Lockdown Troubleshooting

Discussion in 'The Workbench' started by assaf110, Apr 16, 2020.

  1. assaf110

    assaf110 Well-Known Member

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    Hi all,
    Looking for some help troubleshooting a Princeton reverb kit build (Mojotone).
    I have built a couple of tweed amps before (5e3, champ etc.) a bunch of pedals & other stuff, so my soldering & general technical abilities are good. Having said that, a faulty joint or wiring error is always possible.
    The amp have some serious issues, presumably in the power section. I'm getting some wild voltages, as you can see in the chart attached. The tubes seems to glow OK, the speaker make no sound at all.
    If this thread is better off in a different forum, please feel free to direct me to a more appropriate place...
    Thanks & good health to you and all your loved ones,

     
  2. Travis398

    Travis398 Well-Known Member

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    Is the CT grounded? hard to tell on the picture if there is a jumper or if it's isolated.
     
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  3. assaf110

    assaf110 Well-Known Member

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    Do you mean the PT red&yellow tap? Yes, it’s grounded.
     
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  4. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    GZ34 pins 6&7 approx 328v A/C, then if that is OK then you should find approx 300v D/C on the cathode pin 8.
    Thats the first thing to be getting on with for starters
     
  5. South Park

    South Park Well-Known Member

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    That 800 volt reading you might have a voltage doubler from the filter caps trace out the power rail
     
  6. neikeel

    neikeel Well-Known Member

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    Those voltages really are off.
    It might seem daft but double check the orientation of your rectifier wires in relation to the correct pin numbers.

    The HT is very asymetric. What happens if you temporarily unhook the bias feed wire, or simply remove the rectifier and measure ac between the pins and then each limb to ground?
     
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  7. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Just do the basics first as its always a good idea to get the PSU working properly, then the rest will fall into place, dont go looking too deep
     
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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Let's see close up pictures of the first 3 large tube sockets.
    There is not enough detail to trace the wires to each tube socket pin.
    Apparently the wires to the sockets may be positioned wrong....
     
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  9. assaf110

    assaf110 Well-Known Member

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  10. assaf110

    assaf110 Well-Known Member

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    When I take the GZ34 out and measure I get the same - 370v (pin 6), 234v (pin 4).
     
  11. johan.b

    johan.b Well-Known Member

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    Is this one truly going to ground or might it be "not connected" to the ground next to it? 20200416_175006.jpg
     
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  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Fender Princeton gz34 Power Supply.png


    octal pin nos.png
     
    Last edited: Apr 16, 2020
  13. johan.b

    johan.b Well-Known Member

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    Screenshot_20200416-180156_Samsung Internet.jpg
    Insulation pooking through?
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    This red/yellow stripe wire should be connected to ground, I think you are correct.
    [​IMG]
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Red / yellow stripe wire needs to be grounded to chassis.

    [​IMG]
     
  16. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Across the two red wires on the GZ34 what does it say on your meter, dont forget you are reading AC not DC
     
  17. assaf110

    assaf110 Well-Known Member

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    You guys are ace!
    It’s the first time for me using this 3 lug earth that came in the kit. I assumed they were all conected. Well it’s not the case and the PT CT is not grounded. I’ll sort that out & report back.
     
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  18. ampmadscientist

    ampmadscientist Well-Known Member

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    fender princeton PS.png
     
  19. johan.b

    johan.b Well-Known Member

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    The black wire, I assume, is ground for the bias?... lacking ground on both those points would explain the odd readings you've got.
    J
     
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  20. assaf110

    assaf110 Well-Known Member

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    690vac
     
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