Hey all, regarding this here is something to read... http://www.chemi-con.com/u7002/life.php
This article shows that operating an aluminum electrolytic capacitor (which is the type I assume everybody is referring to) below it's rated voltage has only a minimal impact on it's life. A more important factor in determining lifespan is the operating temperature. Additionally, while not discussed in the article, it is very important to understand that for an aluminum electrolytic the rated capacitance is AT the rated voltage... As you decrease the voltage, the capacitance drops off rather dramatically. I no longer recall the general decay rate, but that 100uF 16V capacitor when run at 5V is likely to be closer to 50uF (or less). The odd thing is that these aluminum electrolytic capacitors are a bit of a throw back... They work well at line frequencies and are suitable for linear regulators, but really not much else. Unless you get high frequency capacitors they are just about worthless in modern electronics when used by themselves. In most of the systems I have designed, the electrolytic capacitors were closely coupled to a solid tantalum and a small ceramic to deal with any instantaneous current requirements (perhaps 100uF, 2.2uF and .1uF). When it comes to solid tantalum capacitors I would always recommend running them well below their rated voltage which will certainly improve the lifespan without impacting the capacitance. As for replacing the aluminum capacitors on the motherboards... Here are my suggestions based upon a great many years of soldering and countless board repairs: Generally I do not advice destructive removal of a faulty component, but in the case of Apple's circuit boards I do. Some boards are capable of being reworked without great fear of lifting a pad, unfortunately the Apple boards i have worked on are not this type of board. While far from the worst boards I have worked on (where the pad lifts as soon as heat is applied) they are rather delicate. Carefully cut the capacitor in half and gently remove the can and any additional pieces leaving just the two leads on the pads which should come off without great difficulty. To assist you in the removal of the leads a small drop of liquid rosin will aid in the wetting process... Alternatively you can use a bit of fresh solder. When replacing the aluminum electrolytic capacitors use solid tantalum capacitors as a replacement. Ideally you should use surface mount and not through-hole devices with the leads bent over to match the pads. The reason for this is the force placed on the pads by having an elevated mass is not to be ignored... An unfortunate bump can cause the capacitor and pads to be torn from the board. Once the capacitor has been replaced and the board has been cleaned, I would suggest using a small dab of epoxy resin to ensure the capacitor stays put. Cleaning the board is quite important! People have hemmed and hawed about using a dishwasher or immersion in a water bath, but do not fret. With rare exception most electronics can be totally immersed in water without harm... Providing the board is thoroughly dried prior to applying power. Generally anything not sealed would be a non-submersible part including (but not limited to) some switches and audio enunciators (the little beepers/buzzers you sometimes see in electronics). One important note on water is that ideally you should use distilled (or better distilled & deionized) water for a final rinse. Most of the water we drink has dissolved minerals in it which, while not likely, could cause a problem once it has been deposited on the components. Isopropyl alcohol is a great solvent to clean the circuit boards, but keep in mind that when the alcohol evaporates it does not take the impurity with it. To clean properly with alcohol you must clean the wet alcohol off the board (Kimwipes are ideal for this). If you do not wipe up the alcohol the residual flux or whatever you are trying to clean off the board will simply be thinly spread of a larger area (the area covered by the wet alcohol). If you are really crazy, a thorough water flush followed by a distilled-deionized wash and then an immersion in trichlorotrifluoroethane will do the trick (after drying of course)! In my younger Navy days I once had a radio room flood with salt water and we had a couple dozen transmitters totally immersed. We used the triple wash method and only had 2 or 3 transmitters not come back to life. If there is interest, I can take some pictures of the process (I have a great many boards to recap) and put them... Somewhere. This is not a difficult process but it does take some finesse and it helps to have good equipment. Without re-reading this long mess I think it is all correct and coherent. Derek ex-Navy 2-M, C Level micro tech (for those who know about this sort of thing) On Jan 7, 2013, at 2:59 PM, Jeff Walther wrote: > > > On Dec 30 2012, 12:26 am, James Fraser <wheresthatistanbul- > [email protected]> wrote: >> Hello, >> >> --- On Sat, 12/29/12, Doug McNutt <[email protected]> wrote: >> >>> And while I'm at it, Apple's choice of aluminum >>> electrolytics seems to have used a lot of caps rated at 16 >>> volts where a 6 volt rating would be good enough. It's >>> likely a perceived saving in parts where variety makes >>> automated assembly more difficult. >> >>> Six volt tantalums are cheaper and smaller if you can >>> separate the 12 volt bus from the 5 volt bus. >> >> I'll bite: when you say "cheaper," are you talking on an industrial scale >> (like Apple would use) or on a (typical) hobbyist's scale? > > One of the members of 68kmla.org dug up an old white paper on > capacitors and expected life times and such and the paper concluded > that capacitor's have much longer lives if they are never operated > close to their actual voltage rating. The paper or a link is posted > somewhere in the forums over there. I didn't keep a link, so to find > it you'd have to search. I think JDW may have posted it, but I'm not > certain. > > If you actually look at the expected hours rating on some of the > electrolytics, it is as low as 1000 hours. Even in a home > environment, that time would be consumed quickly and in an office > environment the capacitors would not last out the year. > > The paper recommended voltage ratings approximately three times the > expected operating voltages. So, in a 5V circuit, 16V is just about > perfect. The 1uF 50V capacitors Apple was so fond of are a little > odd, given that they're probably in, at most, a 12V circuit, so for > those I have a 1uF, 35V cap. as the 50V variety was difficult to find > in the small package size (physical envelop) needed. > > So, while it would seem that one would use 6V capacitors in a 5V > circuit, according to some evidence some of us read once, 16V is a > much better choice. :-) > >> I ask because, just out of curiosity, I pulled up an old post that listed the >> caps that Jeff Walther was offering a couple of years back (NB: circa 2010; >> he may or may not be offering the same selection in 2012) and didn't see >> any caps with less than a 16V rating: > > The selection has changed slightly and some new stuff has been added, > but it's still basically the same. My posting is sticky'd in the > "Trading Post" forum on 68kmla. > > I wish I had time to do capacitor installations too, but I just > don't. I have a job, a house, a 10-year-old son, fish, a vegetable > garden, I coach LL baseball two seasons a year, and I have an anemic > thing resembling a social life as well... > > Jeff Walther -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
