Tolerances for capacitor replacement: electronics query

[beachycove]

I am wondering about replacing some capacitors in my SE30, specifically the 16v 47µF ones that mostly are found on the logic board (I've just discovered that several of these have been leaking, so I suppose that 'tis time for a small piece of electronics repair).

Somewhere or other (Applefritter, maybe?), I have read that finding exact matches for the replacement of these capacitors is strictly unnecessary, i.e., that capacitors with different specs can readily be used. Is there someone with the necessary know-how out there who could comment on this suggestion? Would, for instance, the 47µF 50v capacitors that I have lying around work the requisite magic, or 33µF 16v capacitors? Or would installing such components merely fry the machine?

Excuse my ignorance, but I would welcome an explanation. And no, don't worry, my instinct is to find components with the original specs, so I don't have the soldering iron warming up or anything.

 

[equill]

If the voltage rating of new capacitors exceeds that of the capacitors being replaced, and if the physical fit is right, there is no disadvantage other than cost in using same capacitance/higher voltage units. The 47µF caps are mostly doing filter/reservoir work, so accuracy of ±10% in value is acceptable. Putting in 33µF caps will not kill anything, but it diminishes the capacitor's capacity (pun intended) to do its job by a greater margin than 10%, that's all.

If you are considering standard aluminium electrolytics at the moment, why not drop jdw a PM to see what he thinks of his own use of tantalum electrolytics on one of his boards.

de

 

[Franklinstein]

With capacitors, if you can't replace like-for-like insofar as values are concerned, it's best to go higher. Going lower isn't recommended. There are some good sites concerning capacitors that are readily found with Google.

Essentially, voltages are the maximum ratings for the capacitor - installing a 50v cap in a 12v circuit is fine (the capacitor doesn't have to work as hard if it's dealing with a lower voltage rating than it's built for), but installing a 5v cap in a 40v circuit will usually kill them.

The capacitance ratings are important as they dictate how much filtering or charge storing capacity is provided by the component. Any lower than that specified by the circuit may cause issues, such as lower average voltages or current ripples (in AC-DC filters, for example).

Next is temperature ratings. Most caps are rated for 85 C maximums, and they are the most common. However, some applications call for 105 C-rated caps, and I'm sure that some can go higher than that. If the capacitor is going to be used in a hot environment (such as the ones in my car's engine computer), it's best to use the higher temp type. Using an 85 C part where a 105 C part was installed will likely result in a shortened component lifespan.

The specifics for all of the above are generally listed on the component.

Final notes: Make sure to replace electrolytics with electrolytics (and ceramics with ceramics, etc, if they're bad), and to install an electrolytic capacitor in the correct orientation. Also, keep in mind that capacitors with higher values are larger than their lower-value counterparts, so you may not be physically able to install a 50v 47uf component where a 16v 33uf component existed previously. I used to know of a pretty good website where you can order almost any capacitors - I can dig it up if you want.

 

[wally]

...Make sure to ...install an electrolytic capacitor in the correct orientation...

There are exceptions but interestingly the aluminum electrolytics tend have the negative terminals marked with a graphic symbol and the tantalums tend to have the positive terminal marked with a graphic symbol. Count yourself lucky if the symbol is a clear + or -. Do not assume that that bar marking on the end of an SMT tantalum is a long negative sign! Being wrong results in a fried part immediately when power is applied, sometimes explosively. Be careful, and check the data sheet if in doubt, or do a bench test with an extra part!

 

[beachycove]

So the voltage can be higher, but it's best not for the voltage to be lower. 16v or higher it is, then.

In the light of what has been said, a little further clarification of the µF value would also be useful, so if I can impose again: will use of a higher rating (say, 100µF, 16v) also cause problems where the original was 47µF, 16v? Is the 47µF a minimum rating, like the voltage, or something else?

Alas, these are things I ought to know – but I do not.

Many thanks. All this is very helpful.

 

[Mac128]

And now for historical commentary: As equill points out, cost is about the only factor about sizing up. In Larry Pina's excellent book of Macintosh Repair he really goes on a tirade about Apple's choice to use low temp and undersized caps in the original Macintosh designs, a choice they clearly made based on analogue board designs which accommodated larger more robust parts as well (thank gods for engineers). He states they got it right by the final run of the Plus, but even still a few of the caps were undersized, which he recommends upgrading. He blames this more than anything on the high failure rates of the original Macintosh, not the fan-less design. Tom Lee's excellent paper summarizes much of this discussion as well.

