Sansui au 4900 .Realy in protection mode

Discussion in 'Exclusively Sansui' started by luck79, Feb 14, 2018.

  1. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    Hello guys
    Today I have a problem with this Sansui 4900.
    Here Dc Offset
    between
    R 37,39 it's ok +/-3,8 volt
    R 40 0 volts
    R 38 16,4 volts :(



    thank you in advance
    Luciano
     

     

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  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    44,451
    Location:
    Hertfordshire, UK
    Check the main PSU voltages +/-35V approximately.

    Take out all the output transistors and measure them using the 'six way test' - or use a transistor tester.

    Measure all 4 of the emitter resistors 0.33Ω

    That should get you started - good luck.
     
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  3. slimecity

    slimecity Super Member

    Messages:
    2,573
    Location:
    New Zealand
    Just confirming that those values you posted are Volts, and not millivolts (mV)?
     
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  4. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    mV :) ehehe
    sorry
    Thanks

    Hello Hyperion !!!
    4 Resistors is ok
    Tomorrow a lot of work awaits me six way test
    :D ;)
     
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  5. Mr. Nick

    Mr. Nick Member

    Messages:
    65
    If the PSU voltages are good and output transistors checkout fine your next best shot for good DC offset are the IC pair at output stage (2SA798) that are known to be failure prone.
    These are replaced by the modified KSA992 transistor pair per channel.
     
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  6. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    Hello Mr . Nick

    transistor are ok
    I have to test ic01.02 (2sa798)
    diode 601,603 are ko

    I changed them,but does not work.

    thank you for helping me
     

     

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  7. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    ic01.02 (2sa798) test is ok
     
  8. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    transistor MJ15025 ko
    I hope to find the spare part,
    and no other damage....
    this fault on this transistor determines the non-functioning of the relay?
     
  9. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    transistor MJ15025 ko
     
  10. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
  11. Mr. Nick

    Mr. Nick Member

    Messages:
    65
    You are better off to build a "Dim bulb tester" (DBT) to power on your AU4900 next time. Power on through DBT will protect your other components on the output stage.
    https://antiqueradio.org/dimbulb.htm

    There might be other driver transistors blown as well.

    MJ15025 is probably contributing to the 0V problem at R40.
    Find and replace that output transistor pair on the bad channel and next power on through DBT.
     

     

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  12. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    Ok Like a variac;)
     
  13. Mr. Nick

    Mr. Nick Member

    Messages:
    65
    Remember to use regular incandescent light bulb. No halogen or eco. I have used regular 60W with my DBT with good results.
    It's likely that other driver transistors are bad as well when output transistors fail.
     
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  14. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    Thanks for your suggestions.
    I had already devised this alternative to variac.
    Good news the amplifier is fine in tape mode sounds stereo ,but it only sounds on the left ( stereo mode), while in mono it plays in both channels, of course I checked the attacks rcf problem is with aux, tuner, phono
    I verified bias is ok
     
  15. luck79

    luck79 AK Subscriber Subscriber

    Messages:
    25
    Location:
    Palermo
    I checked all the transistors of the F 2555 board and all the Aux, tuner, phono, tape monitor, mode selectors.
    But the problem remains. It is heard only from the left and in stereo only in tape mode.

    someone has some ideas?
     

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