Are these bad caps?

[boinglesnip]

Hey guys, just registered. I'm a newbie... oh no!

So, I have this amp (I know you got an audio section, but this post is to identify possibly bad caps, not diagnose the amps problem). There are a number of axiel type caps on the PCB that look possibly bad to me. The ends of the plastic wrappers look like they might be heat damaged, and there are copper colored bits showing around where the terminals go into the body ofthe cap - Is this normal, or is it because they are blown/bloated?

I happen to have a meter that can read capacitance. Can someone tell me how to use it? A pic of the dial is included. The caps in question are 2.2u rated 63v and 22u rated 25v

And finally for my further education, I have read briefly about ESR meters. Being a bit of a geek, I like to have gadgets to play with What would one of these tell me that my current meter won't? I know it has to do with resistance, but I'm wondering if a cap might show as OK on capacitance, but reveal its fault with an EsR meter?

Thanks for any help!

[selldoor]

Re: Are these bad caps?

Whilst you will get help with the query have you tried using the seach facility. There are lots of references advising what an ESR meter will do.
Have you tried googling your meter make model for instructions?
Probably wont give any accurate result measuring IN Circuit.
Power off set meter on 200 uF to measure the 22u and set it on 20uF to measure the
2.2uF - result should be near to the stated uF value.

[boinglesnip]

Re: Are these bad caps?

Thank you for the reply selldoor.

While yes, I have tried Googling for the required information, I did not find exactly what I was looking for. And generally, I am of the opinion that in order for there to be good references on google, people need to post the questions on forums such as this. How many times have you found what you are looking for/been pointed in the right direction, after reading through a number of forum posts that came up in a Google search? While my post could be seen as a bit lazy, I view it as contributing to the overall effectivness of search engines I guess what slows the research process down is reading through pointless replies in order to find the useful ones!

Anyway, thank you for the meter advice, I bet someone other then myself will find it useful when it comes up in a Google search. Much faster than tracking down a bloated PDF manual! I will get out my desoldering suction device and check the caps. Any idea from the pics on the state of these caps?

[mariushm]

Re: Are these bad caps?

It's hard to tell if those capacitors are bad or not.

The capacity measurement is useful but can trick you - often the capacitors can go bad while the capacity remains within the usual 20% rating and sometimes can even go up.

The ESR measurement is different than simply measuring resistance with a multimeter (which has minimal usefulness, maybe to see if a capacitor is shorted).

The ESR measurement determines an internal resistance of a capacitor by sending an 100kHz (or around that value) signal through the capacitor and measure the resistance that appears only then. Regular multimeters don't have the circuitry inside to generate that signal so they can't measure ESR.

The ESR usually goes up as capacitors go bad so if a 220uF measures 2 ohm for example, there's a high chance it's bad (it depends on the type of capacitor and the capacitor dimensions.. for a 10uF 35v capacitor a 0.8-1.3 ohm ESR is ok, but a 3300uF 6.3v should have 0.1-0.4 ohm ESR)

Such meters are a bit expensive though, usually around 70-100$, so unless you fix electronic parts often, it's cheaper to just replace the capacitors when in doubt.

[boinglesnip]

Re: Are these bad caps?

Exccellent info mariushm. Thanks!

Ok so if 20% is the normal tolerance, then at 22u it should be no more than say 26.4? I removed 2 out of 7 from the PCB, one read 26.3 and the other started off at 29, then slowly dropped to 28.1 - could this be a bad one? It would actually make sense, as this cap looks like it has something to do with powering the valve/tube in the preamp (it's a hybrid amp), and the volume output is very low.

Yea, I noticed ESR meters are pretty expensive, but it may be worth it, as I do want to get into electronics as a hobby. Fixing borken amps is an interesting place to start and learn from for me. But I'd really like to do some PIC stuff that I can interface with a serial port and control some machines with my code

[mariushm]

Re: Are these bad caps?

No, for such small capacitors it's ok to be above 20% of capacity. Those axial capacitors are probably also older, they may be -20% - + 40% or something like that.
Keep in mind that meter can also be about +/- 5-10% accurate.

[Th3_uN1Qu3]

Re: Are these bad caps?

Yea, I noticed ESR meters are pretty expensive, but it may be worth it, as I do want to get into electronics as a hobby.

Buy one, you'll never regret it. I built one myself and it works great, but it took a lot of patience and trying various things until i arrived to the final version. I had more time than money to waste so i went this route. Since you're just starting with electronics, i'd suggest you buy a meter, and it'll serve you for many years to come. An ESR meter comes in handy in almost any piece of electronics you'll poke your nose into.

[Agent24]

Re: Are these bad caps?

Those capacitors are quite old, they won't typically look as nice as the newer ones anyway, without a larger and more in-focus picture it's hard to tell.

They are probably OK for ESR (or it doesn't matter) but if they are old and have been unused they may have a high leakage current

[Gabriel]

Re: Are these bad caps?

I have some of these Philips caps brand new - mine also have the copper colour around the wires on the body and some also look a little burned.

If you are to replace them, buy new as these are at least 20 years old.

[hardwareguy]

Re: Are these bad caps?

I replace these types during any amp service on the power amp sections or main PSU section.

They're cheap to replace and can cause big problems when they fail. Get them out of there.

Appearance alone on these is rarely a good indicator. The sleeves and leads aren't as perfect on these as they are on modern devices.

[Evil Lurker]

Re: Are these bad caps?

Also the rated maximum lifespan (as per some Japanese cap manufacturers) for aluminum electrolytic caps is 15 years tops... after that caps can have problems with bung degradation and the electrolyte drying out.