JCM2000 DSL401 Crackling / Hum

Discussion in 'The Workbench' started by Emtbreid, Nov 16, 2019.

  1. mickeydg5

    mickeydg5 Well-Known Member

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    So then please explain. Maybe I missed something.
     
  2. JohnH

    JohnH Well-Known Member

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    Sure and thanks, I'll have a go. Need to get to a pc to read the circuit again though.
     
  3. Emtbreid

    Emtbreid Well-Known Member

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    Just for FYI idk exactly all the changes that were made but this is a post 2003 401.
     
  4. Emtbreid

    Emtbreid Well-Known Member

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  5. Emtbreid

    Emtbreid Well-Known Member

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    Grateful for your assistance John. Gonna be awhile til I get to a tech, and I seriously doubt I can repair on my own, so we’ve got time.
     
  6. JohnH

    JohnH Well-Known Member

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    Here's how I'm seeing those three jfets:

    They all operate at one time. If their gates are grounded they are on, with low resistance from source to drain, if gates go to a positive voltage, they are off with very high resistance. This happens when changing from clean to OD,

    When gates go to ground which is in od mode,T12 shuts off the clean channel, T13 trims the signal level going to send so it's not so big, and T14 bypasses R126, connecting C91 to ground, resulting in more OD gain.

    So my thought is, that all of these changes are quite significant, and if one of the jfets wasn't working, either as a short of an open circuit, the result would be noticeable. And from the testing, it seems all tonal functions are ok. So it seems like those jfets would be likely ok.

    Also, if the fault created an open or a short in one of the jfets, this fault condition would not appear to function differently to normal in one of the modes (since the jfet is in any case, biased on or off), but only if the other mode was engaged. So if this was the source of hum, then it would only occur in one mode, instead of being constant in all modes

    Any use.?
     
    Last edited: Dec 22, 2019
  7. Emtbreid

    Emtbreid Well-Known Member

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    I don’t understand the technical aspect, but I just want to add that the hum is more noticeable on the clean channel. I can faintly hear it in the noise of the background on the OD channel, but clean is where it’s blatantly obvious.
     
  8. JohnH

    JohnH Well-Known Member

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    Can you check one more simple thing: If you are on clean channel, and its humming, does turning the OD gain down to zero make any difference? Obviously it shouldn't, but there is another circuit called ODMUTE (controlled by T11), that if it wasn't working, would be replicated by turning down OD gain (which is VR5 on the diagram)
     
  9. Emtbreid

    Emtbreid Well-Known Member

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    Turning down the gain does nothing. Turning gain to max changes the sound a bit, makes it more high pitched maybe? Give me an hour; I’ll shoot a video and show you.
     
  10. Emtbreid

    Emtbreid Well-Known Member

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  11. JohnH

    JohnH Well-Known Member

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    Thanks What I was really requesting was, set channel to green and sweep the OD gain down to zero, not the clean gain. .It's to test the ODMute circuit.
     
  12. mickeydg5

    mickeydg5 Well-Known Member

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    Play the guitar in CLEAN first with CLEAN GAIN at zero and then start turning up to max.
    (I mean make guitar sound, not a song, hot licks or anything.)
     
    Last edited: Dec 22, 2019
  13. mickeydg5

    mickeydg5 Well-Known Member

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    @Emtbreid
    Again do this while playing the guitar as before.
    (I mean make guitar sound, not a song, hot licks or anything.)
     
    Last edited: Dec 22, 2019
  14. Emtbreid

    Emtbreid Well-Known Member

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    Stand by for video. @JohnH rolling overdrive gain to zero affects nothing.
     
  15. Emtbreid

    Emtbreid Well-Known Member

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  16. mickeydg5

    mickeydg5 Well-Known Member

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    Sorry I mispoke about channel affect by these JFETs. I forgot T12 is the "mute" for CLEAN channel. So when T12 is ON it should mute the CLEAN channel.

    @JohnH
    The thing we are dealing with here is abnormality. We cannot look at the diagrams and characteristics as if things are normal. They are not which is causing the issue.

    T12, T13 and T14 are stuck ON since T13 has 0VDC at the gate under all channel circumstances as shown by voltage measurements given. All three gates are tied.

    Now just because they are stuck ON does not, I repeat does not mean they are operating according to their normal characteristics.

    If that were the case that things are operating normal then there would be no guitar sound on CLEAN channel with the gate voltage at 0V, which I noticed a while back but did not mention.

    T13 reiteration:
    Above are the VDC readings at T13.
    LEG 1 = drain
    LEG 2 = gate
    LEG 3 = source
    Legs 2 and 3 always stay at 0 volts.
    The drain here only has a bit of VDC.
    There is no supply voltage to the gate when changing channels.

    @Emtbreid
    If you will, please get VDC readings at the legs of T12 in both CLEAN and OD channel modes to double check things there.

    If the same type voltages appear then we know for sure T12 is not operating correctly.
     
  17. Emtbreid

    Emtbreid Well-Known Member

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    I’ll do my best to get them before the night is done.
     
    mickeydg5 likes this.
  18. mickeydg5

    mickeydg5 Well-Known Member

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    You know we looked at voltages on T2 and T13 but not between them.
    So lets also get VDC measurements at each side/lead of components R26 100k, D18 1N914, R125 470k and C90 47nF/63V.
     
  19. SkyMonkey

    SkyMonkey Well-Known Member

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    Did you guys get it sorted, or are you just having a holiday break?
     
  20. Emtbreid

    Emtbreid Well-Known Member

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    Holiday break..... and I had a death in the family....... and I bought another amp in the meantime.
     

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