unstable DC offset on 771

Discussion in 'Exclusively Sansui' started by bourdagespl, Jan 9, 2014.

  1. bourdagespl

    bourdagespl New Member

    Messages:
    17
    Hello everybody,

    I'm new to vintage audio and I found this forum being very usefull!

    I would appreciate some help on setting DC offset on my 771.

    I found that my right channel has some distortion, so by reading on this forum, I tried to check and adjust the DC offset. The left was approx between 2 mV and 12 Mv. The right was approx between 50mV and 60mV. I say approx because the read my multimeter is giving to me is very unstable. I succesfully adjusted the right approx 2 - 12 mV, but I can't get a stable read.

    I have a cheap mastercraft numeric multimeter and I didn't touch the bias current adjustement yet.

    Can someone tell me why I can't have a stable DC offset?

    Thank you for your help!
     

     

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  2. ed52

    ed52 Active Member

    Messages:
    126
    Location:
    Toronto
    I'm working on my 771 with the left channel offset drifting.
    I swapped the adjustment pots left and right and checked for bad solder points.
    Then I swapped transistors one by one with the opposite channel.
    The third transistor change moved the offset problem to the right channel.
    On the driver board, F 1499, in my case Tr1/Tr2 is the problem.


    Before the unit shut down there was some crackle on power on. When it finally became unusable there were loud crackle sounds from the left channel.

    Note: none of the components were hot to the touch nor was the problem changed by applying heat to the components.

    Up to this point my 661 and 881 have not caught the 771's virus. LOL
     
  3. bourdagespl

    bourdagespl New Member

    Messages:
    17
    Thank you for your answer.

    I'm not sure what you mean by "drifting", but mine stay at the same unstable value (24 hours later). By unstable, I not mean that the offset changed even if untouched, I mean that the reading on my multimeter is not stable.


    Sorry for the misunderstood and the poor english.
     
  4. bourdagespl

    bourdagespl New Member

    Messages:
    17
    Oh and by the way, my two channel (left and right) have the same behavior.

    Thank you again.
     
  5. ed52

    ed52 Active Member

    Messages:
    126
    Location:
    Toronto
    Drifting to me means unstable...the voltages vary out of the acceptable range.

    The good channel is unstable by .003 VDC.
    The defective channel is unstable by over 1.000 VDC
     
  6. bourdagespl

    bourdagespl New Member

    Messages:
    17
    0.003 V, so 3 mV???

    My 771 is unstable by 3-5 mV.

    I just adjusted my bias :
    Left : 0.017 -> 0.019
    Right : 0.015 -> 0.019


    So the unstability (3-5 mV) in offset is ok??

    Thank you!
     
    Last edited: Jan 9, 2014
  7. ed52

    ed52 Active Member

    Messages:
    126
    Location:
    Toronto
    The offset specification is 0V, plus/minus .010(10MV). So it appears that a minor offset variation is acceptable but the smallest voltage variation is the best.
     
  8. stereofun

    stereofun Super Member

    Messages:
    1,190
    Location:
    Tucson, AZ
    your offset trim pot could likely be the culprit. Turn off amp, then apply a little deoxit while turning pot in its full motion. Let sit for a minute, then turned it much more vigorously 20-30 times or so. Set trim pot in the middle position, leads connected to the speaker terminals and start up the beast. If you are lucky the pot was just dirty and you can now get a much better reading.
     
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  9. for_p1

    for_p1 Addicted Member

    Messages:
    5,472
    Location:
    NJ
    Offset can vary over time, but should be stable in short term. Something is wrong. If you suspect trim pot, it is easier just replace it.
     

     

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  10. bourdagespl

    bourdagespl New Member

    Messages:
    17
    Thank you all the your help!

    I will check the VR01 and VR02 trim pot.
     
  11. Filius

    Filius Member

    Messages:
    52
    Location:
    Southern California
    Very old post, obviously but I just wanted to let you know that I had the same problem and your suggestion solved it and saved me a lot of time and trouble. My relay kept tripping and the DC offset was unstable on one channel. I sprayed Deoxit on the trim pot, turned it back and forth several times, and the problem was fixed. Thanks!!
     

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