Jcm 900 Problem

Discussion in 'The Workbench' started by Marcell, Sep 16, 2018.

  1. Marcell

    Marcell Active Member

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    Is it a cap that is draining or a preamp tube.. installed el 34 and a bias resistor a few months ago.. maybe want to sell it, but then the noise got to go!
     
  2. tschrama

    tschrama Well-Known Member

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    Sounds like a error in the muting opamps. not tube related.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    When you turn the amp ON:
    Turn the power ON first (switch up). Wait for about 30 seconds.
    Then turn the standby ON. (switch up)
    Now you can play the amp.

    When you turn the amp OFF
    Turn the standby off (switch down) first, wait for about 5 seconds,
    then turn the power off (switch Down).

    You cannot turn the power OFF (switch down) when the standby is still ON (switch up).
    Always turn the standby OFF (switch down) first.

    ------------------------------------------------------------------------------------------------------

    Wanneer je de versterker AANzet: Schakel eerst de stroom in (overschakelen). Wacht ongeveer 30 seconden. Schakel vervolgens de standby-modus in. (schakel omhoog) Nu kun je de versterker spelen.

    Wanneer u de versterker UITschakelt Schakel de stand-by eerst uit (schakelaar naar beneden), wacht ongeveer 5 seconden, zet vervolgens het apparaat uit (schakelaar naar beneden).

    U kunt de stroom niet uitschakelen (uitschakelen) wanneer de standby-stand nog AAN is (schakelaar omhoog).
    Schakel eerst de stand-by UIT (schakelaar naar beneden).
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    He's turning the power off when the standby is still on. At least it looks that way from here. Call me crazy, but...
     
  5. Marcell

    Marcell Active Member

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    NO.. it might look that way.. first flip the standby switch that the powerswitch.. and it doesn't make a difference if I'll wait several seconds or wait a few hours until I flip the powerswitch.. it makes the same sound.
     
  6. tschrama

    tschrama Well-Known Member

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    If it only happens during powering off... dont worrie about it. It is prolly a temporally oscilation ins the switching opamps during a the smallperiod were the opamp receive just i suficient supply voltage.

    lots of electrical equipment have that sound...

    Does the amp sound right besides this weird sound.?
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    OK
    take the preamp tubes out, and test it again.
    Did the noise stop?

    Yes the noise stopped:
    It might be a bad preamp tube. Swap out the preamp tubes one at a time and re test each time.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    In the video:
    The standby is ON (switch up) when you turn the power off (power switch down).
    You can't do that.

    The standby switch must be in down position first---before you turn the power switch off.

    This is why the amp is making the noise.
     
  9. BowerR64

    BowerR64 Well-Known Member

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    Is something in the loop?

    JCM900 noise.jpg
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes, there is probably something in the loop.
    But, the standby is not in the correct position when he turns off the power.

    He is turning off the power when the B+ is still turned on.

    Watch the video --- you will see.
     
  11. BowerR64

    BowerR64 Well-Known Member

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    Looks to me like he is switching it off properly.

    Power light is on so that switch is in the "play" mode and the standby is also in the same position so when both are same diretion the amp should be making sound.

    I then hear him flip the standby switch first but it may not be turning off the B+ i dont know
     
  12. Marcell

    Marcell Active Member

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    I know I had a beer or 6 to much... glad I'm not the only one. Loop is turned of btw. LOL
     
  13. Marcell

    Marcell Active Member

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    High voltage is off... so filements are still glowing.. but beside that wat is powered? Could it be some induction in the powertransformer just like in a car? Bobine power gets interupted by contactpoint or hall inducter... whatever induction in the secundairy spool witch causes the spark.. I am a automotive geek..
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Turn the master volume(s) to zero then shut power off.

    Did the noise go away?
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Has the output transformer ever been replaced?
     
  16. Marcell

    Marcell Active Member

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    Nope... not that I know of.. volumes are always turned to zero when
    I shut it off.
     
  17. fat_lenny

    fat_lenny Well-Known Member

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    My 4100 has always done this when turning off, Ive had it since new in 1993.

    Doesn't matter how long you leave it on standby before turning the power off.

    Ive had caps replaced (not because of this issue) and retubed several times over the amps life, none of this ever changed the noise when powering down.

    To be honest, as its always been there, I assumed it was normal with this amp. None of my other amps do it.
     
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  18. ampmadscientist

    ampmadscientist Well-Known Member

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    Then I might try reversing the output transformer primary wires.
    The 2 wires go to pin 3 on each side of the 2 output tubes.

    What model 900 is this? Is it 4100?

    The reason I am saying this: we just fixed an amp with the same problem by reversing the wires, a few days ago.
    Just say'in, worth a try.

    Read this thread:
    http://www.marshallforum.com/thread...gh-pitch-noise-when-turning-power-off.103596/

    900 output.png
     
    Last edited: Sep 16, 2018
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    Reversing the output transformer secondary wires:

    What's the best that could happen?
    a. the amp will sound much better.
    b. the oscillation will be gone.

    What's the worse that could happen?
    a. The amp will whistle when the volume is turned up.
    b. You will have to put the wires back to original positions.

    So, like I said: worth a try.
     
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  20. ampmadscientist

    ampmadscientist Well-Known Member

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    (oscillation at power off) It's not normal.
    What I'm thinking is: the wires (red and white) were reversed by accident at the winder or at the assembly.
     

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