I've seen these two swollen caps in a symmetric +12, 0, -12V power supply.
The circuit (a theremin) seems to be working properly, but I don't know if that caps are going to cause damage in the whole circuit in the long term. Should I replace them?
closed as off-topic by Nick Alexeev♦ Aug 18 '14 at 0:56
This question appears to be off-topic. The users who voted to close gave this specific reason:
Yes, you should replace them.
It seems to me that the left one still did not release any dielectric fluid, while the right did release some. That fluid is a nasty sticky smelly thing that will pour on the PCB. I don't think it will damage anything but... It's disgusting.
Your circuit works because these are just levelling caps, and it's all analog in there. I bet the release of the fluid diminished the actual capacitance dramatically, thus increasing ripple... But who cares? I mean, you probably can't hear it. Maybe turning the volume up will reveal some malfunction, maybe the relevant chips have decoupling 100nF-ish caps nearby so they don't get much ripple, maybe you like the mains hum as a note and keep doing it so that's why you don't hear anything strange... The difference between "seems to work properly" and "works as designed" may be huge.
Just replace the caps, they are cheap and you can get rid of these old, dried monsters.
Replacing the caps is a good idea.
The deformation is caused by a high pressure within the cap, which means that the electrolyte (liquid!) became so hot that some of it vaporized. For this to happen, you need a failure mechanism like (i) excessive leakage current, caused by overvoltage or a failure of the dielectric oxide layer on the aluminum foils or (ii) an increased internal resistance of the caps, caused by an aging electrolyte. Note that once some of the electrolyte vaporizes, the resistance will increase further, making the effect self-accelerating until the capacitor has lost all of its electrolyte and becomes "dead" - it is physically still present, but won't act like much of a cap any more.
The deformation also increases the risk of the cap developing a short circuit, which might cause further damage to the "upstream" parts of the system (power supply, transoformer, rectifier, ...).