On Tue, Jan 8, 2013 at 12:28 PM, Derek Morton <[email protected]> wrote: > 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, This is a very good explanation. I've already re-capped a Mac IIci board, and I have a IIx that needs re-capping. I'd love to see documentation of the process. - Alex -- ----- 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/
