Arcing cap???

Greetings all,

Think this is the best place for this.

Heard a new term at my new work today that I do not understand or see the logic in. Hope someone can explain it to me.

Basically the company designs and builds valve based guitar amps, and I heard "the cap is arcing" I have not known this before. Can someone give any info? They changed the cap based on sound and the problem was sorted, could this mask an original fault within the design by replacing a cap with a tighter tolerance one?

Sorry if it sounds like a stupid question, but I have never heard of this before, unless the electrolytic has broke down or breaks down at high voltage.
 
A capacitor is two conducting "plates" (usually wound up), separated by a dielectric material
that increases the capacitance value. Arcing is when a short circuit happens between the plates,
and it can cause a high current to flow. Sometimes this short is self-healing in film capacitors.

This is my understanding of an arcing cap too. It either works or doesn't.

My question is how can someone tell a cap is arcing by sound alone? Ie breaking down. And say a cap is faulty?

My guess would be replacing a cap with a higher value one to mask the underlying problem. Just asking for advice as this is a complete new one for me.
 

rayma

Member
2011-04-29 8:37 pm
how can someone tell a cap is arcing by sound alone? Ie breaking down. And say a cap is faulty?
My guess would be replacing a cap with a higher value one to mask the underlying problem.

Arcing could cause a visible flash, or a sound. Once it happens, I wouldn't trust that part again.
Proof of fault of the part is, when replaced with an identical part, the new part works properly.

Some do tend to use electrolytic capacitors with an inadequate voltage rating, because of cost or size.
There could be a manufacturing problem or a circuit design error, if the failure repeats.
If the part is new, it is rather unlikely to be defective, and more likely that there is a design problem.

If an unused electrolytic capacitor has been on the shelf for years, it should be reformed before use.
Immediate failure can happen otherwise.
 
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To clarify I am talking about an electrolytic cap (through hole), in my experience it works or it doesn't. Valves are new to me and I get the harmonics thing.

But I don't see how you can say a cap is arcing with no proof, replace it and say it is fine.

Am I missing something here?

Even though valves are new to me, a cap will either work or not work. Replacing it and saying the cap is faulty sounds like a lottery to me and you are relying on the tolerances of the new cap to hide a underlying design issue.
 

rayma

Member
2011-04-29 8:37 pm
But I don't see how you can say a cap is arcing with no proof, replace it and say it is fine. Even though valves are new to me,
a cap will either work or not work. Replacing it and saying the cap is faulty sounds like a lottery to me and you are relying
on the tolerances of the new cap to hide a underlying design issue.

Repairs are very expensive in technician labor costs, so the tendency is to replace the apparent problem part and move on.
If it's a product prototype however, you want to be very certain of the problem's cause. Parts can be intermittent as well,
which can be problematic in tracking down the source of a problem. This is one reason to burn in production units.
 
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Don't read too much into it. Basic troubleshooting isolates the source of the noise to a point in the circuit. They found a cap breaking down. Replacing it fixed it. Best description of that breakdown was an "arc".

I can often tell that a transformer is arcing inside even though I cannot see it. If it in fact is not actually arcing, it is doing something with the same result, so the description is the same.

Popping is different than just being leaky. A leaky cap lets DC through, whereas one breaking down a different way may withstand the voltage but every now and then allows a bit to siphon off.

Why does it need to be a design issue? Parts fail, even parts used well within their specs and time of life. Especially tube amps, tubes themselves can fail at any time, even new from the box.

Why did that particular cap fail? Probably not from design. More likely some tiny flaw in the dielectric. It is a long strip of something rolled up. Any tiny little blemish in that roll could result in a small void or thin spot. So that is a weak spot. The cap works, but like a cracked rung on a ladder, climb it one time too many and it snaps. Your cap may have been just waiting for some little voltage surge/spike that was just enough to puncture the dielectric.

COmputers have occasional "soft errors". As I sit here typing and you sit there reading, countless cosmic rays pass right through our bodies, and our computers, and for that matter the entire earth. Once in a while they actually hit an atom in something. In a computer memory, this CAN cause a bit to change in memory if it hits an atom in a memory cell. One wrong bit can crash a program, yet reboot and absolutely nothing is wrong with the machine. Well a static discharge or even a cosmic ray could have had a little effect on this part. Sounds crazy, but stuff happens.
 
A leaky cap and caps breaking down I understand, but when someone says it is "arcing" without seeing it arc made me think do I not know something or do they not know something. I'm familiar with the workings of a capacitor thank you :).

I understand parts fail from new and have seen it on large scale from my last job, but they mentioned quite a few have done it now, so I had a quick look and they are different batches, hence my thought of something in the design.