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Commonly Failing Electronic Components
(in vintage computers)


Tantalum Capacitor

Capacitors that use a tantalum based dialectric.  Used mainly to filter small voltage rails.

These either go open circuit or short circuit.  If they go short circuit, the power supply in the computer becomes overloaded and will normally not work (although the power supply's fan may still turn).

These are a reported common failure in IBM 51xx computers and related IBM cards (e.g.  IBM EGA card).  My own experience is that the failure rate is relatively high if the tantalum capacitor has been unpowered for many years (e.g. I buy an 'unknown working status' motherboard from eBay), but the failure rate is very low for a tantalum capacitor that is periodically powered (e.g. I use my IBM 51xx computers at least every few months.)

My observation is that if a tantalum capacitor is going to fail, it will most likely fail on application of power, or shortly after (say, within 30 seconds).

These capacitors sometimes fail quite dramatically, from a shower of spark-like particles to literally exploding, or both.  Three examples are shown below.  The first example produced a spark shower and the only evidence of damage to the capacitor is a small black eye/hole.  I have witnessed that a few times.  The second example is that of an exploded tantalum (that particular example emitted a fragment that burnt my carpet).



Do not assume that because a tantalum capacitor shows no damage, that it is good.  Faulty tantalum capacitors often show no visible indication of failure.

Tantalums have polarity.  They need to be inserted the right way - positive leg into positive hole, and negative leg into negative hole.

On the IBM 51xx motherboards and cards, the tantalums used by IBM are commonly 10µF/16V; they typically have "106 16V" or "10µF 16V" marked on them.
They either have two or three legs.  Two-legged 10µF/16V replacements can be found easily.
Click here for information about the three-legged tantalums on the IBM 51xx motherboards/cards.

Note that the early IBM 51xx motherboards/cards have three-hole positions containing a two-legged tantalum.  Such three-hole positions are not designed for three-legged tantalums.  Click here to see an example.

Do not assume that all orange capacitors on IBM 51xx motherboards and cards are tantalum.  For example, the capacitor circled in the photo at here is not a tantalum.



Dynamic RAM Chips

A common failure in vintage computers is that of dynamic RAM chips.

The failure of any chip in the 51xx motherboard's first bank of RAM (bank 0) results in what appears to be a 'dead' motherboard.
    * IBM 5150: Click here.
    * IBM 5155: Click here.
    * IBM 5160: Click here.
    * IBM 5170: Click here.
Of course, many other things can cause the symptom of 'dead' motherboard.



Line Suppression Capacitor

Another common failure in vintage computers is that of the line suppression capacitor.
'Line Suppression' refers to the functionality that the capacitor provides, not to the capacitor's dialectric.
May also be referred to as 'mains suppression capacitor', 'EMI suppression capacitor', 'RFI suppression capacitor', 'line filter capacitor', 'mains filter capacitor', etc.

These capacitors are in the power supply.  They absorb radio frequency interference (RFI) in/out of the AC power (line) connector, and absorb incoming power surges/transients.  Because they have to tolerate surges/transients of varying degree over a long period of time (example), they are specially designed to take that kind of punishment.

There are two classes of line suppression capacitor, X and Y, and those can be divided further into sub-classes, e.g.  X1, X2, Y1
More information on classes is in the 'futher reading' links below.

Aged line suppression capacitors tend to rupture.  An example of an old RIFA made capacitor is pictured below.

For RIFA made capacitors like this one, the rupture is accompanied by smoke and a strong foul odour. The odour will persist until the capacitor is removed.  The capacitor contains fluid, which may leak out. Do not touch the fluid (it may be carcinogenic, or otherwise toxic). The "X" in "0,1µF@X" probably indicates that this capacitor is of class X  (expected because it sits between line and neutral).

See here for a WIMA made capacitor that ruptured.

When sourcing a replacement for a faulty item, note that you need to match three things:
1.  Capacitance (e.g.  0.1 µF);
2.  Maximum working voltage (e.g.  275 volts AC) (can use something rated higher);
3.  Class (i.e.  replace X with X, replace Y with Y).

Further reading: 'Line-Filter Capacitors' section of my.execpc.com
Further reading: EvoxRifa document: Capacitors for RFI Suppression of the AC Line   (1996) (includes safety standards)
Further reading: EPCOS - EMI suppression capacitors   (2009) (includes safety standards)
Further reading: 'Noise suppression capacitors on AC mains' section of www.capakor.com

Aluminium Electrolytic

Note: Aluminium is also known as aluminum (no second 'i').

Aluminium electrolytic capacitors (big and small) in power supplies and in monitors.  Over years, the electrolyte within slowly dries out.  The capacitance goes out of tolerance and/or the ESR (equivalent series resistance) increases.

Sometimes poor engineering causes failure well before expected.  For example, failure of C11/C13/C14 in the power supply of the IBM EGA (5154) monitor was common (due to overheating).

The vast majority of failed aluminium electrolytic capacitors that I have found have no visible indication of failure/deteriation.  Some examples follow of units that I have found where there is a visible indication.  The second example whistled as it failed.  I consider the fourth example to be very rare.  



If you are testing aluminium electrolytics using a capacitance meter, note these two things:
1.  Aluminium electrolytics typically have a tolerance in capacitance of +/- 20%;
2.  You may measure the capacitance still within tolerance, but the capacitor is faulty because its ESR has increased beyond the point where it can be tolerated within the particular circuit it is fitted in.  

When sourcing a replacement for a faulty item, note that you need to match four things:
1.  Capacitance (e.g.  100 µF);
2.  Maximum working voltage (e.g.  50 volts);
3.  Maximum temperature (e.g.  85 degrees Celsius).
4.  ESR - replace a low ESR capacitor with another low ESR one.

You can substitute a larger working voltage capacitor.  For example, if a failed capacitor is rated at 16 working volts, you can substitute a 25 volt one.

You can substitute a higher temperature capacitor.  For example, if a failed capacitor is rated at 85 degrees Celsius, you can substitute a 105 degrees Celsius one.

Some aluminium electrolytics are designated as 'low ESR'.  As the words suggest, the capacitors are made to have a low ESR (as compared to 'normal' aluminium electrolytics).  Seen a lot in switchmode power supplies.  My understanding is that there is no industry standard and accordingly, that makers themselves decide on what is 'low ESR'.  The words 'low ESR' may (repeat: may) appear on the capacitor.  Some makers simply use different coloured lettering on their capacitors to signify 'low ESR'.  For technical reading on the 'low ESR' subject, see http://www.illinoiscapacitor.com/pdf/Papers/low_ESR_fact_or_fiction.pdf

85 degree capacitors are common in vintage computer equipment.  Some capacitors in vintage switch mode power supplies are rated at 105 degrees.  Some capacitors in vintage switch mode power supplies are rated at 85 degrees, but should have been made 105 degrees.

Aluminium electrolytic capacitors used in computers have polarity.  They need to be inserted the right way (positive leg into positive hole, negative leg into negative hole).  Markings on the capacitor will indicate polarity, and on new aluminium electrolytic capacitors, the shorter leg is the negative one.