Thank you Doug! NOW it makes some sense as to why cleaning the board of residue might restore funtionality - because the cap may be unaffected! So much time is given to this topic, yet the basics are often overlooked. I am ignorant of this topic in a scientific way, so a simple explantion like this really clicks. ATTA BOY!
Well, kinda sorta. The cap is most *definitely* affected -- the guts are there for a reason, so when they leak out, stuff happens. :)
Here's more of an explanation (warning: Extreme nerdliness ahead; can't help it -- it's an occupational hazard):
1) The cap leaks its electrolyte (thanks to poor manufacturing). The electrolyte is electrically conductive, so its presence on a circuit board naturally wreaks havoc. Cleaning off the electrolyte takes care of that havoc.
2) The electrolyte is necessary for maintaining a high capacitance (the dielectric is effectively a nanoscopically thin aluminum oxide layer maintained by the electrolyte; the conductive electrolyte also acts effectively as one of the plates). So, when the electrolyte leaks out, the capacitance goes down (easily by a factor of 10 to 100 or more, making it behave more like an open circuit, for all practical purposes). The reason the mac continues to function after the residue is cleaned off is simply that most of these capacitors are redundant "power supply bypass" capacitors. That is, they are simply placed across the power supply terminals to act as short-term energy reservoirs to handle transient current drains. There are typically many more of these caps than are strictly necessary, so even if, say, half of them go away, the mac will likely not care. That's why it is usually unnecessary to replace all of the caps (it's a tradeoff between risk of damage from further leakage, and risk of damage from your monkeying around with a soldering iron).
3) One example of where a failed (open) cap *does* matter is when it happens to be directly in the path of a signal (e.g., the audio). You'll get weak, scratchy, or no sound if that cap goes. Cleaning will not help that problem, of course -- you must replace the cap. To find the cap in question, connect one lead of an ohmmeter to the speaker terminal, and use the other one to find the cap that's connected to it (likely near the speaker, or near the sound chip).
--Cheers, Tom -- Prof. Thomas H. Lee Center for Integrated Systems, CIS-205 420 Via Palou Mall Stanford University Stanford, CA 94305-4070 http://www-smirc.stanford.edu 650-725-3709 voice, -3383 fax -- 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 ---------------------------------------------------------------
