Missing Capacitors, but "Shuttle XPC SG45H7" works fine. Side effects ?

Capacitors sort of absorb any fast, small variations in the voltage across them, by charging/discharging against the circuit's impedance. That's how they absorb 'noise'. They can also be thought of acting as local reserviours of power. Imagine a componenent suddenly increases current draw... if the power has to be supplied entirely from a long trace back to the regulator, that trace will see a voltage drop. A local capacitor can supply the immediate and transient current demand itself, making power transfer to the demading component quicker and reducing (or slowing down) the voltage drop along the trace, a drop which causes noise and issues for other components. The faster current changes, the more noise is created so capacitors slowing it down is very helpful.

Now... if a capacitor is missing, while not ideal if the designer intended it to be there, it isn't necessarily a cause of immediate failure. It might have no effect, or it might have a little effect on reliability depending on current activity, ambient temperature, whether the Moon is aligned with Saturn, etc.. Or it might be in a critical location where its action is essential. Capacitors around regulators are essential to maintain regulator function, but reserviour capacitors further along the power rails somewhat less so.

As to failure modes of solids... those have been around since the 1960's so it's a real shame they aren't used more. But then, why make a board that lasts 20 years if you can make one that lasts five, and make sure the user needs to replace them. Solid aluminium capacitors (of the type you show) have no specific failure mode. They can live decades, even at high (within spec) temperatures. I guess evetually mechanical failure from solder joints, repeated thermal cycling or the like could damage them. Tantalum capacitors (another type of solid) are a bit more prone to failure, and age by growing microscopic crystals which gradually decreses capacitance and increases leakage current. They still live a long time though, unless you over-volt them. That's when those things tend to act like little thermite charges.

As normal electrolytic capacitors age, the electrolyte slowly 'vanishes' which leads to a loss of capacitance and, more importantly in power applications, increase in ESR. The Equivalent Series Resistance is the small amout of resistance present in the capacitor (they all have some). As this increases, the power dissipated by the capacitor while soing its job increases, leading to increased temperature, quicker degradation of electrolyte, and eventual failure.

P.s. Many years back when the capacitor plague happened ('fake'/substandard capacitors being sold to several manufacturers) I must have replaced blown and swelling capactiors on at least 50 motherboards and other things.

 

75% of the capacitors in electronics are there for "nothing".

Obviously "nothing" is a joke, but not 100% a joke.

I mean that, say any digital board has one capacitor placed next to each IC to cleanup his power supply from tiny variations due to peak current.

Well those were likely vital in the days of RTL / TTL / early MOSFET logic from the 70s/80s.

Now everything is way more robust even short circuits in output pins usually don't destroy the component, and all the logic is usually extremely resilient to power supply noise. Also (aside CPUs/GPUs and alike) everything works with currents that are 1/1000 of what we had in 70s.

But the capacitors next to the ICs are mostly still there, it's just inertia, and lack of will to experiment seriously on thing that everyone accepted blindly since the secondary school.

 

Sorry but what's the value of knowing what a missing capacitor capacity was, if the final goal isn't to replace it/them?

The board works, and three capacitors are missing. End of the useful info, for anyone,but a electronic technician.

 

Wow - I made a lot of spelling mistakes in my previous post. Sorry, was on a new PC install and had forgotten to add a dictionary to Firefox. I can spell quite well but my fingers are rebellious and outright jokers at times.

An answer I often use myself

I've taken a closer look at your MB. By the looks, that heat sink next to the missing capacitors hides a few MOSFETs, driven by the little regulator ICs between the (missing) capacitors. The cubes on the other side marked Magic R30 are 0.3uH inductors, used by the regulators along with (some of) the capacitors. Given that, I'm a little surprised the MB is functioning. I would guess they are general reservoirs/filters, probably for the common, input side of the regulators. Since the PSU also has some big capacitors on the output stages, it's clearly enough for things not to get upset. A close-up photo of your MB, showing part numbers if they are printed, might be educational.

When younger, one of my favourite uses of very large electrolytic capacitors pulled out of old TVs and the like was to wire them to a chunky PSU, backwards, and retreat to a safe distance

 

Knowledge is never useless.

But there are different kinds of "useless" knowledge.

In that case knowing the details of capacitors that once were there, is close to be really useless. On the other hand approaching the problem slightly differently can be very productive (even if not immediately useful).

Say understanding why the capacitors were there, understanding if they are/were in parallel or meant to serve a single phase, understanding how much uF a missing condenser should be w/o having a friend that send a photo, could be a very nice piece of "useless" info.

 

They are clearly placed, one per phase, on a four phase converter and very likely (being 16 V/270uF and being placed on the opposite side of the CPU), they are placed on the 12 V line BEFORE the conversion process, that's why they are "useless", given the 12V is already leveled multiple times, in the PSU and other places in the MB (eg. the fourth surviving one).

Likely they exploded because a fault in the PSU, or were just torn off because a "too vigorous" dissembling process.

In short surely replacing them doesn't hurt, but likely w/o them replaced the MB will last another 20 years.

P.S. notice that you also miss the heat-sink on the mosfets (assuming you didn't remove it to take the photo) that could be serious a problem, especially using higher end CPUs

 

Yep, all that sounds spot on. Including the missing heatsink being more an issue for longevity. They'll get a fair bit off heatsinking through the soldering of the back to the MB of course; clearly enough to run OK so far. Given we've decided the capacitors are definitely regulator input reservoirs, value isn't critical (including, apparent;y, 0uF )

My bet is someone gave the board a pretty good whack with or against something. Those capacitors really have been torn off, lol.

 

How do you know they are missing? Is there any indication they have been de-soldered? Or is the pad clean where they appear to be missing?

What's a heatspreader? Do you mean heat sink?

OK, from the drawing, I see what you man by a heatspreader, I call them heat sinks. However, they would not use a heat sink for nothing, they usually have a chip under them. If the heat sink is missing, is there anything under where it should be?