/05, Sean Billings wrote: >And another thing! There was mention on this thread I think about why >capacitors fail, pointing to a finite life etc. could it also not be that >the extended periods of inactivity these machines went through before being >'rescued' by people like ourselves could contribute to the failure?
Aluminum electrolytic capacitors are made from two layers of aluminum foil that are separated by an insulator made of porous "paper" saturated with an acid - it was once sulfuric or chromic acid but the choice is proprietary. After the capacitor is made it has its electrodes formed by applying a DC voltage that is slowly increased. The process makes current flow and the anode aluminum oxidizes to form an insulating layer of Al2O3 which increases in thickness during the forming process. It is that very thin layer of oxide that is the insulator that makes the aluminum plates be a capacitor. The remaining acid is just a conductor. When capacitors like that dry out the thickness of the insulator becomes large and the capacity gets smaller such a capacitor is trash. When they are left for a long time without a DC biasing voltage the oxide can de-dissolve into the electrolyte. In some cases slow application of increasing voltage can reform such a capacitor but that's hard to do in-circuit because there is no way to start with a reduced voltage. The failure being seen in classic Macs is a simple leak of electrolyte which doesn't affect the capacitor very much but it raises hell with the printed circuit board underneath. The capacitors leak because they were cheaply made with insufficient testing of the design. -- --> On the eighth day, about 6 kiloyears ago, the Lord realized that free will would make man ask what existed before the Creation. So He installed a few gigayears of history complete with a big bang and a fossilized record of evolution. <-- -- Compact Macs is sponsored by <http://lowendmac.com/>. Support Low End Mac <http://lowendmac.com/lists/support.html> Compact Macs list info: <http://lowendmac.com/lists/compact.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/compact.macs%40mail.maclaunch.com/> --------------------------------------------------------------- iPod Accessories for Less at 1-800-iPOD.COM Fast Delivery, Low Price, Good Deal www.1800ipod.com ---------------------------------------------------------------
