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Marshall JTM/1962 , of 1998, amp clipping

Discussion in 'Electronic Repair' started by N_Cook, Nov 4, 2012.

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  1. N_Cook

    N_Cook Guest

    Output ok up to 1 watt out then output clips both sides of sine. All valves
    test fine for gain and internal leakage. Signal is fine up to the wiper of
    the treble, ie before the splitter/driver, that signal stays sine well into
    clipping of the output. Changing the splitter is the same. Running at
    clipping and dropping the HT via variac makes no difference , changing
    output bias makes no difference to the clipping. The splitter valve cathodes
    measure about 34V and the grids about 23V at point of clipping. All the
    surrounding Rs measure ok. Output Tx primary measures 139R red to white.
    Capacitor problem? amp was subjected to a knock that dropped the output of
    the rectifier valve , since changed, a session before this problem emerged.
     
  2. N_Cook

    N_Cook Guest

    v3p1 anode is 220V and v3p6 anode is 168V so leaky .1uF letting through some
    of the neg bias from the output bottles , I assume
     
  3. Phil Allison

    Phil Allison Guest

    "Nutcase Kook"
    ** ROTFL...
     
  4. Phil Allison

    Phil Allison Guest

    "Nutcase Kook"

    ** The output tranny may have shorted turns.

    I test them by driving the speaker winding with an audio generator set at
    400Hz and checking the AC at the anode of each EL34 / 5881 with a scope.

    The amp is powered off for this, of course.


    ..... Phil
     
  5. N_Cook

    N_Cook Guest

    I forgot to say swapping output bottles did not swap the problem side.
    Trying a 600 ohm sig gen backwards into "8 ohm" so about 50mV pk-pk there,
    0.36V on the (from low V3 anodes) problem side "white anode" and 0.5V on the
    red other . I assume they should be much closer than that. I suppose the
    conclusive test is swapping red and white wires and see of the low splitter
    V3 anode voltage swaps halves.
     
  6. Phil Allison

    Phil Allison Guest

    "Nutcase Kook"

    ** The OT is fucked.




    .... Phil
     
  7. N_Cook

    N_Cook Guest


    Looks more and more like that
    I forgot about the feedback and simply swapping white and red creates an
    ultrasonic oscillator via the phase change. Changed that .1uF cap and same
    clipping problem.
    So nowhere else to go , I may try disconnecting the feedback , take some
    readings with red and white in correct positions and then swap white and red
    and see if the problem side changes.
    Any other ideas to try with the Tx in the amp and wiring in place?
     
  8. N_Cook

    N_Cook Guest



    I may as well try removing the output valves and trying on 80 percent mains
    or so ,and check their cathode signals, but looks more and more like a turn
    or few , shorted turns in the Tx
     
  9. Phil Allison

    Phil Allison Guest

    "Nutcase Kook"

    ** In my workshop, I constantly monitor the AC current draw of any piece of
    gear on the test bench.

    Faulty OTs are immediately obvious as the power coming out of the amp, when
    under load and drive, is SFA - while the AC current draw is full on.

    Also, the resistances of each half of the OT primary differ by more than
    10%.



    ..... Phil
     
  10. N_Cook

    N_Cook Guest


    I forget whether measured cold or warm but 61.3R and 66.4R, which seems
    normal enough, will try again cold and also with a 1KHz RLC for "R" and L
    of both
     
  11. N_Cook

    N_Cook Guest

    a corroded break rather than shorted turn/s? now taking Tx readings
    DVM White 61.8R , Red 75.3R was 61.3/66.4
    RLC(L) 20.9mH, 34.0mH
    RLC(R) 215 imp, 338 imp
    Things certainly not right but what is wrong inside? tugging the tails makes
    no difference
     
  12. Phil Allison

    Phil Allison Guest

    "Nutcase Kook"

    ** Valve OTs do not get hot enough to melt the enamel insulation on the
    wires, as often happens with power trannys. What brings about their demise
    is the very high voltages that exist during operation into a loudspeaker,
    particularly when driven into clipping.

    Voltage peaks can then occur on the anodes of the output tubes that range up
    to thousands of volts - resulting in insulation breakdown and arcing
    between adjacent layers of the primary winding.

    Sometimes an OT will test OK with a multimeter or at low power only to arc
    internally as soon as full power operating conditions are applied. This is
    due to carbonised material in the area of the previous arcing reigniting in
    a new arc.

    Marshall amps using EL43s are the prime example of this kind of OT failure.

    Some amps, notably Music Man and some Boogie models, have high voltage
    diodes fitted from output valve anodes to ground to prevent the sort of high
    voltages that damage OTs.


    ..... Phil
     
  13. Guest

    You either got raised resistance of the cathode resistor(s) of the splitter, low voltage to an element of the outputs or shorted something on the output.

    Do you know how to use a scope in the DC mode ? Whatever the cathode voltage is on the outputs, remember that and set the trace to a line somewhere, over it. If the grid drive gets up near that level then the problem is after.. Has to be in the load or the bias of the output stage then.

    If however you see that the output grid drive clips before getting near thecathodes' DC level (of the outputs of course) then look to the splitter stage. The way to tell if the cathode resistor value(s) has gone high is thatthe plate voltage will be high. Usually.

    J
     
  14. Phil Allison

    Phil Allison Guest

    <>

    You either got raised resistance of the cathode resistor(s) of the splitter,
    low voltage to an element of the outputs or shorted something on the output.

    Do you know how to use a scope in the DC mode ? Whatever the cathode voltage
    is on the outputs, remember that and set the trace to a line somewhere, over
    it. If the grid drive gets up near that level then the problem is after. Has
    to be in the load or the bias of the output stage then.

    If however you see that the output grid drive clips before getting near the
    cathodes' DC level (of the outputs of course) then look to the splitter
    stage. The way to tell if the cathode resistor value(s) has gone high is
    that the plate voltage will be high. Usually.


    ** OTOH - the amp might just be hexed.

    Don't you think ?



    ..... Phil
     
  15. N_Cook

    N_Cook Guest

    You either got raised resistance of the cathode resistor(s) of the splitter,
    low voltage to an element of the outputs or shorted something on the output.

    Do you know how to use a scope in the DC mode ? Whatever the cathode voltage
    is on the outputs, remember that and set the trace to a line somewhere, over
    it. If the grid drive gets up near that level then the problem is after. Has
    to be in the load or the bias of the output stage then.

    If however you see that the output grid drive clips before getting near the
    cathodes' DC level (of the outputs of course) then look to the splitter
    stage. The way to tell if the cathode resistor value(s) has gone high is
    that the plate voltage will be high. Usually.

    J

    +++++

    The owner reckons he knows someone across the country with a spare Tx and
    ability to fit it.
     
  16. Guest

    So what now, he ships the thing there ? I think it would be better to ship the Tx than the whole amp. Of course some people are nuts.

    Really though, if this thing runs class AB, if the Tx is shorted (turns) causing this, I would imagine the output tubes would redplate if you keep on pushing it no ?

    Maybe you should plug the primary of that transformer into the mains and see if it smokes.

    HEY, that is not stupid as it may sound really, but I would definitely remove it from the cabinet first. Think of it, you put 220/110 at 50/60Hx to itat the primary ? That shouldn't bother it at all. If it does, it's bad anyway. Difference is, this is a source with some balls (current) behind it. The impedance SHOULD keep it from smoking.

    It's called follaw the smoke, I've done it meself sometimes.

    J
     
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