It's like some sort of greedy profit strategy:

Cost savings from use of inferior parts, minus

Savings lost to warranty repairs, plus

Profits earned from Apple Care, minus

Profits lost to warranty repairs, minus

Cost to minimally upgrade high failure parts, plus

Savings from fewer warranty repairs as the newer parts last just past the warranty period ends, equals:

more profit than if they'd used the proper parts to begin with ...

...otherwise why do it that way, when they clearly knew better?

 

[equill]

As always, it depends whether manufacturing or marketing is driving the company. Some of Apple's model conceptions reflect the latter, and it could well be that it also influenced build quality at times.

de

 

[wally]

... will use of a higher rating (say, 100µF, 16v) also cause problems where the original was 47µF, 16v? Is the 47µF a minimum rating, like the voltage, or something else?...

For bypassing, it's a minimum capacitance rating, at which the noise on the logic board reduced to an acceptable level. The usual capacitance tolerance on modern electrolytic parts is +-20%. Further increased individual or total capacitance will slightly further decrease the medium frequency noise, but too much total capacitance (thousands of microfarads) will slow the power supply's corrections to any detected deviations from nominal voltage (increased deviation duration, and in extreme cases cause voltage instability: step load transient noise exceeding the 125 mV amplitude /5 mS duration spec). You have some latitude here, remember that the power system design has to tolerate the unspecified additional capacitance of any optional SE/30 PDS slot card bypassing. NuBus cards have a capacitance per card upper limit spec of 1513 uF on the +5V, 536 uF on +12V, and 698 uF on the -12V, for example, to keep the power supplies in an stable operating region of reliable regulation.

For coupling (logic board C3, C4) bigger capacitance is better (for bass response). For analog board coupling, bigger is usually better, unless the capacitor is part of a resonant circuit or used to linearize a waveform. On the analog board C9 can be bigger, while C15 may need to be low ESR but right valued for linearity.

Aluminum electrolytic capacitor manufacturers often offer a variant part type that is physically smaller than their standard parts for a given capacitance and voltage spec. These should be avoided for replacement purposes and most new designs because they usually will have higher internal resistance, reduced AC current capability, and shorter operating life.

 

[JDW]

Others in this thread have offered you a wealth of technical information that I need not repeat. Suffice it to say, your reason to post here all boils down to your need to replace caps on your SE/30 logic board. No doubt you are inclined to replace only those which you physical see leaking. But I would advise you to replace them all.

Yes, I have replaced all the caps on two different logic boards in the past. It's worth doing. Why? Because many of the caps have leaked, even if you cannot see it. They all have aged too, so they all really do need replacing. That's the only way to be sure a system crash now and then is not being caused by a cap you neglected to replace.

I used electrolytics on one of my boards and tantalums on another. Both boards function the same, for now. But Tants will give you longer life. Indeed, the tants will keep going even 25 years hence. Electrolytics will leak at some point (although it make take more than 15 years for that to happen).

You will find that you cannot replace the biggest two caps with Tants, simply because they don't make tants that big. So use electrolytic for those. I would also suggest that you find the exact capacitance size, as it's not that hard to do so. Voltage doesn't matter at all, so long as your replacements are the same or higher voltage that the one's you're replacing.

So long as you have the right soldering tools and the right replacement caps, it's not really that hard a job. (And if you want to read more about capacitor replacement on older compact Macs, this thread may be enlightening.)

 

[beachycove]

I think that, having sized up the problem in the light of these posts, I'll shoot for the tantaliums, 47µF 16+v, and that I'll also replace whatever else is on the board capacitor-wise while I'm at it, following the last piece of wisdom posted. I thank you all again for your advice, which has been much appreciated.

One more contribution would be welcome. My soldering experiences are limited for the most part to cruder devices, but not being an unhandy klutz, I am happy to give the SE/30 fix a go. In this light, is the removal of the old capacitors a task requiring anything other than the usual: an hour or so, a steady hand, a low wattage iron, some braid to soak up the old flux, and a beer? It looks a slightly delicate operation, but presumably I just remove the cannisters' terminals by desoldering them at the surface, one by one, and all will be well.

 

[JDW]

No doubt you have a standard soldering iron. I used a special tip on my soldering station that has two movable prongs sticking which allow you to apply heat to both sides of a component -- making it much easier and cleaner to desolder SMD parts or surface mount Al Electrolytic caps, as are used on the SE/30 logic board. However, it is technically possible to use a standard soldering iron to removal all the caps. It just takes longer and you may break a trace or two.

