Kenwood ka3500 distortion both channels

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by E-rock, Feb 11, 2018.

  1. E-rock

    E-rock New Member

    Messages:
    28
    Hello,

    Just obtained a kenwood ka3500.

    Upon initial testing I have distortion buzz on both channels. Music playing

    Speaker offset is 9mv 30 mv

    I removed the case and deoxit all pots and the like and no change.

    If I turn it up to loud it will cut out.

    Would this be in the power supply stage as both speakers are effected?

    I have a dvm, some electronics knowledge (enough to get me in trouble) and a Hakko station. And a service manual not a lot of direction there.

    Any suggestions on what to test or investigate?

    Thanks in advance!
     

     

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  2. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    976
    Location:
    Dallas, TX
    Buzzing in both channels typically indicates a problem in the power supply. The SM provides nominal voltages (AC for anything before the rectifiers, and DC after) for the power supply from which you can compare to your unit. It's highly recommended to use mini grabbers with your DMM for taking any measurements.

    These measurements require you measure the amp while powered on so please be careful.

    Compare your voltages to the following and report back
    Power Board X07-1470-10
    Nominal voltages according to the SM
    Pin13 27.5 VAC
    Pin14 27.5 VAC
    Pin8 24.5 VDC
    Pin9 -24.5 VDC

    Also measure across each of the large, main 6800uf caps. One should measure 39 VDC the other -39 VDC.

    That should be a nice start to finding the issue The more you help us, the more we can help you.
     
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  3. E-rock

    E-rock New Member

    Messages:
    28
    Will do,

    This is the direction I needed.

    Will update findings.

    Thank you,

    Erik
     
  4. E-rock

    E-rock New Member

    Messages:
    28
    Ok got a minute to check these,

    One of these things are doin their own thing lol...


    Pin 13 25.1 v
    Pin14 24.5 v

    Pin 8 24.6 v

    Pin 9 25.9 v

    C30 -40.9 v
    C29 26.2 v

    Assuming this cap could be it or? Diode or transistor in its circuit?


    Thanks,

    Erik
     
  5. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    976
    Location:
    Dallas, TX
    Double and Triple check your reading at Ce29 in Volts DC. One lead to the positive terminal, the other to the negative.

    If it truly reads ~26 V you'll need to replace the main bridge rectifier De1 which is supposed to be a S5VB20. Replacing the main bridge requires some true desoldering skill to remove without tearing up the traces on the board. I don't recommend it as a first repair, but as I've learned from years on these forums people are gonna do what they're gonna do. If you are truly up to the task the S5VB20 is 200V 6A part which you can replace with a much more hardy part like say this 35A part which will fit nicely in place of the original. Polarity is critical during installation of the bridge rectifier so make sure you know what you're doing there, and ask questions if you don't.

    Do not assume that this is the only thing wrong, further troubleshooting may be required.

    Once your repairs are complete your unit should have it's electrolytic caps replaced. Particularly in areas of heat and stress, but ideally everywhere. If you intend to take this on, here's my best advice:

    - take photos of everything to top to bottom. Cover yourself from error. Focus attention of areas of wiring/pin connections (not if but when they break off you'll be damned glad you can go back in time), semiconductor pin orientation, screws/fastening, capacitor polarity, and anywhere a mistake could be made.
    - do not count on the SM to be accurate. If there are discrepancies with parts the installed part takes precedent (assuming it's original).
    -do repairs in small sections and check for functionality often using a DBT (dim bulb tester) before going to full power.
    - if you're apprehensive about something...ASK BEFORE YOU ACT.
     
    Last edited: Feb 13, 2018
  6. E-rock

    E-rock New Member

    Messages:
    28
    Ok will double check c29.
    Pretty sure my readings are correct.

    I'm assuming the removal of the bridge rectifier is difficult because it's 4 pins?

    I have rebuilt many crossovers so have experience in removing components.

    Would it be wise to rebuild the whole power supply at least caps and diodes as well or just one at a time and test with dim.
    (I was already in the process of building one )

    Also once the bridge rectifier is replaced is there adjustment procedure etc?

    Does the replacement you posted have any orientation differences from stock?

