Disc capaitor leak question

[vigilante397]

You tell one person you know how to solder and all of a sudden everyone in town wants you to fix their things

So I'm working on a power supply for a player piano system (long story) and the owner says it worked fine for years then just stopped powering on. I opened it up and one of the covers for an electrolytic was rattling around inside. Cap itself was probably fine but I replaced it for good measure. But there were a couple 4.7 nF ceramics that looked suspicious. They both have a hardened black substance all over them, which leads the layman to believe goop has leaked out. But I have seen people glue capacitors to the board to stop them from vibrating, and there are several components on this board glued down but they all use a white paste. Also, all of that paste was applied relatively neatly, whereas this almost looks like it was spilled onto them.

They measure at 3.5 nF and 2.9 nF which leads me to believe they may be leaking, though I'm still doubtful that it's a big enough difference to stop the circuit from working altogether. Thoughts?

[bluebunny]

I don't think you're gonna get goop from a slice of ceramic.    And there's a beautifully straight line on the black stuff, which suggests it's been carefully painted on (or dipped).

[FiveseveN]

Those are high tolerance caps so the measured values are probably OK.

[anotherjim]

I don't think that cap type can leak - shouldn't be anything like that inside them.

Agree the values you measured are likely correct. That gloop looks like a variety of sealant/potting compound rather than something that leaked.. They could have been replaced either by manufacturer or a 3rd party repairer and the original white glue wasn't available. Boards can be held in stock a while before they are used, by which time a change has been made to the design and they are "reworked" to latest rev before fitting in the product. The resulting differences in appearance & workmanship can fool you into suspecting something is amiss.

That said, if they are used on the AC line power side, they could be varistor surge protectors...
http://uk.rs-online.com/web/p/products/2896780/

[antonis]

I tend to agree with Jim..

Perhaps one of them is connected across Hot and Neutral and the other across Hot and Ground (which should be wrong in case of step-down transformer excistence..)

You may test the supply without them in place and verify it..

[vigilante397]

Thanks everyone I was kind of thinking that from the beginning but I was hoping something was wrong with them because I don't see anything wrong with anything else

[duck_arse]

my 2c says they are way high volts ceramics, like 1kV or more. if they are across high volts, the goop may be finger fry protection. can you ungoop enough for some markings?

[vigilante397]

my 2c says they are way high volts ceramics, like 1kV or more. if they are across high volts, the goop may be finger fry protection. can you ungoop enough for some markings?

Yup. They're rated at 400V, so that could very well be it.

[thermionix]

Yep, no liquids/goop inside a ceramic.