If you use a standard soldering iron, you should be using a 35W iron or better. I wouldn't do the job with a 25W iron myself. You will then need to apply heat to one side of the SMD cap, then "flip that side up in the air" when you've applied sufficient heat. Then you have one side of the SMD cap hanging in the air with the other side still soldered. Desolder the other side and you can remove the cap. But you will put strain on one trace when you flip the cap up, which is why it's always best to apply heat to both sides at once. But you can only apply heat to one side of the cap with a standard soldering iron, so you really have no choice. And if you break a trace, you can repair the trace if you have a steady hand (which is basically making the connection between the side of the cap with the broken trace and whatever component that trace leads to).

The only other thing I can forewarn you about are exploding caps. If you use a standard soldering iron, this may not be a problem unless you just apply way to much heat to one side. But I blew a couple caps with my tweezer-style soldering head because the cap got too hot. Don't worry, it's only a tiny explosion. But sometimes a part of the cap may fly into the air, so you may wish to wear goggles or close your eyes.

This all may sound a little frightful at first, but that's only because you haven't done it before. But I've done two boards, and others on this site have down boards with a standard soldering iron, so we can attest to your high chance of success. The only reason you'd fail is if you don't have a steady hand, you have really bad eyes, or you give up easily.

It probably will take you an hour or slightly more to remove the caps to your satisfaction. You will then need to examine the open pads to make sure there are no breaks (being a perfectionist, I prefer to use a DMM's continuity checker to verify electrical contacts are sound after I desolder caps and then again after I solder in the new caps). I would also advise you to clean your board after you desolder all the caps in order to clean up leaked cap fluid. I personally recommend 99.5% pure Dehydrated Ethanol alcohol for this task (sold even on Amazon.co.jp here in Japan), which is what we use at the office to clean circuit boards and is perfect for cleaning electronics of any kind. But I've heard it's hard to find in the US because so many Americans have the tendency to imbibe cleaning agents like this. Even so, it's worth looking for because Long's Drugs style rubbing alcohol has salts and other impurities that will leave a slight film on your circuit board, even if you cannot see the film. I suppose that flux remover may also work too, but I haven't tried it. Anyway, you really need to clean the board after desoldering the caps or else the residual leaked fluid on the board could cause capacitance between pins of certain ICs on the board, causing system instability. Using Ethanol, I did a very thorough cleaning job in about 20 minutes.

Hope this helps!

 

[chris]

91% or higher isopropyl alcohol works as well, no salts, just alcohol and purified water(quoth my trusty bottle's label)

 

[MacJunky]

It just takes longer and you may break a trace or two.

You had better stress that "may" there buddy.

If you use a standard soldering iron, you should be using a 35W iron or better. I wouldn't do the job with a 25W iron myself. You will then need to apply heat to one side of the SMD cap, then "flip that side up in the air" when you've applied sufficient heat. Then you have one side of the SMD cap hanging in the air with the other side still soldered. Desolder the other side and you can remove the cap. But you will put strain on one trace when you flip the cap up, which is why it's always best to apply heat to both sides at once.

Or you can use desoldering braid on both sides to significantly reduce the solder there if not remove it completely aside from a thin coating(less solder = faster to heat up and weaker). Then with much less solder to deal with often just flipping the iron between the two joints works well or on some occasions just heating up one side and even very slightly pulling can break the other joint. Since there is less connecting it there is less strain on the pad as the remaining solder flexes more easily.
When I started I did it more like you mention JDW and I had a good success rate until my cat ended up surprising me and I lost a good 1CM of trace while overclocking my PDQ (my tweezers were on the resistor)( I was able to fix it.. used a tiny strand of wire that I pulled from the wire left over from an old set of headphones.).

Since I started doing things differently I would be willing to bet that if something was to surprise me like that again I would most likely not loose any pads or traces.

Oh, and I was (and still am) using a Weller WTCPN with TC201 pencil and PTA7 tip for jobs like that. It works well and I like it. I also like how it is not very ugly and is all one unit. (datasheet here: http://www.divshare.com/download/2469822-98b )

Heh, I only recently made the jump to non-lead based solder and I am not entirely happy about it.

And no! I have not burnt myself yet.

