Help repairing Akai AM-M939

Discussion in 'Solid State' started by tallman5353, Sep 14, 2018 at 8:44 AM.

  1. tallman5353

    tallman5353 New Member

    Messages:
    10
    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 " Subscriber

    Messages:
    42,085
    Location:
    Fort Dodge, Iowa
    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

    Messages:
    56
    Location:
    Germany
    Speaker Relay ?
     
  4. avionic

    avionic " Black Knights " Subscriber

    Messages:
    42,085
    Location:
    Fort Dodge, Iowa
    Relay won't fix the bias issue.
     
  5. tallman5353

    tallman5353 New Member

    Messages:
    10
    Thanks avionic, checked both resistors. Both measure the same at 5ohms attached to the board. Any other ideas?
     
  6. mbz

    mbz AK Subscriber Subscriber

    Messages:
    2,020
    Location:
    Melbourne, Victoria
    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 " Subscriber

    Messages:
    42,085
    Location:
    Fort Dodge, Iowa
    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 at 3:31 AM
  8. avionic

    avionic " Black Knights " Subscriber

    Messages:
    42,085
    Location:
    Fort Dodge, Iowa
    Out of circuit.
     
  9. tallman5353

    tallman5353 New Member

    Messages:
    10
    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 " Subscriber

    Messages:
    42,085
    Location:
    Fort Dodge, Iowa
    OK, those look good. Your test leads are probably adding .4 or .5 Ω to the measurement.
     
    Last edited: Sep 15, 2018 at 2:22 PM
  11. avionic

    avionic " Black Knights " Subscriber

    Messages:
    42,085
    Location:
    Fort Dodge, Iowa
    Time to start taking voltage readings.
     

     

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