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Sansui 7070 pops

Discussion in 'Exclusively Sansui' started by CDFixer, Feb 12, 2018.

  1. CDFixer

    CDFixer Number 1 USC Trojan Fan

    Messages:
    1,913
    Location:
    Orange County Ca
    i am working on a Sansiu 7070 that has a slight pop when switching the selector switch, it also pops when phono 1-2 selector switch is pressed.

    There is a circuit consisting of tr602, 603, 604, d601, 602, and 603, that is supposed to prevent this.

    I’m beginning to wonder if I’m chasing a ghost, does anyone have this problem? It only occurs when volume control is NOT at zero.
     

     

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  2. Tom B

    Tom B AK Subscriber Subscriber

    Messages:
    1,432
    Location:
    Bay Ridge, Brooklyn New York
    many times, switch popping is a result of low level DC on one side of the contact. You may want to check for DC on those switch contacts, and trace backwards (or fowards) to the nearest coupling capacitor. Usually it is a 1uf, 0.5uf, 2.2uf or 3,3uf, cap used for coupling from section to section. You may also have a resistor between the cap and the switch like at the end of the phono amps. Check and see if there is any DC on the leg of the cap closest to the switch contact. By any DC I mean anything above low millivolts. It should be close to 0, as anything like 10 mv or above can be noticeable. Replacing it (them) with high end Nichicon Fine gold, or similar caps give good results.
     
    tnsilver and UncleBingo like this.
  3. tarior

    tarior Dirty pool, old man? Subscriber

    Messages:
    6,508
    Location:
    Kelso, Washington
    ^^^What he said. Although I would go with Nichicon KL (low DC leakage) instead of FG. Or Panasonic stacked film caps for anything 1uF or less.
     
  4. CDFixer

    CDFixer Number 1 USC Trojan Fan

    Messages:
    1,913
    Location:
    Orange County Ca
    I have eliminated the problem, note I did not say fixed.

    This circuit attached is causing me the problem, it is suppose to prevent noise while changing the selector switch.
    I removed the circuit from the receiver by opening the connection between the collectors of TR603 and 4 and the volume control wiper which stopped the pop when changing the switch. The circuit should mute the receiver while changing functions.

    What is happening is when the selector switch is thrown or the phono select button is pressed the voltage at the base of TR602 reduces to about 11 volts, this is enough to turn on TR602 and the collector transitions from 0 to near 12V this voltage across R614 drops to a little over a volt then through the diodes D602 and 603, the voltage is at the bases of TR603 and 604 transitions from 0 to about .575V which turns on the transistors and should apply ground to the volume control wipers. This is all happening like it should but what happens is a spike to 5MV from 0 on the wipers of the volume control causing the 'pop'.

    Anyone got any Ideas, other than leaving this circuit disconnected?
     

    Attached Files:

  5. LBPete

    LBPete Rolling Along Subscriber

    Messages:
    11,480
    Location:
    Long Beach, CA
    Could you have leakage to ground through one or both switches? That could cause a voltage drop. There may be some kind of conductive contaminate in the switch. Jumper across the switches and see if it still pops.

    - Pete
     
    Last edited: Feb 13, 2018

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