Problem fixing an Amplifier - replacing electrolytic capacitors

Thread Starter

jmaarroyo

Joined Jun 21, 2011
6
Hi guys!
I'm new in this forum.
And I'm trying to fix and Old Onkyo a809 Stereo Amplifier.
Well everyhing started because, when I was listening some music in the amp, right chanel sometimes started to make some scratchy sound and sound got distorted.
I was reading in some pages that this is caused because some electrotic capacitors in the power suply leaks some AC voltage, this amplifier was made I think in 1992, so capacitors have almost 20 years, Original capacitors are 18000uf - 63 volts, I didn't find them, so I replaced them with some different value, 22000uf - 63 volts,
New capacitors are half of the size than old capacitors
It took me 2 hours to replaced them because of the cables, after that I turned it on, and it sound for a while, new capacitors got burned, and blown
Why does it happen?, is it because they're differente value, or because they're small size?
Any ideas would be appreciated.
I hope I could restore this old amplifier, it has a lot of power.
 

t_n_k

Joined Mar 6, 2009
5,455
This might seem a stupid question on my part but did you check the polarity - correct orientation - when you replaced the capacitors?
 

Thread Starter

jmaarroyo

Joined Jun 21, 2011
6
Yes I did, I even took a picture, to not forget polarity and did a small diagram.
I think that the problem is that old capacitors are very big, and say "18000uf - 63WV", so I think new capacitors didn't support the load, they're 1/3 of the size, a809 is a high current amp, that's why they got burned.
Is it my theory right?
thanks for your help!
 

t_n_k

Joined Mar 6, 2009
5,455
Yes I did, I even took a picture, to not forget polarity and did a small diagram.
I think that the problem is that old capacitors are very big, and say "18000uf - 63WV", so I think new capacitors didn't support the load, they're 1/3 of the size, a809 is a high current amp, that's why they got burned.
Is it my theory right?
thanks for your help!
If the AC current is high in the caps then it's possible they are failing due to I^2R losses.

Is that what the capacitors are likely to be doing in the circuit - carrying large AC signal or power related currents? Are they for DC filtering for power level decoupling?

A partial schematic might aid the discussion.
 

Thread Starter

jmaarroyo

Joined Jun 21, 2011
6
Yes they're for DC Filtering.
I'll try tomorrow, using old caps, and I'll see the results.
can I post a file in this forum or just a link?, I downloaded the schematic diagram, may be this could help.
Thanks a lot for your support.
 

t_n_k

Joined Mar 6, 2009
5,455
The simplest way is to include a graphics file (e.g filename.png) with your post.

Use the paper clip icon above the message window to start the process - it's fairly straightforward.

Surprising that a DC filter capacitor blows up due to excessive ripple current. I guess it's possible. Was the amplifier working really hard when things went haywire? Are you confident that the working voltage of the new caps is 63V. Might be worth checking the DC bus voltage isn't too close to or exceeding the cap's working voltage. Time to get the multimeter out and have a closer look. Take care with the covers off the unit and mains power on.
 

Thread Starter

jmaarroyo

Joined Jun 21, 2011
6
Hi guys!
Thanks a lot for your support!
Well I replace new caps for older caps, the only thing that got damaged was a fuse.
So I replaced the fuse and turn the amp on.
And it's working again!, thanks god!, I would have missed this amp.
I detected the problem and I saw that the "Input Selector" is motor controlled, so I used a bit of WD-40 to clean the input selector and the problem is gone!
I almost mess my amp!

So my theory is that new caps didn't produce enough power to support the amp.
Or may be they are defective.
I promise be more careful, hehehe!
 

SgtWookie

Joined Jul 17, 2007
22,227
When purchasing replacement electrolytic capacitors, you should use a high-volume authorized distributor, such as Mouser, Digikey, AvnetExpress, etc. Using authorized distributors ensures that you get the genuine articles that you are purchasing, and that the capacitors haven't been sitting on the shelf for years. Electrolytic capacitors that have been sitting unused for a period of time will eventually lose their dielectric, and will have high leakage current when placed back into service. This high leakage current can cause the capacitor to heat up, and the electrolyte to boil, rupturing the package forcefully.

Old capacitors that have excessive leakage current can frequently be re-formed via charging them from a current limited source, but you're better off to replace them with fresh caps from a reputable distributor.
 

Thread Starter

jmaarroyo

Joined Jun 21, 2011
6
Thanks for your advice "SgtWookie", In fact I was looking for a cap same size, and capacity, but I couldn't find it, Amplifier was made in 1991, so caps almost have finished their lifecycle, it's a shame that here in Mexico you can't find good cap brands, I think my amplifier has limited life, I will try to find a dealer, or may be someone who could restore the amplifier later, like carver amps.
I made a search on ebay and there are some similar capacitors, not the same capacity.

SamYoung
http://cgi.ebay.com/2x-18000uf-71v-85C-SamYoung-AHS-audio-snap-in-capacitor-/270722218000

ELNA
http://cgi.ebay.com/ELNA-71V-18000uF-M-Hi-Fi-Audio-Capacitor-JAPAN-x-2-NEW-/400106769898

There's a difference of 3 volts.
Any site do you know, where I could request good caps?

Thanks a lot!
 
I've noticed that while modern capacitors are almost universally marked with a stripe on the (-) lead, some older capacitors have the (+) lead marked. This can be confusing and if you are merely matching stripe for stripe, you should go back and confirm that the polarity was indeed correct when installed.
 

SgtWookie

Joined Jul 17, 2007
22,227
Funny, I don't remember ever seeing aluminum electrolytic caps having their positive terminal marked even in equipment made as far back as the 40's.

Tantalum caps are the odd duck; they have their positive terminals marked. I wonder who the genius was who caused that to happen. Almost had to be someone in the sales end of the business.
 
Funny, I don't remember ever seeing aluminum electrolytic caps having their positive terminal marked even in equipment made as far back as the 40's.
I've seen that many times before. Never saved any to photograph but here is the most recent example, swapped out from a portable boom box, which I haven't tossed out yet. Definitely an electrolytic can and not a tantalum.

 

#12

Joined Nov 30, 2010
18,224
My 2 cents...

Recent research into ripple current found a capacitor manufacturers equasion that says the expected lifetime of the cap is significantly increased by purchasing a cap with UP TO twice the necessary voltage rating.

In addition, the physical size of the cap is how it relieves itself of heat. Purchasing a higher voltage cap will probably get you bigger case sizes and thus make them run cooler.

This whole thing might come down to your statement that the new caps were half the size of the originals. Post #1, line 9
 

castley

Joined Jul 17, 2011
31
The capacitors you used may have deteriorated with age. Electrolytics that haven't been used for some time should be placed on a lower voltage with an ammeter and a resistor in series. The leakage current will drop of in time. Keep increasing the voltage
until the leakage current is under 5 Ma. Your capacitors probably blew up because of the high leakage current.
 
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