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KR-4070 fails DBT - where to begin?

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by jberger, Jul 18, 2018.

  1. jberger

    jberger AK Subscriber Subscriber

    Messages:
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    Location:
    Long Beach, CA
    Testing a recently acquired KR-4070, and it fails its dim bulb test (stays bright). What's the order to test; output transistors first, look for overheated components, disconnect power to power amp section and test again? Suggestions?
     

     

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  2. gort69

    gort69 AK Subscriber Subscriber

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    I've never worked on one, but if it were mine I'd do a visual check of the power amp section looking for burned resistors, and don't forget to look closely at the emitter resistors. If you see burned stuff there I'd remove and test the outputs. If all looks normal, I would check for the +/- 39VDC at the filter caps to rule out a shorted bridge rectifier. If that checks out OK, then I'd remove and test the outputs. Sometimes they take out other components and sometimes you get lucky and they just go pfffft. Hopefully someone with experience with this receiver will speak up.
     
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  3. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
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    Location:
    Dallas, TX
    What wattage bulb did you use to perform the DBT? A 40W bulb would be the one I'd use.
     
  4. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
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    Location:
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    If you suspect the output transistors you can usually quickly identify if they are bad by checking them in circuit (power off). Outputs typically fail-short emitter to collector. Just be sure to not short /slip your leads across pins, mini grabbers are highly recommended here.
     
  5. jberger

    jberger AK Subscriber Subscriber

    Messages:
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    Location:
    Long Beach, CA
    I have a B&K tester, that can go in or out of circuit (with grabbers). Tested with 60W, and it never dimmed.
     
  6. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
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    As you suspected it's drawing excessive current.
    Where to start..
    Visually inspect the unit for anything burnt, leaking or bulging. Use the tester on the outputs and drivers and see if you find something faulty. Test the main filter capacitors for ESR and capacitance. Use the diode function of your DMM to test the rectifier bridge for shorts.

    Conrad Hoffman has a a great write-up here worthy of reading 5 times over concerning troubleshooting.
     

     

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  7. jberger

    jberger AK Subscriber Subscriber

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  8. Watthour

    Watthour Electron Rancher - JS3600

    All good advice. If a 60W lamp in the DBT remains bright, there is a short. I've been through at least a few of these, and still have one in the collection. There is nothing substantially different in this model compared to any other complementary amp. It is a very conventional design. The only differentiation is in some of the component selection - Specifically, in that the main rectifier is a single bridge, and the outputs are "batwing" (XM20) devices. The bridge can be replaced in kind or with a build-up of discrete devices, and the XM20s can be replaced with TO-220s with very little modification.

    I've also found that the main filter caps may be a little on the light side, but they work acceptably as designed. The Alps VF-5 power switch can suffer the same problems that all of them have, but they normally fail open, not shorted. The emitter resistors are also a little on the light side (wattage). The tuner sections in these are superb. The chassis is fairly open, and most things are easily accessible.

    The prime suspects have already been identified by Gort - Main rectifiers and filter caps, and main output transistors. All can be tested in-circuit for shorts.

    A secondary concern would be the trimpots for idle current adjustment are an open design, and prone to oxidation. Check them for opens, and either clean/exercise/lube them or replace them. If you clean them, return them to their original positions before powering up the unit.
     
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  9. jberger

    jberger AK Subscriber Subscriber

    Messages:
    2,107
    Location:
    Long Beach, CA
    This is excellent guidance. Thank you.
     
  10. quaddriver

    quaddriver 120 What's per channel Subscriber

    unsolder the outputs and go from there. for guidance on any and all replacements, I have a thread about a 4070 that went dark, circa 2017ish? 2016ish? I forget. But I have done 3 now with the subs mentioned - including one that caught fire.

    and fix the $%#% power switch while in there
     
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