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Old 16th November 2011, 12:11 PM   #1
RichIOM is offline RichIOM  Isle of Man
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Default Measuring caps without ESR

Hi,

Does everyone rely on an ESR meter to truely identify a faulty cap or can I use a standard multimeter/multimeter with capacitor test?

I'd like to test electrolytic capacitors in an amp I'm trying to fault find on:
Amp repair help (which if anyone is able to further asist with I would really appreciate).

I don't have the luxury of being able to budget for an ESR just yet, and I wondered if the industry relies on these or whether you can just as effectively work without one?

The caps all look to be ok, but they are at least 11 years old. I'm only really relying on the fact that the tops of them look fine, it's hard to tell if the odd one is bulging or not because some are more stumpy in size than others, there is certainly no obvious bulge.

Thanks.
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Old 16th November 2011, 04:16 PM   #2
cbdb is offline cbdb  Canada
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11 years is not that old. The caps are probably fine. If you scope the (power supply) caps voltage while the amp is putting out 50% of rated output and the ripple voltage is less than a few volts the caps are probably fine.

In old caps its not the ESR that becomes the problem. They start to leak current, and or have a much lower capacitance than rated. Charge the cap and see how long it takes to discharge (with no load). It should take a long time (5 minuets or so ). Tghis will show you if it leaks. The cap value is a little more difficult but doable. Discharge the cap thru a known resistance and use the time constant to figure out the capacitance.

Last edited by cbdb; 16th November 2011 at 04:20 PM.
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Old 16th November 2011, 05:04 PM   #3
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Hi RichIOM
Most of us actually do without any kind of capacitance quality evaluation like ESR testing. The age of good, industry quality electrolytic caps used within their ratings in linear power supplies and general audio application has been suggested here as 20 years from the purchase date. I can agree with that, noting that many cheap caps can fail much sooner due to inferior construction and materials. High temperature operation in some designs will also shorten cap. life considerably. If your amp runs hot and the parts too, consider a 5-10 year replacement cycle for these.

Don't be discouraged here. The simple expedient of using reliable parts and doing systematic replacements by a sensible time limit like even 20 yrs will likely be better insurance and you may never need be concerned again.
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Old 16th November 2011, 05:11 PM   #4
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If the series model capacitance (Cs), which is probably what your meter measures, is good/sufficient, the power supply will work fine. Ripple is dependent almost entirely on Cs and other losses would have to be huge before they mattered at all. Now, if other losses were huge, you might get (believe it or not) a very high reading for Cs, well above the marked value. Low is bad, unexpectedly high is bad. So, bottom line is, for mains operated supplies (not switchers), if the capacitance is about right, and if the thing isn't leaking or bulging, it's probably good. For switchers you really should be measuring loss, but selecting caps for switchers is a whole topic in itself.
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Old 16th November 2011, 05:35 PM   #5
sbrads is offline sbrads  United Kingdom
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If there are caps parked next to hot components then they could have dried out and gone low in capacitance without any noticeable bulging. A couple of years can be long enough for this to happen even with decent quality caps. I've lost count of the number of times I've seen this design faux pas.
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Old 16th November 2011, 05:58 PM   #6
RichIOM is offline RichIOM  Isle of Man
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Thanks for the input, the reason I'm looking at the caps as the fault is because I can't see any real fault with the amp, other than it's not very loud even when turned up.

It used to be loud and now isn't on the same equipment and speakers, it's my brother in-laws amp and I have the same speakers so I've since hooked it up to my equipment with my speakers and the volume is just the same.

There is one cap that may be suspect purely on the fact it doesn't kick when I drain it using a car lightbulb, where as the same rated cap next to it, gives off a real crack and a spark when I connect the bulb to it.

Both caps are rated at 8200uF.

I've attached a photo so you can see how close they are in proximity to one another which is why I would expect them both to be carrying similar charges?

Click the image to open in full size.
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Old 16th November 2011, 06:41 PM   #7
Mooly is offline Mooly  United Kingdom
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I very quickly skimmed your other thread.

My honest feeling is that there is something else making you think its not as loud. You have measured the amp output and confirmed its to spec. That pretty much rules out the 8200uF caps.

I am not familiar with the amp at all but anything like equalisers or custom sound settings can alter the level greatly. I don't really think the caps are at fault. Any source equipment that may have been changed and has a different outut level ?

If you want to prove the caps to yourself then perhaps swap them over.
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Old 16th November 2011, 07:59 PM   #8
RichIOM is offline RichIOM  Isle of Man
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Thanks for your feedback. I know what you mean by something else making me think it's not as loud, as many people can have this feeling when they get used to a system etc.

