Sansui 5000A Current Bias Adjustment Question

Discussion in 'Exclusively Sansui' started by WillRobinson, Jun 11, 2018.

  1. WillRobinson

    WillRobinson New Member

    Messages:
    8
    Hi all, I have a 5000A with F1040-1s that I just completed skippy124's mod on. The final step is to adjust the bias current. That is where I've run into trouble, I'm following the service manuals procedure of connecting 10 ohm 1 watt resistors to each speaker terminal, removing the quick acting fuses and measuring DC milliamps at the fuse holders. The problem is I'm onny measuring 0.86mA no matter how I adjust the bias pots, I thought perhaps the pots were dead, but removed one and it tests fine. What am I doing wrong? Any help would be greatly appreciated.
     

     

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  2. tom3

    tom3 Super Member

    Messages:
    1,851
    Location:
    So. Ohio
    I'm assuming you did both boards? I'd first check voltages at the collector on the output transistors. Looking for about 80 volts or 40 volts or so. Then I'd double check the refit. Something didn't get hooked up right?
     
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  3. kevzep

    kevzep Its all about the Music Subscriber

    Check the fuse in the multimeter......
     
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  4. WillRobinson

    WillRobinson New Member

    Messages:
    8
    Thanks for the replies! After studying the scymatics and even more stairing at the boards I realized I had D801 and D802 installed incorrectly, the layout of F1040-1 is slightly different from the F1040 in skippy124's guide so I had the common connections wrong.
    So I fix that issue, bias is set to spec now. I also set the center voltage to half the supply voltage as I've seen suggested, now DC offset reads 0.01-0.02mV. After testing with headphones it sounds fantastic! I just need to replace a few bulbs now and she'll be back to former glory.

    Thanks again!
     
  5. kevzep

    kevzep Its all about the Music Subscriber

    Good stuff, oops....yep, put those diodes in the right way.....nothing to see here......Done that before!!
     

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