JVM410js-StrangeGainproblem [SOLVED]

Discussion in 'Marshall Amps' started by Hollar, Feb 21, 2020.

  1. Hollar

    Hollar New Member

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    HI all,
    A mates amp died on him and Im trying to help out. I have electronics background but not specifically amps.
    After much messing around the amp works perfect, as in all the knobs seem to do what id expect. But it only ever works with the guitar on full vol. Strange yeah? Lowering the gtr vol a fraction(tried various guitars, various leads same thing) loud scratchy sound and then nothing, no gain from the amp at all. Almost like a gate threshold is switching in. Problem is not intermittent but directly related to input signal level.
    Tried the same thing with a sig gen sinewave on the input. I tried various amp modes and ch. All the same so its something right back at the preamp stage.
    Low input signal no sound, get just enough signal and bang shes off and running with heaps of that expected Marshall power.
    With gtr vol full I tired to see if the threshold mode were working. All checked out, knobs did the expected thing amp was throttled back when signal died off. So I don't think thats at fault.
    Im waiting on some preamp valves to play swaps with but thought Id yell out in case anyone had any ideas?

    All fuses are fine, HT volts look good etc
     
  2. Hollar

    Hollar New Member

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    A little more info.
    Something strange in that input jack PCB.
    It has two leads a grey one(carrying signal to the main board and the valve preamp).
    and a blue one which carries +_8v to the TL072 and a buffered signal back to the main board.Which feeds THAT4301 and the VGATE signal to the uP.
    If I remove the blue lead, things behave themselves. I can vary the amp output using the gtr vol pot(with a suitable amp gain and vol set).
    If I inject the gtr signal direct into the grey lead(ie bypass the input jack pcb) again all is well.

    So looking at the output of the TL072 which to me is a simple non inverted opamp. If I vary the gtr pot I get crazy DC shift on this output.(passive gtr so there aint nothing dc going in) At full gtr vol the opamp output centres around ground and I see nice gtr signal (oh BTW Im using a CRO now) but anything other than full vol and I get DC crazy which I suspect is heading down that blue lead into the THAT4301 and causing all sorts of drama further down?

    Im going to have to wait now for a TL072 to replace it in the theory that its at fault.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes, a gate threshold is switching in. It's got a noise gate at the input....It doesn't look like there should be any DC on the output of the TLO72.
    Some schematics are posted here:
    http://jvmforum.com/phpBB3/viewtopic.php?f=33&t=7866&sid=722ffceab4d242cd3c37c9f9a8a34636
     
    Last edited: Feb 22, 2020
    Nik Henville likes this.
  4. SkyMonkey

    SkyMonkey Well-Known Member

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  5. Nik Henville

    Nik Henville Well-Known Member Premium Member

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  6. South Park

    South Park Well-Known Member

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    Check the fed back circuit on that op amp . It how you control the gain
     
  7. Hollar

    Hollar New Member

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    Just reporting back for completeness.
    It was indeed the TL072 input op amp that drives the noise gate. Replaced chip and all is well.
    I suspect over voltage was placed on the input as there is no form of protection for the opamp.
    thanks all.
     
    Nik Henville likes this.
  8. Sir Don

    Sir Don Well-Known Member

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    Glad you got it up and running.
     
  9. Lance Chambers

    Lance Chambers Well-Known Member

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    Did you check the fuses and make sure they are the correct type/rating?

    :cheers:
     
    Nik Henville likes this.
  10. Nik Henville

    Nik Henville Well-Known Member Premium Member

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    Just rip 'em all out and replace with 50amp ones...
    [​IMG]
     
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