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Problem fixing an Amplifier - replacing electrolitic capacitors

Discussion in 'Troubleshooting and Repair' started by jmaarroyo, Jun 22, 2011.

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  1. jmaarroyo

    jmaarroyo

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    Jun 22, 2011
    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.
    Do yo think any other components got damaged?, I hope only capacitors be the only problem.
    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.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I would suspect that when you replaced these capacitors, you installed some of them the wrong way around.
     
  3. jmaarroyo

    jmaarroyo

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    Jun 22, 2011
    I was thinking a bit.
    And I think I know what the problem is.
    Old capacitors has a legend that say "18000 uf 63WV(M)", and very big size.
    New capacitors are very small, may be 1/3 of the size, and a bit different, so I think they couldn't support the load, that's why they got burned, I think I'll have to ask for them, maybe on ebay.
    It's a shame.
     
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    The physical size isnt the problem as long as the capacitance value is the same or a little higher and the voltage rating is the same or higher they will work perfectly. Capacitor manufacturing has improved in recent years and they can make them physically smaller than the "old days"

    As steve said, its much more likely that you installed some or all of the replacements around the wrong way. The DO have a positive and negative side and if reversed can fail spectactularly!!!

    Dave
     
  5. jmaarroyo

    jmaarroyo

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    Jun 22, 2011
    Nop, I replaced them correctly.
    I turned it on, and the amp started to sound, but only 10 seconds and then, caps got burned.
    That's why I think the problem is on capacitors that are very small.
    because too much power was demanded
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Well, if you know the problem, I'm not sure why you bothered asking.
     
  7. jmaarroyo

    jmaarroyo

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    Jun 22, 2011
    Sorry to bother you guys.
    But I haven't think in the possibility of caps don't have too much power to support the amp, may be like a resistor if you use one that has small power(1/4 instead of 1/2), it can get burned.
     
  8. duke37

    duke37

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    Jan 9, 2011
    From the value of the capacitors, I assume that it is a transistor amplifier so it can not be old!

    If the fault occurred on one channel only. then the power supply was not the trouble.
    You may well have a PSU problem now. Have you checked the rectifier, the capacitors will not like AC and neither will the rest of the amp.
    To take matters further you should put fuses in the transformer output and limit the input with either a Variac or a series incandescent lamp. Go gently.
     
  9. duke37

    duke37

    5,334
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    Jan 9, 2011
    Resistors will fail if they dissipate too much power.
    In the case of capacitors, the dissipation depends on the ripple current which will vary with the output power that is demanded by the amp.
    You need to check that one of your amps has not failed short circuit.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What you need to do:

    1) take some photos of the old capacitors showing the markings.

    2) take some photos of the new capacitors showing the markings (pick a pair old/replacement)

    3) Photos of both sides of a board (with dead capacitors) in enough detail to trace the circuit would be useful too.

    Did *all* the capacitors die? or just some of them?

    What is the power rating of the amplifier and how loud was it playing just before the capacitors died?

    Does the board show the voltage/capacitance/polarity of the capacitors? Or did you have to remember where and which way the old ones went in? (did you replace them one at a time?, or all those of the same type at once, etc?)
     
  11. daddles

    daddles

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    Jun 10, 2011
    One of the first rules of troubleshooting is to drain the right swamp. You replaced capacitors, but you didn't indicate you had any measurement data to indicate those capacitors were bad. In fact, there's a relatively simple way to measure the ESR of the capacitors in circuit with a scope or AC voltmeter and a function generator -- and you'd have a good idea whether those caps were actually bad.

    Unfortunately, replacing components is one way other components can be damaged -- making troubleshooting harder.

    The usual troubleshooting method when you have one channel working and one not is to methodically compare the good channel to the bad and find where they differ. This can lead you to the problem area. And, a good rule of thumb is to always verify your power supply is operating correctly first (an "obvious" thing emphasized by the late Bob Pease).
     
  12. jmaarroyo

    jmaarroyo

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    Jun 22, 2011
    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!, 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!
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    caps don't produce power.

    Either they were the wrong voltage, or placed the wrong way around, or the things you replaced were not capacitors, or you made some other error.

    That's highly unlikely presuming that you purchased them from somewhere even half reliable.

    Good idea.

    If you want to learn something you can post pictures of the caps and the ones you replaced them with and we can try to tell you what went wrong.

    In any case, you have discovered the benefits of investigating a fault before repairing it.
     
  14. jmaarroyo

    jmaarroyo

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    Jun 22, 2011
    Thanks Steve!
    Well first time I replaced caps, I turn the amp on and was working at low volume, I increased volume and one cap got blown!, and a fuse got burned!
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you want to learn something you can post pictures of the caps and the ones you replaced them with and we can try to tell you what went wrong.
     
  16. jmaarroyo

    jmaarroyo

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    Jun 22, 2011
    Well
    I took some pictures with my phone.
    you could see a cap that is a bit inflated!, luckily didn't explode because a fuse got burned!.
    Old caps are huge!, I hope they could last at least two years, my amp was made in 1991, so caps are 20 years.
     

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