 

[equill]

It is worth repeating that straight-chain (ethanol, propanol) and branched-chain (iso-propanol) aliphatic alcohols have no inherent advantage over water when water-soluble salts from capacitors have to be removed. Further, against their use is that they are more expensive than water, more flammable than water, and in the case of anhydrous ethanol, extremely difficult to keep dry because it is so avid for water, even that from surrounding air.

Thorough washing, using a hand-operated spray bottle, with clean warm (<50°C) water containing a drop of household non-ionic detergent in each pint or so, followed by equally thorough rinsing with clean warm water, is effective. If any stubborn deposits remain before the rinsing step, a cotton bud (tip) is much preferable to a toothbrush for their removal.

After the rinses (plural) with clean water, anhydrous ethanol can do a good job of dissolving and removing the residual water. However, this is an expensive and flammable way of doing what standing the MLB vertically in a warm dry location for several days can do equally well, or a current of warm dry air can do more quickly.

Industrial PCB-cleaning uses sonication in aqueous media, a sophisticated form of 'dishwashing'. Volatile non-aqueous solvents are not used except to remove water-insolubles. Above all, flammable, narcotic or toxic solvents are not recommendable for use when there is no effective positive ventilation of the working space.

de

 

[Mac128]

Or you can use desoldering braid on both sides to significantly reduce the solder there if not remove it completely

This is my preferred method as well. However, I was never able to get a braid to work very well. My favored choice is a vacuum de-soldering tool (though more expensive than braid), sucks the hot solder right off the board leaving the leads extremely clean, then you can do as MacJunky says to loosen the residual solder.

FYI, if they are bad caps, RAM, ICs, resistors, etc., you can just clip the leads at the base with needle-nosed wire-cutters and then simply pull the pins one at a time. No need to delicately de-solder or risk exploding caps! ... unless you physically can't get under them to cut the leads, or are trying to salvage something. So much faster to yank those bad components. It takes much less heat and time to solder the new leads to the board.

 

[beachycove]

FYI, if they are bad caps, RAM, ICs, resistors, etc., you can just clip the leads at the base with needle-nosed wire-cutters and then simply pull the pins one at a time. No need to delicately de-solder or risk exploding caps!

Can this really be done with the little aluminum electrolytic caps?

 

[JDW]

FYI, if they are bad caps, RAM, ICs, resistors, etc., you can just clip the leads at the base with needle-nosed wire-cutters and then simply pull the pins one at a time. No need to delicately de-solder or risk exploding caps!

Can this really be done with the little aluminum electrolytic caps?

No, it cannot be done with SMD caps (which are the majority of caps on an SE/30 logic board).

At some point you will need to wade through the mountain of advice presented in this thread, and just select one method presented by someone who has successfully done cap jobs before. "Tyler" is someone other than myself who I know has done many boards before, but he has yet to respond in this thread. Even so, the information presented here is sufficient for you to get the job done. And as far as I am concerned, I can only report what worked for me (being no novice to working with and repairing electronic devices).

 

[tomlee59]

I have not done a thorough re-cap of any compact mac board, but I've done plenty of comparable operations on other circuit boards. JDW has a large amount of experience with this, and so you should pay close attention to his advice.

Although I have access to some pretty advanced soldering equipment at work, I have fairly rudimentary tools at home. Still, I make out ok for the most part. I have removed surface mount caps using two cheap soldering irons (one on each lead, natch). If you work quickly, you can easily avoid the exploding cap problem, as well as minimally damage the traces (since you won't really be putting much mechanical stress on the pads). That said, protective eyewear is always a good idea when soldering. We had a close call with a student soldering a little too close to his face. A bit of solder spat up into his eye. His contact lens saved his eye. I'd rather not rely on that sort of luck.

 

[JDW]

To supplement what Tom Lee has written, I can only add that I experienced "exploding caps" only on one of the two boards I did, and there were only two caps that blew up, and the reason why is because I used a tweezer-style desoldering tip that applied a lot of heat to both sides of the cap itself. Suffice it to say, you will likely NOT have an exploding cap. And even if you use tweezer style desoldering tips, so long as you don't apply too much heat, the caps won't blow up. I put extra heat on because I wanted to cleanly yank off the cap without any possibility of breaking the trace (which could happen if I didn't apply sufficient heat to melt all the solder but then "yanked" up anyway).

So really, there's not much you need to worry about. Just be sure to solder in your replacements in the correct position polarity-wise!