Author Topic: When electrolytic caps go bad, do they usually internally short, or go "open"  (Read 1797 times)

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Offline gooseEL34

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Item is a Polk Audio PSW202 powered subwoofer. 
Blows 2A fuse immediately.
There are 2 4700uF / 35v caps in the power supply.  Tops are obviously not blown and spewing goo, but the PCB is covered in black goop (like guitar pedal makers use to coat components and "protect" their design).

So.  If I cannot see any obvious sign of damage, I "should" be able to set my VOM to resistance and if I touch + and - leads and get a low resistance (40ohm or under) show it bad, right?

Can I do that in circuit usually, or do I need to desolder to test it?


Offline Flump

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check the rectifier

Offline mrgregs

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Here is a list of capacitor failure modes with their relative probabilities. Caps should be tested out of circuit and discharged.
As for the goop in guitar pedals, it's not usually to protect the designs, but more because if a lead separates, it's always the manufacturer's fault and not the guitarist who stamps on it as if they're trying to push it through the stage, throw it in an already full gig bag from a distance or kick it across a stage whilst it's still connected (or fling it at bandmates and missing), oh no.

Offline 0xdeadbeef

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ESR Meters can measure the capacitance and ESR in circuit. E.g. this one here:
Trying is the first step towards failure - Homer J. Simpson

Offline cncjerry

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It's probably the output transistors. One of them is shorted.

Offline Mephitus

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ESR Meters can measure the capacitance and ESR in circuit. E.g. this one here:
Or might I recommend building one? I plan to make one soon. Likely something like this:
A true gentleman must be prepared for anything. - Pepe le' Pew

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