However, this system isn't mine it's my brother in-laws, who said it wasn't as loud as it used to be and sounded pretty flat until it's wound up.

I would have thought as you if I didn't then take it from him and listen to it myself. I have the exact same speakers, so I could rule that out.

I've heard it on his source equipment first, when he was showing me that it wasn't as loud. The only way I can describe how it's not that loud, is that it only starts to get loud at -25db, the dial starts at -72db, so theres a big gap before it get loud. We were even able to turn it into the +db before it was the sort of level I would expect of -20db/-15db. I would have expected +2db to be deafening, over a 5 speaker surround setup with a sub from this amp, in a small room.

It only starts to sound decent in my spare room (not big enough for a double bed), at -25db, on a pair of bookshelf 8ohm Infinity Alpha 5 bookshelf speakers.

Spec for Speakers here:

Infinity - Home Audio

One other fault I did see at my brother in-laws but haven't further tested on my own setup, as I've only been testing with a pair of speakers, was that not all speakers displayed on the display when it was on a surround setting.

So although all speakers we playing of equal volume when I put my ear up to them, the display wasn't showing all of them as lit, so I don't know if this factors into the fault in some way.

The caps in the photo aren't power supply caps, they're the two largest caps in the centre of the motherboard (I call it this because each of the input banks at the back have their own circuit board, like PC cards).

As the source has been varied and tested on every input to the amp, I think it's safe to rule the source out.

Thanks for all the help, I just really want to make progress with this as it's one of those faults I really want to fix and it's a really good learning experience for me, so I really do appreciate the input.
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Old 16th November 2011, 08:38 PM   #9
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It is really useful to load the amp with the proper impedance. I use 10 ohm 225 w variable resistors, then I measure the output of the amp on the AC volts scale. 35W into 8 ohm should be about 17 VAC. If the amp is not putting that out, it has a problem.
If the caps are putting AC ripple at full power, then they are probably low capacitance. I've never owned a meter that could measure 8200 uf, or even 100 uf. The capacitance scale on $30 sears meters tops out at 1 or 2, realistically. I mostly measure B+ on tube amps, if it sags at the 4th section but not at the first it is the cap, not the tube. (had to recap my ST70 3 times for low B+, and two rectifier tubes). The same principal applies to solid state amps, only they don't have 4 gang caps typcally and SS rectifiers either work or they don't. SS rectifiers don't sag.
A good Simpson 266 VOM with 100kohm/volt 2VAC and 20 VAC scales helped me debug for 25 years before I ever found a working oscilloscope I could afford. Modern DVMs in the $30 area aren't sensitive to non-power line frequency AC, and have provided me with enough stupid answers I quit trying to use them. At 125 VAC and above at 50 and 60 hz, they are fine. The $119 Fluke meter the factory loaned me had the same problem. Besides all DVM's I could afford average over 2-4 seconds, and signals come and go faster than that.
If you actually use a scope, the voltage you calculate from the P=IV and V=IR equations is 0.7* the peak to peak voltage you see on the scope, roughly.
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Last edited by indianajo; 16th November 2011 at 08:43 PM.
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Old 17th November 2011, 06:36 AM   #10
Mooly is offline Mooly  United Kingdom
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I really don't know what to suggest to you.

If all channels behave the same then I think you can rule out an "audio" fault as such.

The next step has to be to see if it meets its rated output for its quoted input sensitivity. You have a scope so that's easy but you need to be careful you don't damage anything.

You need to apply the correct input voltage (sine wave at say 400hz) remembering that the RMS figure is what you see on the scope (peak to peak) divided by two and then multiplied by 0.707

I would initially test with no load because most commercial gear has limited heatsinking and is not specified to run for long at full output into say 8 ohms. See if the output meets its spec for the given input. If you then reduce the output to a more manageable figure of say 1 or 10 watts RMS you can apply the specified load impedance and the output should be maintained.

You are quoting the readout from the display (-db figures). This is where it gets tricky. Are you certain the amp has not got a loudness function etc and that this has in the past been on but now is not ?

Is the volume control a mechanical (motorised maybe ?) conventional potentiometer or is the volume control purely electronic and the front control just a "rotary encoder" which tell the microprocesser which way to "electronically" reduce or increase the volume ?

If the later then you next need to check that the output follows an approximate logarithmic law as it is increased. I wouldn't read to much into the display reading as I am sure it is not referenced to anything. The reading will alter with volume position whether a signal is present or not and so in terms of actual level its meaningless.

Now those caps....

Two equal value large caps next to a bridge rectifier. You say they are not PSU caps. What do you think they are ?
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