    Erik
     

     

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  7. E-rock

    E-rock New Member

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    28
    Also the bridge rectifier you posted seams unobtainium.

    I'm only finding original used on eBay right now....

    Any direction other than louder digikey?

    Don't want to put a junkyard part in it :)
     
  8. gort69

    gort69 AK Subscriber Subscriber

    Messages:
    1,023
    Location:
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    The screen I see says Digikey has 135 of them in stock.
     
  9. E-rock

    E-rock New Member

    Messages:
    28
    Oops

    My bad it said obsolete....

    Looking at the rectifier legs were black not a good sign I assume.

    Checked my readings they are correct 26v

    What is the difficulty removing?

    Legs are elevated would I not clip them off and desolder pins individually?
    P
    Want to make sure I'm not missing anything.

    I see pin orientation now it was under the dust lol.
     
  10. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    976
    Location:
    Dallas, TX
    I do my best to salvage any part I remove as a matter of good practice and do nothing that's not reversible. If that means you can clip the legs, have at it.

    Typically the thicker the leg, the larger the pad. The larger the pad, the more solder to be removed. The more solder to be removed, the more difficult the removal. That's what I'm getting at. To put it bluntly, I've seen too many hack desoldering jobs to leave you without caution.
     
  11. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
    976
    Location:
    Dallas, TX
    When troubleshooting I try to replace as little as required to help ascertain the problem before moving forward.

    Study the datasheet of the replacement to ensure correct orientation. The one I linked you has the positive (+) pin out of alignment with the rest.
     

     

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  12. E-rock

    E-rock New Member

    Messages:
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    Understand completely.

    Thank you

    I have a good station and am use to soldering led mods reflow and the like.

    Will order the rectifier.
     
  13. E-rock

    E-rock New Member

    Messages:
    28
    Copy the previous,

    I did not see the pin markings as it was dust bunny city on top of the rectifier.
    Rectifier legs were black.....

    Thanks again to all!
     
  14. E-rock

    E-rock New Member

    Messages:
    28
    Don't know if the Bridge is bad?

    Here are the readings out of circuit.
     

    Attached Files:

  15. hopjohn

    hopjohn Kenwood Krazy Subscriber

    Messages:
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    Location:
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    A positive DMM diode test will not definitively determine it's failure. Ce29 should be seeing the same voltage as Ce30. If it's not the rectifier then I don't know what else it could be. If anyone else wants to share there diagnosis step right in. @rjsalvi ?
     
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  16. restorer-john

    restorer-john Addicted Member

    Messages:
    5,737
    Location:
    Australia
    If one diode in the bridge is breaking down under voltage and going open, he would be getting one rail which was essentially half wave rectified. The average voltage present with a few hundred mA (the quiescent power amp, VA, and preamp stages) being pulled would be similar to his reading.

    Take RMS voltage of transformer secondary as 27.5V. Multiply by root 2 to get peak. Multiply by .637 of the Vpeak (for a HWR) gives 24.7- close to what he has.

    Bridge is still suspect IMO, as any true load large enough to pull down a FWR rail that hard would blow the 3A line fuse and cause some major heating someplace in the amp.

    PS, I've repaired some Sansui amps where one or two diodes in the BR go out and the amps still work - just.
     
    Last edited: Feb 14, 2018

     

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  17. hopjohn

    hopjohn Kenwood Krazy Subscriber

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    Yep, I've seen the same. Thanks for your analysis.
     
  18. E-rock

    E-rock New Member

    Messages:
    28
    Understand.

    Will replace and update.
     
  19. rjsalvi

    rjsalvi Active Member

    Messages:
    494
    Location:
    San Diego, CA
    Sorry, I missed this, but both you and restorer-john are all over this. I too would've also taken a close look at De1 (and De2) before suspecting a bridge rectifier diode, but if De1 & 2 check out, the only players left are the BR diodes.
     
  20. llwhtt

    llwhtt Super Member

    Messages:
    1,383
    Location:
    SoCal
    What about a dead filter capacitor? That would cause a buzz and low DC voltage. Check the AC voltage on both filter capacitors.

    Craig
     

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