Bias issue on TSL yes I know but this is weird

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

  1. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Take tubes out.
    Power off / power un-plugged.
    Set meter for ohms.
    Red probe on pin 8 of output socket.
    Black probe on chassis ground.
    What is the readings of all 4 sockets?

    R21 should not blow, that would be kind of unusual.
     
    Last edited: Sep 27, 2019
  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    standby, or live?

    yeah, that's why Mick asked if I had shorted something out...
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Unplug power and do the test with amp off.
    Whenever you are using an ohm meter, the power is disconnected from the mains.
     
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  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Dude, change that thing out.
     
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  5. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    from rear of chassis: pin 8

    left: 2 ohms/2 ohms
    right: 2.38 m'ohms/2.38 m'ohms
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Right
    2.38 m ohms? is that mega ohms?
    Can you show a picture?
    There is definitely something still questionable going on here...
     
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  7. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    yes, mega... (on the right, adds 'M' (or 'K') ohms & decimals automatically)

    ohm.jpg

    R9 also measured 137.9k ohms.

    which is why I ordered the resistors last night. They say shipping date is friday.
     
  8. mcblink

    mcblink Well-Known Member

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    s0807.gif


    This is cool to watch.

    Okay, sorry gents, carry on...
     
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  9. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    I took a couple more snapshots...

    R9: 186.9 k'ohms

    ohm.jpg

    the only difference from yesterday & this, is that, yesterday, I hadn't removed the tubes & I hadn't put juice into the amp yet (after a couple years sitting)
     
  10. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    this is the crispy critter R21:

    ohm.jpg

    supposed to be 100 ohms...
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Well then the all resistors is fubar it sure looks like to me.
    The resistor should be replaced with same size wattage rating.
    1% would be just dandy.
    The resistor is a fuse.
    If the tube draws too much current the resistor will blow. This protects the output transformer.
    Putting in the over-sized resistor could mean the transformer or tube becomes the fuse instead of the resistor.
    The resistor limits the current.

    R21
    Are we looking at different schematics? with different part numbers on the drawing?
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    That is an excellent picture thank you.
     
    Last edited: Sep 28, 2019
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    99.7 is closer than 1% which means the reading shows good.
    The resistor won't read exactly will always be 1% 5% 10% 20% plus minus, and all those are readings normal.
    But the 100 ohm resistor is toast what made the 100 ohm blow?

    Because if you put in a new resistor it may blow again for the same reason.
    You need to explain why the resistor blows in the first place, before putting a new resistor in.
     
  14. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    if you follow back here:

    Bias issue on TSL yes I know but this is weird

    then look at the following post, I posted a snippet of the schem. R21 is on the output board w/ the speaker jacks...

    The spec on the resistor measure good, but, since it's crispy, I'm going to change it when the part comes in...

    Everything is on hold, until I get the replacements, which my order says delivery friday...

    When I get those in, then I can retest everything & see where we stand...
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    The 100 ohm resistor is the 24 volt supply for the footswitch circuit.
    You got something shorted in that switching circuit that is causing that resistor to burn up. (maybe a shorted footswitch cable is what it's looking like)
    If you replace the resistor without finding the short, the resistor is just going to burn up again.

    So lets check for a short between pin 3 and pin 6 of the footswitch cable...
     
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  16. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    ok, yeah, I've never hooked up the footswitch, although I do have it...
     
  17. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    I took a look around & can't locate the footswitch right off. I'll have to hunt it down... I found the ones for my JMD:1, JMP-1 but not the TSL. Funny, it's one of those things where I've seen it hundreds of x's, while not looking for it, but, obviously, it's been moved & stashed...
     
  18. ampmadscientist

    ampmadscientist Well-Known Member

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    That cooked resistor may have happened before you bought the amp?
    If it's not the FS then it's something else making the resistor overheat?
     
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  19. Gunner64

    Gunner64 Well-Known Member Premium Member

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    I do know that my tsl60 had a bad cable on the footswitch, and have heard of other tsl footswitches with bad cables. It's been awhile, but I think they were solid wire, and didn't stand up well to the flexing of the cable., maybe that would have contributed to that resistor cooking.
     
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  20. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    yeah, probably...

    I'll have to hunt down the footswitch. I got it w/ the amp, but, I never used the FS, because I was always just right there dialing it in. At the time, I had just got 1 of those Alessandro bias meters. I also just got an Ampeg VL-501 (Lee Jackson). I went to change the tube, when I found the initial issue (high bias on 1 side). When I couldn't get it to bias, I plugged in the VL-501 & just ran w/ it & didn't mess w/ the TSL. I figured I'd just get back to it.

    I actually had a few ideas, like changing the trannies & adding a choke. I was also debating changing the VPR to a pot (variable), instead of a button (fixed), as Wilder told me it was just a fixed PPIMV. I was going to have Wilder do it for me, but then the drama started back then, right before he left...

    So, it never happened & the amp got shelved...

    This thread made me think about it again & since I was at storage, I grabbed it real quick & did my 1st open it up & look & here we are...

    IDK where the pedal is right off though. I've had so many projects going on around here, things get moved & stashed...

    I think, when I went to try the amp out (at the guy's house), he had the pedal hooked up & it worked fine...

    Doesn't mean that there isn't some sort of problem w/ it that needs to be sorted out before I plug it back in...

    When I get these parts, there will be plenty of extras, but, hopefully, I can swap them out & everything will be ok, or, I can further inspect it, to see if the numbers prescribed earlier check out...
     
    Last edited: Sep 28, 2019

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