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Vintage jvc ax-4 amplifier

cedric_06 Posted messages 966 Status Member -  
 b -
Hello, I have been the happy owner of a JVC A-X4 amplifier for a few years now (5 years) connected to my computer constantly for a minimum of 8 hours a day, and it's starting to show signs of fatigue. Let me explain: when I turn it on, nothing happens, but after a certain period of time, the amp seems to kick in with a loud thump in my speakers that almost disassemblies them, then it crackles, and as a result, I have to disable the equalizer in order for the sound to go back to normal. However, this bothers me because disabling the equalizer makes the sound linear, and I prefer to personalize the music I listen to.
Anyway, I hope I was clear enough and that you can help me.

Thank you.

7 answers

  1.
    fabul Posted messages 42235 Registration date   Status Moderator Last intervention   6 081
     
    Hi,

    In your opinion, could it be a problem with the equalizer controls?

    Do you turn it on with the equalizers disabled?

    When you say nothing happens, is that before the relay clicks?

    Have you opened it?

    Cleaned the controls?

    I would recommend DeoxIT Faderlube F-5 to clean/lubricate the faders (slider potentiometers)

    But the problem might not stop there.

    The first thing I do when I receive a device that’s a bit old is a complete cleaning.

    When I say complete, I mean complete.

    The only thing that doesn’t end up being rinsed thoroughly after cleaning with certain products is the transformer.

    Do you have a multimeter?

    Photos of the inside would be welcome.

    PS: Service Manual (EN)
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  2. cedric_06 Posted messages 966 Status Member 34
     
    Yes, there's a click that happens as if there's a bad connection, then the sound comes back but it's crackling, so I have to disable the equalizer. But otherwise, without the equalizer, it doesn't cut out even with prolonged high volume. No, I don't have a multimeter, and it was full of dust which I cleaned off with a brush, but under the dust, I noticed a suspicious stain on a rectangular component (it's not a transistor), and there are no swollen or exploded capacitors. As for the photos, I'll try to get them done as soon as possible.
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    1. cedric_06 Posted messages 966 Status Member 34
       

      All the buttons work perfectly with smoothness and no sticking; it must be coming from the circuit.

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    2. b
       
      The click is normal. It's the delay relay at startup that prevents a "thump" in the speakers, if something isn't running (smoothly) that could damage them. For example... destroying the speakers, but I don't think they would be at much risk the first time.
      Now for the failure, we should look elsewhere, but I don't think I can help you. I don't know anything about electronics.
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  3. cedric_06 Posted messages 966 Status Member 34
     
    The equalizer circuit is in perfect condition, everything is very clean, no corrosion on the solder joints except for this stain that looks sticky on this 8-pin component in a line. The component is green and it says "super A" on it.
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    1. fabul Posted messages 42235 Registration date   Status Moderator Last intervention   6 081
       
      It must be thermal paste, it's a STK, it acts as a transistor.
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    2. cedric_06 Posted messages 966 Status Member 34
       
      Ah okay, that's weird, yet no radiator is stuck on it :/
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    3. fabul Posted messages 42235 Registration date   Status Moderator Last intervention   6 081
       
      I may have been confused, but I would be curious to see.
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    4. cedric_06 Posted messages 966 Status Member 34
       
      Well, as soon as possible, I'll take the photos; for now, I don't have a camera.
      I hope I can manage to get it back on its feet because I care about it a lot.
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  4. rovolt Posted messages 1 Status Member 1
     
    Hello cedric_06, I had a similar problem. It's about a chemical capacitor that shorts out and then reforms in the protection circuit of the power stage.
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    1. cedric_06 Posted messages 966 Status Member 34
       
      I wanted to heat all the changes, in that case, the problem would be solved, but I would like to know where it is located, you said on the power map where there are the transistors.
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    2. cedric_06
       
      Is it about the power part?
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  6. fabul Posted messages 42235 Registration date   Status Moderator Last intervention   6 081
     
    For the switches, you would be better off using DeoxIT D-5

    It is also used in rotary potentiometers, before lubrication with Faderlube.

    You can clean the solder side with acetone and a toothbrush, finishing with window cleaner and water.

    Window cleaner and water may be sufficient for the side of the components, just a quick rinse.

    Yes, a brush helps.
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  7. cedric_06 Posted messages 966 Status Member 34
     
    Hello, still no photos but I have new information to give you, probably important:
    the two large electrolytic capacitors in the middle have leaked
    is it serious?
    their references are 56v8700µF

    I didn't see them directly because they leaked from underneath and on top, they're fine, not even bulging.
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    1. fabul Posted messages 42235 Registration date   Status Moderator Last intervention   6 081
       
      To be replaced, but are you sure it’s not glue? Often, it can be confusing.

      Check if the 10,000uF / 63V Gold Tune are the same size

      http://www.partsconnexion.com/capacitor_ele_nichicon_kg.html
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    2. cedric_06 Posted messages 966 Status Member 34
       
      he has a brown stain all around and under one of the condensers there's a bubble
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    4. cedric_06 Posted messages 966 Status Member 34
       
      No, nothing like that fortunately, but the tasks look somewhat like this, yet it doesn't prevent one of the capacitors from having bubbles and something resembling rust on one of its three legs.
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    5. fabul Posted messages 42235 Registration date   Status Moderator Last intervention   6 081
       
      Use acetone and a Q-tip, scraper to clean,

      Measure the capacitors,

      Note which side is positive / negative / none

      Negative goes to the chassis (ground) normally.
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  8. cedric_06 Posted messages 966 Status Member 34
     
    Hello, a question: is WD-40 a good product for cleaning the circuit and the potentiometers?
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    1. fabul Posted messages 42235 Registration date   Status Moderator Last intervention   6 081
       
      Hi,

      No, it could damage them in the short or long term.

      CAIG Laboratories, Inc. has a whole range of "DeoxIT" products specifically for that.

      DeoxIT D5, DeoxIT D100 (deoxidizing cleaners)
      DeoxIT Fader (Faderlube) F5, F100 (cleaning/lubricating)
      DeoxIT Gold ... (protectors)
      DeoxIT Shield ... (protectors)
      ...
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    2. cedric_06 Posted messages 966 Status Member 34
       
      ok thank you
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