electrolitic capacitor question

[amtpdb]

Hi:
I am a amp for a friend and am just learning on my own. The sound can be traced through but the volume is not amplified. I was looking at the capacitors. I was testing the case to ground on the electrolitics and  some of them show the voltage from ground to the top of the can. Some of them don't. Some of the new ones I have don't show any connection from the pins to the case. Some of the ones in the amp are showing the running voltages (some are -93 volts). Is this correct or are these caps bad? Any help will be appreciated.
Don

[valentinc]

      I don't understand exactly what you're saying...

      About what amplifier are we talking about ? Do you have a schematic ?

      How are you "tracing the sound" ? Do you have an oscilloscope ?
 
      What have you measured between the caps and the case is totally irrelevant... You need yo track down the problem to a smaller block of the amplifier and then look deeper in that to see what component/components are bad...

       And a golden rule in repairing stuff is: don't assume anything...

[Rigby]

If you don't have the proper tools, the easiest (and, if you don't have the tools, the least expensive) way to check for good caps is to assume all electrolytics are bad and replace them with new ones of the same ratings.

Some polarized cap designs probably do short the can itself to the negative terminal, but I'm not experienced enough to say this for sure.

Also, it would greatly help us if you could do your best on the grammar and stuff.   You may be very young or not a native English speaker, I don't know, just do your best and we'll do our best to help.

[papo]

Some polarized cap designs probably do short the can itself to the negative terminal, but I'm not experienced enough to say this for sure.

Yes that's not uncommon. I leaned it the hard way quite some time ago, when repairing a flyback supply I checked whether one of these fat electrolytics was getting hot. With my finger, in front of a student. 230 V mains right there, even though I was sure I knew what I was doing. Yes that was stupid and careless.

I've never seen caps where the can is connected to the positive lead but under no circumstances would I rely on that.

Regards
Matt

[valentinc]

     It may be the easiest way to just change all the capacitors in the amplifier, but that's an empiric way to repair things... And the big filter capacitors are not so cheap... I think it's cheaper to build a simple capacitance meter (there are a ton of schematics of the web) and just check... You would not need an ESR meter for this kind of application...

     And from my experience, faulty capacitors are not such a common problem in audio amplifiers, like they are in switching power supplies, for example... More common problems are faulty transistors, and even resistors...

     A golden rule for caps is that if they have buldges they certainly need replacement... Or if they have leaked...

     

[amtpdb]

I appreciate all the help I have gotten from all of you on this project. I replaced one mosfet and re soldered many of the connections. I used a simple tracer that uses a small amp and was able to get it going again.
I have a cheap capacitor checker, but was told that I needed a esr meter. I plan to look into building one in the near future.
Thanks again for all the input.
Don

[valentinc]

         Congratulations for the repair !

         You can build this ESR-meter: https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/

         I think it's the simplest one that can be done, and it's also very cheap to build...

         And about the usage of an ESR meter, as I mentioned, the ESR does matter most in switching power supplies, where an increased ESR can increase the output ripple outside the normal limits... From what I have seen so far, I never seen a capacitor that has a high ESR and the other parameters (like capacitance, dissipation factor) remain normal... Usually, as a electrolytic capacitor leaks or dry out, its capacitance drops significantly, its ESR increases, its dissipation factor increases...

         In switching power supplies they tend to fail beacause they work at high ripple currents and the manufacturers don't always consider to use capacitors that support ripple currents much higher than the actual currents in the circuit... They work at their limits, and of course, that is why they tend to fail... On the other hand, in 50/60 Hz systems, like audio amplifiers, they rarely fail...

[amtpdb]

valentinc
Thanks for your help and for the link. I do enjoy learning. Sometimes it is hard though.
Hope you have a nice week ahead.
Don

[rjk5378]

Another option for an ESR meter is the Blue ESR Meter marketed by Anatatek (anatekcorp.com). It is a digital version of the old Bob Parker analog ESR meter. You can get one for US$99, or save $25 or so and get the kit. I put together one of these. Not too difficult, all thru hole soldering, and calibration of the assembled kit is easy and doesn't require more than a good multimeter.

This is quite valuable for checking electrolytics. Sure they are bad if bulging or if leakage is in evidence, but the lack of these things doesn't guarantee that they're good. Another benefit to an ESR meter (or at least this one - I assume it's also true of others) is that you don't have to pull the caps from the circuit to check them.

Good luck.

- Bob, KY3R

[amtpdb]

Thanks for the suggestion. I'll look at it.
Don