Electrolytic Cap Testing

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Hello everybody!
Sorry if the thread is slightly OT, please moderators move it if it is.

I am working on a solid state guitar amp (not diy, just repairing). It has a loud hummmm when something is plugged in the input.
I checked for bad solder joints (since I've got another amp like this that had a cold joint on a main filter cap due to vibrations - it's a combo amp so few centimeters from the cone) but nothing. I think it has the power supply filter cap a bit leaky (two 4700uF 50V connected in series), but I haven't anything to test them, other than my DMM. They're not shorted, but they read 17k and 6k on the ohm scale and they're slowly increasing. Many other caps, even larger (discharged, obvious) measure 1.8/2Mohm increasing (for example a 2200uF 35V, that isn't so distant by value).

How can I test them?

By the way all the other things in the amp works, for example I expected a diode faliure due to leaky filter caps instead they measure good, even the power transformer.

I've made a circuit with a 12V power supply and an incandescent bulb. Put the bulb in series with the cap to limit current: I thought that if the cap is leaky the bulb shouldn't go off after the initial inrush... instead the cap charges and the bulb does shut off. So maybe it's not leaky but simply greatly reduced in value???

I know what are you thinking, that I'm concentrating only on filter caps but maybe the problem is elsewhere, but anyway I'd like to know how to test electrolytics.

Thank you all for the answers!!!
I think you are headed in the wrong direction. If you have a bad capacitor, I would expect to hear hum constantly, not just when something is connected to the amp. I would more sustect some sort of ground loop or perhaps a missing ground. I don't think there is anything wrong with your capacitors. The best way to determine if the capacitors are good is to have a look at the ripple with an oscilloscope.
I would agree with you, but the other (identical amp) I repaired (btw Fender Deluxe 90) with a cracked solder joint on the filter caps, hummed only with something plugged into the input jack. It must have some sort of switched jacks, maybe... and those filter caps are only for the output section, because the preamp has its own rectification/filters/regulators...

Sorry I don't have a scope...

But I'll try to see if there are bad joints that I haven't seen..


But what about testing caps? You said that the best way is checking in circuit with a scope, but how about caps per themselves? I mean out of a circuit...
Joined 2004
Paid Member
Hi Giaime,
There are cap checkers, LCR meters properly. Good ones are extremely expensive. I use an HP.

With no gear, connect a known good one in circuit and apply power. IF the circuit then works, the old one was bad.

From how much you get into things you really need a 'scope. Even the inexpensive (CDN$ 500~600) 'scopes will do everything you need. Don't buy a super cheap one. It's almost not worth buying surplus stuff. If you even consider buying a new meter with a cap function - don't. Buy the 'scope, you'll never be able how you got along without it.

anatech said:
Hi Giaime,
Sorry to hear you are unemployeed at the present. Do you know someone who can give you access to a 'scope occasionally? This can make your life much easier.

Hope you find a job soon.


Thank you Chis! I'm still studying at the moment, and I would like to go to the college... I'm 18... Not much money (I've just bought a CD player on eBay... NAD514 someone knows about? it sounds amazing...)

Yes I can have access to a scope, either at school or at my friend's repair shop... I plan on buying one, maybe used on eBay... I found a nice vintage one (with tubes :D) for little money, but I thought that it can't be precise, it had to be restored... caps jobs etc... so I didn't buy it.

I've got the amp at school, since is owned by the reharsal room we have at our high school, and the headmaster didn't allow me to bring it home to repair it, so I have to repair it in the physics lab... :eek: near the one tube regen radio I built...

I'll let you know what I will found...

Thanks to all!
Joined 2004
Paid Member
Hi Giaime,
Ahhh, when I was young and 18. I'm 46 now. Enjoy your life as it unfolds. Do not ignore the love of a good woman. One lesson I can pass on.

Your situation is good then. Buy a new 'scope later when it's time for you. When I was young they were very expensive. I bought army surplus and made my own leads. That 'scope was only good for 500KHz. That was okay for then, but not now. Since the price of test gear is so low by comparison it does not make any sense to buy used. The test leads in new condition is very important!

I have older tube 'scopes that I restore. They are a hobby that I should really give up. This would make my wife very happy (and that is extremely important!).

Take care Giaime,

Thank you Chris, thank you very much for the kind words... Yes I will put down the soldering iron sometimes to enjoy the love of a good girl... ;) You know, women's operating point is harder to understand, instead of tubes... :smash:

But I think we're going a little bit OT...

But many thanks to you and all the other beautiful people here at diyaudio!!!!
jacco vermeulen said:


he lives in Venetia, Italia. :D
Personally i regularly check the girls at the Venice beach webcam

I think we're a little OT, but anyway...

let me say that there isn't a Venice beach: there are many beaches near here but Venice hasn't got a beach. Maybe you refer to Jesolo, the Lido of Venice... :D
I can assure you that here girls are not like you see in movies or something... :whazzat: they're more difficult to understand and complex... just like a bad designed solid state cheap amp.

Maybe we'll see this thread in the Texas Section... good! I like Texas...
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