Bias issue on TSL yes I know but this is weird

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

  1. Jethro Rocker

    Jethro Rocker Well-Known Member

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    I think any meter accurate to like 5 Mv would be totally fine. A digital meter should absolutley do it.
     
  2. Jethro Rocker

    Jethro Rocker Well-Known Member

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    Yes good point. I keep volumes at zero as well. Also on full power not VPR on.
     
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  3. anitoli

    anitoli Well-Known Member

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    But if there is a VHF oscillation occurring the meter may not read properly. I have witnessed this first hand with ha fender dual showman.
     
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  4. Lance Chambers

    Lance Chambers Well-Known Member

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    I bought mine from Amazon.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Do not troubleshoot with the tubes installed.
    Pull the tubes out.
    Turn the power on and leave standby in "warm-up" position.
    Set meter for DC volts.
    Connect black probe to chassis and red probe to pin 5 of output tube socket.
    Read all output tube sockets pin 5.
    Report the DC readings of all output tube sockets pin 5.
     
    Last edited: Sep 20, 2019
  6. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Do any of you search for information?

    This is classic crappy cathode bias monitor resistor behavior. Those 1 ohm 1W resistors crap out constantly.

    Check R6 and R9, both 1 ohm at 1W.
     
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  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Here is a partial schematic.

    upload_2019-9-21_0-48-49.png
     
  8. Jethro Rocker

    Jethro Rocker Well-Known Member

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    No I don't. I went with what I kjow about mine. Past that, it wasn't really my issue. Amd I'm lazy. And surly...:D
    And not that clever.
     
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  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Well the way I look at it I test the bias first with no tubes installed.
    if the bias won't hold steady there is nothing else worth doing first but replace pots / board or modify board.
    True the 1 ohms do go bad,,,but the adjustment pots have gone bad in almost 8 out of 10.
     
  10. South Park

    South Park Active Member

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    Pull out the power tubes. Check the voltage from the grids to the high taps. That bias drift is from a bad circuit board
     
  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I know, I know.
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    It could be the bad board issue but I would rule out the cathode resistor first.
     
  13. John T

    John T New Member

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    it is set to DC only
     
  14. John T

    John T New Member

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    I'm wondering if I swap the tubes (left two with the right two) if the problem will follow to the right side. I will try that later today.
     
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  15. Gunner64

    Gunner64 Well-Known Member Premium Member

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    It could just be a tweaked bias pot.
     
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  16. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    do it, it doesn't cost anything...

    neither does what ampmad suggested...

    nor, what Mickey suggested.

    Just make sure you are careful inside the amp. The TSL supposedly drains cap's, on power down, but, you should always check for yourself, to be sure. If the drain isn't working, you can get a lethal shock...
     
  17. Jeff Hudson

    Jeff Hudson Active Member

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    You may have two matched pairs instead of a quad, get a new matched quad and see what happens, unless you have access to a tube tester.
     
  18. RickyLee

    RickyLee Well-Known Member VIP Member

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    As the pros here mentioned, you need to check/monitor the bias supply voltage going to each side of the push pull/pins 5 on the power tubes. You can check it in standby with tubes in, or just pull the tubes. On these types of issues, you definitely need to stop playing the amp until you figure this out.

    As Micky stated, it sounds like your ground reference on that odd high reading side is bad, meaning the 1 ohm bias resistor is bad.

    I would thoroughly check the bias supply circuit front to back as well, mainly those trim pots.

    If the 1 ohm bias resistor(s) measure fine with amp OFF, then it could be acting up when in operation only. If bias supply voltage to pins 5 is correct, then you can install a jumper across that 1 ohm bias resistor giving you good cathode to ground connection. Then you have to check the bias on that side via BiasRite type tube sockets or check bias by output transformer windings voltage drop divided into the ohms of the output transformer winding on that side. That will point to the 1 ohm bias resistor as a problem if bias measures correct at that point.
     
  19. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    Welcome to the forum new MF’r I am confident you will get your answers here on this great forum!
    Cheers Mitch
     
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  20. John T

    John T New Member

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    Ok so I did the stable bias mod with a new bias board form the UK and still the same issue. Left side (V7 V8) goes right to 22V which the standby is switched. The other side is rock stable where it should be. Marshall suggested removing connector 20 from the lead board but that did not change anything. I tried removing C46 across the pins on V8 as I heard this can cause issues but no luck. I swapped tubes 1 and 2 with 3 and 4 and the problem did not follow. I check the bias with the tubes out and the right side is rock steady at.1mv. The left side goes to -12mv. I put an ohm meter on the pots when I was working on the board. Both pots read the same value. Any ideas where that voltage could be coming from? I have heard that the coupling caps (c6-c7) could be leaking but I think they were bypassed with the stable bias board.
     

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