Bias issue on TSL yes I know but this is weird

Discussion in 'Marshall Amps' started by John T, Sep 20, 2019.

  1. John T

    John T New Member

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    OK 2002 TSL 100w. Just got this amp and I am very aware of the issues with the bias. So I am checking it and one side I can adjust the POT and get it to 90mv which does increase after time like we know. But the left side will not adjust with the POT and the meter reads 15.00 volts not MV but V! Bad POT?? Not red plating at all so what would you experts think? I am not an expert.

    Thanks

    JT
     
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  2. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    Hmm. Are the tubes 2 matching pairs? Shouldn't read V like that. Something amiss.
     
  3. John T

    John T New Member

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    Well like I said I am not the original owner but by looking at the tubes they are RUBY tubes and I do not know if they are matched. But even if they were not matched 15.9 volts seems a tad high. I double triple checked my connection to the amp and I can not see where I made a mistake. Ill try to post a YTube video if I can.
     
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  4. John T

    John T New Member

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  5. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    Oh thats extreme yes. Could be something at the bias points or circuit. That's where I bow out!

    Sorry was able to watch with audio. That helps! I don't get this at all...
     
    Last edited: Sep 20, 2019
  6. John T

    John T New Member

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    The meter is auto ranging. Yes DC volts. Read Ma at 200 left side .1 right side .2. The thing that makes me think I'm not screwing up is the right side works as advertised. I'm just puzzled why the left side reads so high. I am planning on pulling this amp apart in the next several days and fixing the bias issue with a replacement board but this is weird. I'll let you know what i find if anything.
     
  7. Lance Chambers

    Lance Chambers Well-Known Member

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    You have the volt meter on the wrong setting. Change it to the 100 mA setting then redu the bias.

    FYI: 70% saturation would be 34-36mA per tube. Set each side to 72-76 maximum.
     
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  8. South Park

    South Park Well-Known Member

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    Try to trace out the bias circuit from the high voltage tap to the grids .
     
  9. John T

    John T New Member

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    ok why does it work on one side and not the other?
     
  10. Lance Chambers

    Lance Chambers Well-Known Member

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    One side is too high thats why the reading is off. The TSL bias adjust is a push-pull type. Turn clockwise will lower that side then raise the other.........vice verse!

    Make small adjustments.
     
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  11. John T

    John T New Member

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    Putting it on a mA setting does nothing on either side. The right side works like it should setting 90 Mv. the left side does nothing. I will try to look at the amp further tomorrow. It still sounds great though.
     
  12. Lance Chambers

    Lance Chambers Well-Known Member

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    Important........verify all fuses are the correct values.
     
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  13. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    Yes but not 15 volts. Way too high. Should read in Mv range. Plus the TSL is recommended at 90 per side. I set mine around 84. It should never read even 1 volt.
     
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  14. John T

    John T New Member

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    If I turn the right side all the way down to 24Mv the left side goes up to over 15V. If I turn up the right side to 100Mv the left side goes down to 14.8 V
     
  15. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    In layman's terms it's f&cked! :D
    Great sounding amps though.
     
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  16. Lance Chambers

    Lance Chambers Well-Known Member

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    I use a Fluke 21 and set it to 300. I also use a socket meter on each tube.

    TSL's are finicky! :wallbash:
     
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  17. John T

    John T New Member

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    they are
     
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  18. John T

    John T New Member

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    I have looked at those on Amazon and really did not think I needed one. Do you think they are worth it?
     
  19. anitoli

    anitoli Well-Known Member

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    Set the meter to read DC only. Is it the same? If there is an oscillation issue an auto ranging meter will read f'ed up.
     
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  20. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    @ampmadscientist has a post somewhere around here, where he tells you to pull the tubes & take some readings. Maybe he'll be along shortly...
     
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