Electronics > Beginners
ESR Confusion
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Noidzoid:
As I am very new to all this I do realise I am probably missing a very important point.

I decided to replace a pair of capacitors in an audio amplifier as one of them was bulging at the top and was making a loud buzzing noise. 

Upon removing the suspect one I found that it rattled when shook almost like it contained a large ball bearing.

Before soldering in the new one I thought I would do a side by side test using my newly acquired MESR-100.  To my surprise they measured EXACTLY the same.

As I stated at the beginning of this post I am probably missing something but to my untrained eye it seems the purchase of the MESR-100 was pointless.

BTW the amplifier works great now having only singled out the dodgy capacitor because of how it looked. Had I relied on the meter to make a decision I would still be looking for the fault.

I would be interested to hear comments from anyone more experienced than myself (so almost anyone really ha!)
wraper:
ESR increases only when most of electrolyte is gone, it may bulge or not depending how tight a rubber seal is. Give it some time and bulging cap which measures fine would die and measure high ESR.

--- Quote ---Upon removing the suspect one I found that it rattled when shook almost like it contained a large ball bearing.
--- End quote ---
Large snap in caps may have a lot of free space inside. Paper/foil roll inside usually is only attached by thin, flat, flexible leads. Depending on how much free space there was inside, and position of that roll, its not surprising it could rattle. You could take that cap apart and look yourself.

--- Quote ---was making a loud buzzing noise.
--- End quote ---

This is something strange. Electrolytic capacitors normally don't buzz regardless if they are fine or faulty.

--- Quote ---To my surprise they measured EXACTLY the same.
--- End quote ---
The same what? And what was the capacitance and voltage rating? Maybe they were equally dead. EDIT: for some reason I thought you compared two old ones, I guess you mean old and new.
Noidzoid:
Yes, old vs new
6800uF 50v

Big buzzing that was loud with no volume could be amplified through the speakers. Could the capacitors I replaced have been causing something else to buzz? Although I was pretty sure the noise was  coming from the bulging one as when I put my finger on the top of it I could feel it vibrating.
wraper:
Maybe there was lost/broken connection between actual cap and terminals inside the case, hence it buzzed and rattled. Or broken solder joint (if it's single layer PCB). And maybe connection was restored when you removed or shock it. Anyway, it's something strange. If you still have it, you could peek inside out of interest.
tautech:
SMPS PSU's are more prone to buzzing if componentry has drifted out of spec but that's not to say linear PSU's won't buzz either. Being their caps are operating at vastly different frequencies, SMPS are more likely to whine in the KHz region whereas linear a hum @ 2x mains frequency.
I'm guessing being old enough for the main filter caps to be shot it's a linear PSU and they usually don't use low ESR caps. The ESR value selected by designers has much to do with the application and this needs be considered when selecting replacements.
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