Need Help With JVM410H, Burnt Resistor

Discussion in 'The Workbench' started by AlinSt, Aug 23, 2019.

  1. AlinSt

    AlinSt New Member

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    R27.jpg Hi guys, any expert around? Today while changing the tubes i was trying to balance the bias between the pairs and the right pin measured around 210 V and that amazed me knowing that i should have around 60 mv there. Then i noticed that R27 is blown. Any idea what could cause this and if i will be safe just replacing that resistor? I bought this amp from a friend, i really like it but i need to be sure that everything will be ok after removing this failed resistor[​IMG]
     
  2. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    Welcome to the forum new MF’er.

    Congrats on the new amp. AmpMadscientist will be able to help Im sure!!!

    Calling @ampmadscientist ... new member needs your expertise!!!
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    The burned resistor is the symptom not the cause.
    The circuit board may be bad.
    Do not play the amp.

    You need to make several measurements to narrow down the location.
    Do not put new tubes into the amp, do not turn the standby to "operate." The new tubes will burn up.

    If you need
    I can walk you through measurements later / over the weekend, but DO NOT play this amp or put the tubes in.

    Trouble shooting is done with NO tubes installed. This will prevent the tubes from being destroyed.
     
  4. AlinSt

    AlinSt New Member

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    Thanks a lot, i will not play it but i did put a new TAD quartet not knowing that was toasted, trying to biasing i found this issue. I hope they are still ok... :) Please let me know what to measure to narrow it down.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    Does this schematic match your amp?
    R102 and R106 should be 1 ohm?
    Could you check that and report back?
    Power un-plugged, power off.
    If it does match, take the tubes out and measure those 2 resistors, I think you may have a blown resistor.

    Set meter for ohms.
    Measure across each resistor should read 1 ohm or less?
    upload_2019-8-24_13-42-42.png
     

    Attached Files:

    Last edited: Aug 24, 2019
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    On your amp the 1 ohm resistors may be R26 and R27.
    Depends on which board it has.
    Anyhow remove the tubes and test the 1 ohm resistors cause that seems to be what's matching your description.

    MarshallJVM410PA.png
     
  7. AlinSt

    AlinSt New Member

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    Already did that, R27 is gone, it gives no value when measured. R26 is ok
     
  8. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    Take it to a tech and let him/her figure out why the resistor blew.
     
  9. AlinSt

    AlinSt New Member

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    It's not that simple, here we don't have techs, only some passionate guys that are self-taught but with no real technical background. I have a friend that has the technical background but he doesn't do this type of work
     
  10. john hammond

    john hammond Well-Known Member

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    1 ohm resistors are used as bias measuring points from pins 1-8 to ground in some amps , but they can also double as a sort of fuse in that location.
    It is plausible that a tube popped and took this resistor with it.
    it wont hurt the amp to replace the resistor, fire up amp quickly and check all voltages from output tubes.( and then preamp tubes if all is good)
    if there is, for example, no voltage on pin 5 of either of the output tubes, shut it down quick, as ampmad has stated..you'll just cook off your new tubes. You probably have..20 - 30 seconds?
    Pls note the 1 ohm resistor could be used to ' fuse' another part of the amp.

    edit.- you could also simply measure for voltage on pin 3 of the output tubes with no tubes in the amp. If a 1ohm resistor has been used as a fuse for that tube...and its fried...you'll get no voltage reading on pins 3 and 4 because the ground reference is no longer there ( the 1 ohm resistor is the reference to ground )
     
    Last edited: Aug 25, 2019
    sloan_amps likes this.
  11. john hammond

    john hammond Well-Known Member

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    i just checked that diagram. r27 is a current limiter.( fuse)
    ampmad is right on the scent of this one.

    the crazy bias reading could be because the ground reference for the bias sub circuit is either gone, or partially gone.
    I cant tell off the diagram how the amp gets its bias, so i think voltage readings from pin 3 from those output tubes is a good next step.
     
  12. AlinSt

    AlinSt New Member

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    I guess with no tubes and with the amp full on!
     
  13. john hammond

    john hammond Well-Known Member

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    yes with the amp full on.

    carefully take voltage readings off the pins, you dont even have to worry about pin numbers.

    Just note what voltages you got and report back here, if the theory is correct, youll get some high voltage numbers off three output tubes , but only perhaps that erroneous 200v bias reading off the other ( as well as some heater voltage from all of them, which are low). just put the black lead of the multmeter to ground and use the red lead.

    And don't kill yourself, if you don't like playing with live amps, take it to someone who is used to it.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    We should make more tests, but the resistor blows because:
    1. Bad output tube.
    2. Wrong speaker impedance load. (like a bad speaker cable, bad connections, bad speakers etc...)
    3. Bias failure.
     
    myersbw likes this.
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    Yes, you should take more readings.
    You do this with the standby set to "warm-up" first.
    Yes the pin numbers DO matter.

    If you want to make the tests let me know, and I can walk you through that.
     
  16. john hammond

    john hammond Well-Known Member

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    ampmad ive butted in with this one, its all yours again.
     
  17. AlinSt

    AlinSt New Member

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    Of course I want to find out what is the reason for the failure. Please let me know what to measure and how.
     
  18. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    Take out power tubes.
    Turn on the power, but leave standby in warm-up position.
    Set meter for DC volts.
    Black probe connect to chassis. Red probe connect to pin 5 of output tube socket.
    Each output tube socket should have negative voltage on pin 5.
    Write down the voltages you are reading and report them for each socket.

    step 2
    After you record the DC voltage reading for each socket pin 5, turn the bias adjust control for each socket, and make sure the voltage will adjust up and down on each socket pin 5.
    There are 2 bias adjust controls...
    After you verify the DC adjustment is working for each output tube socket pin 5, set the voltage back to where you found it originally.

    Were you able to complete this test?
    What was the readings?
     
  19. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    All the more reason to make the effort and find a tech. I know it might be a little more effort and time, but it might be better in your situation.

    JVM amps just don't blow resistors all the time. Additionally, you didn't say in your first post that you were having problems with the amp or that you made an error in how you were using the amp, so blowing a resistor in your case is curious.

    Is the amp still under warranty? If so, then you may have some good options before you go sticking your hand in that amp and/or start spending money on it.

    I know you said in your 1st post that you bought the amp from a friend. If the amp is still under warranty and your friend is willing, ask him to send the amp to Marshall so they can have a look at it. The reason you may need your friend's help is that (if I remember correctly) Marshall's warranty is not transferable if the amp is sold to another person.

    Lots of things to consider when using the warranty. Age of the amplifier, length of the warranty coverage, modifications, etc. It's worth looking into using the warranty before you start spending time, money, and effort.

    Whatever you decide to do, I wish you best of luck with your amp situation. Let us know what you find.
     
    Last edited: Aug 25, 2019
  20. MonstersOfTheMidway

    MonstersOfTheMidway Well-Known Member

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    There are many resistors inside a JVM, and each can have a different task depending on the circuit. Additionally, if this JVM had a bad output tube, perhaps a fuse would have blown first before a resistor. In my experience with the stock JVM amps, resistors don't blow often (it's more likely that a fuse would have blown than a resistor if a component, like a tube, failed).

    I'm not saying a resistor can't blow for the reasons you gave (which are all reasonable), but it's not common to blow a resistor in a JVM from my experience. And given that the OP didn't say he made a mistake like using the wrong speaker impedance load or that he used a bad tube, blowing the resistor seems very curious.
     
    Last edited: Aug 25, 2019

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