Help repairing Akai AM-M939

Discussion in 'Solid State' started by tallman5353, Sep 14, 2018.

  1. tallman5353

    tallman5353 New Member

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    31
    Hi there,

    Recently picked up a nice matching akai system from a garage sale, the amp being the AM-M939
    Unfortunately upon firing it up the right channel appeared to be barely working, crackling some music at times.

    I took the cover off and did some diagnostic, no outstanding dry joints, all caps and transistors don't appear burnt out. I've cleaned all of the pots and switches for balance, volume, tone etc with deoxit.

    I did swap the output transistors from each channel to the other, the problem persists still on the R.
    I also tested the idle current, the left channel measures correct at 20mv, however the right is a flat 0v. Adjusting the VR pot does nothing.

    Anyone have advice? I had a peek at the service manual schematic, but it is a bit of a maze to me.
    Thanks in advance.
     

     

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  2. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    Welcome to AK.. On the bad channel. Verify that the originally installed outputs are not shorted. Then verify that the emitter resistor block R39 is still serviceable. Check that both resistors within the block are not "open"..
     
  3. sp58

    sp58 Member

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    Speaker Relay ?
     
  4. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    Relay won't fix the bias issue.
     
  5. tallman5353

    tallman5353 New Member

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    Thanks avionic, checked both resistors. Both measure the same at 5ohms attached to the board. Any other ideas?
     
  6. mbz

    mbz Super Member

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    Would be nice to know what the bias transisrtor is seeing.
    If possible measure TR901b b/c/e voltages to GND/chassis.
    This can be done at the jumpers as per pix, ie, inturn,
    measure each jumper to chassis, expect about +/-2Vdc

    upload_2018-9-15_17-6-54.png
     

     

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  7. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    The should have been . 33 Ω each. They may be open. Need to remove it from the circuit and remeasure.
     
    Last edited: Sep 15, 2018
  8. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    Out of circuit.
     
  9. tallman5353

    tallman5353 New Member

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    Ok, have pulled them out, readings are shown in pictures attached. Readings are the same for both resistors
     

    Attached Files:

  10. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    OK, those look good. Your test leads are probably adding .4 or .5 Ω to the measurement.
     
    Last edited: Sep 15, 2018
  11. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    Time to start taking voltage readings.
     

     

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  12. tallman5353

    tallman5353 New Member

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    Ok, measurements taken. Definitely a discrepancy between the two tr901's
    These are measurements taken from 901b
     

    Attached Files:

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  13. tallman5353

    tallman5353 New Member

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    And these from 901
     

    Attached Files:

  14. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    Nice photos...Which is the emitter? Base? collector?

    Just list the voltages. photos are not necessary.
    TR901 ---- 901b
    E----------------- ?
    C------------------?
    B------------------?
     
  15. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    Next voltage measurements TR1 and 1b. TR1 is a JFET(junction field effect transistor)
    Expected readings...Drain,Source & Gate

    D-- 0v
    S-- 10.5v
    G-- 0v
     
    Last edited: Sep 22, 2018
  16. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    These look good..
    Not so good.
     

     

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  17. tallman5353

    tallman5353 New Member

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    Ok, TR1 readings:

    TR1
    D-- 9.4v
    G-- 0v
    S-- 16mv

    TR1b
    D-- 9.7v
    G-- 0v
    S-- 20mv
     
  18. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    TR1&2 look OK , Next E,C & B of TR6 & 7.
     
  19. tallman5353

    tallman5353 New Member

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    Ok, i think i have the ecb the right way around.

    TR7b
    E 47v
    C -0.6v
    B 48v

    TR6b
    E -47v
    C -0.6v
    B -48v

    TR7
    E 48v
    C 1.25v
    B 47v

    TR6
    E -48v
    C -1.25v
    B -47v

    Could these transistors (6b & 7b) be the culprit, or is it further up the circuit?
     
  20. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    I'd try replacing TR901b and zener diode D3b.
     
    Last edited: Sep 23, 2018

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