Full volume after 2 minutes

Discussion in 'Amp Tech Center' started by DucDone, Aug 5, 2020.

  1. DucDone

    DucDone Tele-Meister

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    Hi. I have this odd problem/issue with my Soldano Slo amp. After it's turned on, it comes to full power after about 2 minutes, as you can see in the waveform in the picture. What could this be? I have changed tubes and that didn't change. I was hoping I wouldn't have to take it out of the cabinet, so if anyone has a suggestion?
     

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  2. Wally

    Wally Telefied Ad Free Member

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    Are you saying that no matter where the gain and volume controls are set the amp produces full output? What if all controls are set to zero? Is this with an instrument plugged in?
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Once the situation occurs, what happens if you pull preamp tubes? Start with V1 and pull in order with the phase inverter being last. Does the situation change with the removal of any 12A-7 tube? I am going to think that you have an osicillation is occurring and taking the output to full roaring output. Have you put that scope at different points in the circuit to see where things are going wild?
     
  4. DucDone

    DucDone Tele-Meister

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    The amp acts natural at all points, the only difference is that the volume is low the first couple of minutes, then it output what would be the normal output. No issues elsewhere. It reacts to all of the controls in a normal way, The volume f x is turn the sound up and down as usual, only the first couple of minutes it's about half the volume.
     
  5. Jon Snell

    Jon Snell Tele-Meister

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    The first place I would look is here;
    Screenshot 2020-08-05 at 14.20.26.png as the LDRs are the common denominator here.
    Check/replace C37
     
  6. DucDone

    DucDone Tele-Meister

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    Could I measure LDRs?
     

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  7. Jon Snell

    Jon Snell Tele-Meister

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    Yes. With the amplifier switched off and no latent high voltage on the smoothing capacitors, measure the resistance of the LDRs. They should be very high resistance and when the amplifier is powered up change value when each LDR is selected. Screenshot 2020-08-06 at 14.01.34.png To prove the LDRs, link the resistor parts out temporarily with a 10k resistor bridges across the LDR side. Hopefully if my diagnosis is right, there will be full volume straight away. If LDR1 is linked out the volume will be reduced. Just think of them as variable resistors, the more light, the lower the resistance.
    Your layout is not to be trusted as at first glance the power switches are drawn wrong!
     
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  8. Wally

    Wally Telefied Ad Free Member

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    Imho, the amp needs to be on a tech’s bench so that voltages could be observed during the time of low volume versus the time of normal operation. If you are that tech, put it on the bench. How old is the amp? SLOs have been around long enough for the electrolytics...like the one Jon Snell points to....are weak and in need of replacement.
     
  9. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    So Jon your diagnosis is the switching supply voltage is causing the amp to "switch" as it starts to function after two minutes? Or is something else being dragged down?

    Trying to learn.
     
  10. Jon Snell

    Jon Snell Tele-Meister

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    What we must do first is diagnose what is actually happening.
    By measuring the off load resistances, they should be very high to reduce the level of signal.
    Now, when we put current through the LED part of the LDR, the resistance should go down, allowing for more signal or if current is instant, acting a little like an electronic switch. A Light Dependant Resistor.
    Once we know they change value as current is passing through the LED sections, we know they are working as expected.
    So, if the resistance changes over say 2 minutes when the amplifier is working and first switched on, they are either all faulty or there is an issue with the drive to the LEDs. Until that point is reached we are clutching at straws.
    I hope that explains it for you.

    I just read what I typed through and amazed myself as I am dyslexic and it even makes sense to me!:D
     
    Last edited: Aug 6, 2020
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  11. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Has it ever worked properly, or has it always had this two minute lag?
     
  12. DucDone

    DucDone Tele-Meister

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    It has worked properly for a long time.
     
  13. DucDone

    DucDone Tele-Meister

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    How should the power switches be drawn? I did this build myself, and bought the stuff from C3 amps.
     
  14. Jon Snell

    Jon Snell Tele-Meister

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    Transpose the lower pair of wires, both power and standby to the centre pins. Screenshot 2020-08-11 at 12.58.59.png or link bottom to centre.
     
  15. Wally

    Wally Telefied Ad Free Member

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    So, a build that has worked for a ‘long time’. Jon Snell’s path if investigation is interesting. As for the switches, they must be wired differently from your drawing shown in post #6, or the amp never would have powered up, would it?
    Have you gone through the test procedure Jon Snell outlines for those diodes? if that investigation does not point to the problem
    and since you say the amp has worked properly ‘for a long time’, one wonders what a ‘long time’ is? When did you build this amp? if the diodes are good, one might suspect electrolytic caps??
     
  16. DucDone

    DucDone Tele-Meister

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    I built this amp in around 2017. I have ordered new LDR's, next up are the filter caps. It's a very strange thing, the sound is the same at lower volume as it is when the volume gets to full.

    I replaced a 10k resistor on the filter board, it had brown "burn marks" on it.
     
  17. DucDone

    DucDone Tele-Meister

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    Found the reason for the volume drop/increase, it was one of the LDR's, as shown on the pic.

    Thanks for the help Jon Snell-
     

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  18. Wally

    Wally Telefied Ad Free Member

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    So, this is the LDR that is connected to the control grid of the second triode in V2, correct?
     
  19. DucDone

    DucDone Tele-Meister

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    It seems so, yes. I have enclose the pdf for the build.
     
  20. DucDone

    DucDone Tele-Meister

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    Here:
     

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