Can one faulty cap in parallel with working cap cause problems?

[Capsaican]

If there is an electrolytic capacitor on a circuit board that is faulty, is it possible to place a working electrolytic capacitor (with the same specifications as the faulty one) in parallel by soldering it on the underside of the board and have the circuit brought back to normal operation, or will the faulty capacitor still cause problems?

[bp1]

Re: Can one faulty cap in parallel with working cap cause problems?

You need to remove the faulty cap and get it replaced. Soldering the cap in parellel will increase the cap value eg faulty cap value plus the new one in parrellel

[junger]

Re: Can one faulty cap in parallel with working cap cause problems?

The short answer is no, however if the faulty cap is o/c then it should be ok

[PCBONEZ]

Re: Can one faulty cap in parallel with working cap cause problems?

Faulty capacitor will almost always still cause problems.

The times when it wouldn't cause problems would be when the bad cap is 100% open circuit which is very rare.
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[Capsaican]

Re: Can one faulty cap in parallel with working cap cause problems?

Thanks for the replies.

The reason I'm asking is that I have a device that has a LAN connection which is not working properly. I've read online that replacing a capacitor may solve the problem. I don't have the equipment to desolder the capacitor so I thought by just holding the new capacitor onto the underside of the board I would be able to see if there was a difference. It did make a small difference, I was getting less timeouts, but still nowhere near as it should be.

[junger]

Re: Can one faulty cap in parallel with working cap cause problems?

ok, the reason for the improvement is that the secod cap is helping with the smoothing, however the faulty cap is probably still passing current though itself causing shutdown/reset, can you cut one leg of the faulty cap?

[Capsaican]

Re: Can one faulty cap in parallel with working cap cause problems?

I will try to cut one of the legs of the capacitor.

What I've noticed since the degradation of the connection is that the LEDs on the port of the network switch that it is connected to constantly flicker even though there is no traffic. Is this a symptom of a faulty capacitor?

[PCBONEZ]

Re: Can one faulty cap in parallel with working cap cause problems?

That's impossible to say without knowing a lot about the device and where the cap is.
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Sounds like it's searching for signal and not finding one but that's just a guess.
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[severach]

Re: Can one faulty cap in parallel with working cap cause problems?

Some capacitor types fail shorted. Placing a good capacitor in parallel with a shorted capacitor will accomplish nothing.

High frequency circuits such as SMPS circuits are very distance sensitive. A capacitor with 1 inch leads will make so little improvement to a high frequency circuit that it may only be visible on a scope. Capacitors far from a noise source have little effect. This means that some capacitors are more important than others so some understanding of the circuit is necessary to know which capacitors are key.

Electrolytic capacitors usually fail with high resistance so they disappear from the circuit as they fail allowing better capacitors in parallel take the load without interference from the bad ones. In high frequency circuits parallel capacitors must be near the noise source to take the load.

If the leads are short or the frequency is low, the bad capacitor is not shorted, and you're testing the only key capacitor that is failing, a good cap placed across the leads works well to diagnose a bad capacitor. Unfortunately it is quite difficult to keep a steady hand on the capacitor to keep it in continuous contact so it's only a good technique when the feedback is instant, such as watching the voltage on a meter, the wave on a scope, or the device powers on. Counting bad packets is way too slow. As many bad packets will result from shaky disconnects as will come from the bad capacitor. I test by tack soldering the capacitor to the solder filets to keep the leads short and eliminate the poor connections.

[Capsaican]

Re: Can one faulty cap in parallel with working cap cause problems?

I guess the only way to really find out is to remove the capacitor. I'm a bit scared to do it, though.

The device is a satellite receiver, the Dreambox DM600. The problem, I think, is the 220uF 16V capacitor.

[PCBONEZ]

Re: Can one faulty cap in parallel with working cap cause problems?

The other problem is caps don't usually go from good to bad in an instant.
They fail over some amount of time which might be short or long.
In other words when problems start showing up the caps are usually partially shorted or partially open.
It's hard to say what will happen with another in parallel.
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[Capsaican]

Re: Can one faulty cap in parallel with working cap cause problems?

Transfer over the LAN did actually degrade over a few weeks. When it was working, I would get a transfer rate of about 3MB/s. It went down to about 200KB/s and stayed that way for about a week. It then went down to about 50KB/s. Now, a few weeks later, I am lucky if I can transfer a file a few kilobytes in size with success.