How to tell a bad cap?

[AudioGuy88]

Hi everyone,

I had a Sunfire True Subwoofer which was making a buzzing noise when it when it was on with no signal. It made more of a buzzing noise when it had a signal.

I figure it was bad caps in the power supply so I replaced all the electrolytic caps. This fixed the problem and there no noise at all, so I'm very happy with the result.

I used both a multimeter and a multi-function tester to check the old capacitors. However all of them measure well within spec.
The caps I replaced were
2 x 1500uf 200v
4 x 1000uf 25v

They all measured well within 10%, but at least one of them must be faulty. How can I find out which one is faulty?

[stj]

Re: How to tell a bad cap?

you cant tell with a multimeter, you need a component tester or esr meter

[momaka]

Re: How to tell a bad cap?

you cant tell with a multimeter, you need a component tester or esr meter

I think that's what he said he did: used both a multimeter (with capacitance function test, I presume) as well as a "multi-function tester", which I presume is what he means for those Chinesium transistor testers / component testers.

In any case, if the caps are reading within 10% on the capacitance and ESR is good (for big caps like that, I think you should be seeing 0.10 Ohms absolute maximum), then the caps might actually be good and perhaps the buzzing issue was from a bad/cold solder joint somewhere that happened to "fix" itself when you took the subwoofer apart. Or it could be that dreaded tan/brown conductive glue (very commonly used on subwoofers), and you just happened to knock it off from wherever it may have been causing signal feedback issues due to leakage.

Without pictures and more info, though, it's hard to say what exactly happened.

[AudioGuy88]

Re: How to tell a bad cap?

I think that's what he said he did: used both a multimeter (with capacitance function test, I presume) as well as a "multi-function tester", which I presume is what he means for those Chinesium transistor testers / component testers.

That is correct. I am using the TC1 tester
https://www.ebay.com/itm/263291447651

In any case, if the caps are reading within 10% on the capacitance and ESR is good (for big caps like that, I think you should be seeing 0.10 Ohms absolute maximum), then the caps might actually be good and perhaps the buzzing issue was from a bad/cold solder joint somewhere that happened to "fix" itself when you took the subwoofer apart. Or it could be that dreaded tan/brown conductive glue (very commonly used on subwoofers), and you just happened to knock it off from wherever it may have been causing signal feedback issues due to leakage.

Without pictures and more info, though, it's hard to say what exactly happened.

There was the brown glue on the bottoms of the big caps. However they were only around the edge and not touching anything pins/pads.
With the small caps I measured:

Small caps
905uF - 925uF 0.2 ohm ESR 1.3% Vloss

Big caps
1400uF 0.18 ohm ESR 0.9% Vloss

Are those numbers enough to indicate if the caps are good or not?

[momaka]

Re: How to tell a bad cap?

^ Going by those numbers, the caps seem good. In particular, the capacitance is within range and V_loss is low an all of them, which means the caps aren't electrically leaking (high V_loss = high internal electrical leakage, which usually but not always goes hand-in-hand with abnormally high capacitance.) ESR seems OK overall. Perhaps a little high for the 1000 uF caps, as I imagine they should probably really be around 60-70 mOhms... but depending on the calibration of your TC1 tester, that's probably a normal reading.

So all in all, at least I can't see anything wrong with those caps.
It is possible that one of them may be breaking down only when in use... but highly unlikely, though.

If you really want try this test: charge each of the above caps to about 12-15V... maybe 20V, if you dare. Then remove the power supply and short out the cap's leads with a metal object (preferably something you won't mind getting a tiny burn mark on - so don't use your fancy screwdrivers.) If the caps are remotely good, they should give out a nice spark. With 12-15V, you may not see that much at that capacitance. But at 20V, you should definitely hear a small pop and see a spark.

BTW, also if the brown glue under the big caps was touching some other components around the caps and any of the leads of the caps, that could have been the source of the noise too, since when that glue turns brown (or dark brown / black), that's when it starts going slightly conductive. It should really be only tan in color.

Last but not least, what brand and series were the caps?
Another thing I can think of is perhaps on one of the big caps, one of its leads may be starting to corrode from the electrolyte inside (can happen with cheap caps sometimes.) And somehow, the cap is still working in the TC1 tester, but otherwise may have been on its way out.

[Per Hansson]

Re: How to tell a bad cap?

A capacitor can have leakage that only shows up when it is exposed to its rated voltage.
I repaired an old LEM amplifier that was like that: It had 4x 15000uF 80v caps.
The unit worked fine with a 300w incandescent light bulb in series.
But when I gave it straight mains the 10A fuses on one of the transformers secondary blew!
I identified the bad capacitor and brought it up on my Variac: up to around 50v it was fine.
But after that you could hear load pops inside the can, and the higher you got the worse it got...
The failed cap tested just fine on both my "component tester" and also Keysight U1733C LCR meter at any frequency...

Here is a nice tester for measuring leakage:
#346: Basics of Capacitor Leakage / using the Antique Wireless Association DC Leakage Tester
You can of course just use a bench power supply and measure the current going into the cap once the voltage stabilizes instead.

[momaka]

Re: How to tell a bad cap?

A capacitor can have leakage that only shows up when it is exposed to its rated voltage.
I repaired an old LEM amplifier that was like that: It had 4x 15000uF 80v caps.
The unit worked fine with a 300w incandescent light bulb in series.
But when I gave it straight mains the 10A fuses on one of the transformers secondary blew!
I identified the bad capacitor and brought it up on my Variac: up to around 50v it was fine.
But after that you could hear load pops inside the can, and the higher you got the worse it got...
The failed cap tested just fine on both my "component tester" and also Keysight U1733C LCR meter at any frequency...

That's very interesting.

I've only seen this phenomenon with loose capacitors on my bench - particularly with a few old, used ones that I was trying to reform to their full rated voltage (very old Samsung caps from the 90's and some G-Luxon LZ from an ECS motherboard.) In both cases, the caps would leak a ton of current past a certain voltage, but were OK below it.

So I guess what you're suggesting may be the most likely cause here.

[sam_sam_sam]

Re: How to tell a bad cap?

This is a very interesting topic and I just bought the board to build my own but I am going to change a couple of things one I going to use a digital volt meter modules and not use an external voltage meter unless I have to but will do some testing

I really building it so I can check main filtering capacitor on switching power supply and lower voltage output capacitors as well

[AudioGuy88]

Re: How to tell a bad cap?

Thanks everyone for their replies.
I don't have a scope or variac so unable to test it.
But I've got the answer I need. It seems in some cases testing a cap with a tester alone is not enough.
Thanks