Testing resistors in circuit

Discussion in 'The Workbench' started by CROWEMAG, Sep 19, 2020.

  1. CROWEMAG

    CROWEMAG Active Member

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    On a JCM 800....is there a list of which resistors you cannot teat in circuit? Say R2 (on a 1986 2204). Schematic showing 1Mohm my resistor is reading 91.6kohms. The resistors color codes is for a 1Mohm.
    Thanks in advance!
     
  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Generally you always want to pull one leg free to test ANY resistor in circuit if you want any sort of accurate measurement. Any resistor in parallel with the one being tested WILL throw off the measurement considerably.
     
  3. CROWEMAG

    CROWEMAG Active Member

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    Awesome. Thanks man!
     
  4. anitoli

    anitoli Well-Known Member Gold Supporting Member

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  5. CROWEMAG

    CROWEMAG Active Member

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    Ok so I tested with jack in high input. OL.

    without the resistor reads 96.1k. Should be 1Mohm. I’d hate to lift one leg to test if this a normal reading. Anybody with any personal experience measuring R2 of a 86 2204? Thanks!
     
  6. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    OL would mean the meter can't read the value as it's too high.
     
  7. South Park

    South Park Well-Known Member

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    The resisters that go are on the power Side of things . The resisters on the signal side see very little power . That resister on the jack is just a small ground for the guitar signal so it has some where to go . That value is good
     
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  8. william vogel

    william vogel Well-Known Member

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    I don’t think it’s possible to read it without lifting it. What is the amp doing? Is it really noisy?
     
  9. CROWEMAG

    CROWEMAG Active Member

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    No sir. Not noisy. It just doesn’t seem to have the gain I feel a stock 2204 should have so I was going through testing all the resistors to see if something was wildly out of range.
     
  10. CROWEMAG

    CROWEMAG Active Member

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    Thanks for the reply, man!
     
  11. william vogel

    william vogel Well-Known Member

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    Did you just get it or has the gain gone away? If it’s new to you, check to see if it’s got all 12ax7’s. You probably already know this.
     
  12. South Park

    South Park Well-Known Member

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    The 2204 has lots of gain in the front end but the tone stack cuts things down to Norman . a good overdrive pedal will put you in that high gain sound .
     
  13. CROWEMAG

    CROWEMAG Active Member

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    I’ve had this amp for a few years. It’s always been less gainy then others I’ve had. I really want to discover why!!! All 12AX7 tubes.
     
  14. william vogel

    william vogel Well-Known Member

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    I’ve only had experience with 4 hole Plexi amps, sorry. I’ve always wanted to give an 800 a try. They sound great.
     
  15. CROWEMAG

    CROWEMAG Active Member

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    If I were to want to troubleshoot why my 1986 2204 has less gain that other 800's I have had, where would I start? I have tried numerous preamp tubes in all positions, changing one by one. I have printed off blank voltages charts and plan to test all positions when I get home. I halfway hope there is a part or two that have drifted out of spec making the difference.

    I am getting 459vdc on pin 3 of each power tube and will report the V1-V3 readings later tonight. Any help would be greatly appreciated.
     
  16. william vogel

    william vogel Well-Known Member

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    I’m guessing the reduced gain is noticeably in the preamp vs power amp?
     
  17. CROWEMAG

    CROWEMAG Active Member

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    Correct
     
  18. william vogel

    william vogel Well-Known Member

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    What test equipment do you have available?
     
  19. myersbw

    myersbw Well-Known Member

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    Well, if you were to actually pull all the tubes out of the 2204 you'll see that you effectively "open up one end" of most of the resistors or at lease break open the circuit of some in series so you can get some accurate measurements on that board.

    From my experience, many differences end up being poor jacks, poor cables, or, basically, oxidation points that stifle the full signal from coming through. Especially when the amp gets to this age.

    Speaker sensitivities make a huge difference. Your guitar pickups do, too. What you haven't detailed is what factors have you held constant so you can prove your amp is weak? Did others bring over heads and plug into your same cab? Same cables? Was the same guitar used in every one you tested? All those things are important....more than what most give credit for.

    Also, the term "gain" is very subjective these days. Some allude to it being overall volume (in basic circuit classes, we liken gain to the voltage gain an electronic stage yields when the signal passes though...but, these days some folks refer to "gain" as the amount of clipping/distortion they get).

    Always make sure you do a good jack & pot cleaning if you find a reason to remove the chassis from the cabinet. And, be REALLY careful of overspray! DeOxit is a heavy viscosity. If you spray too much on a jack, it can accumulate as a droplet between jack contacts and actually make a short (or greatly reduced impedance) when a plug is inserted. I've seen that produce a very weak signal.

    Cheers!
    Brad
     
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  20. CROWEMAG

    CROWEMAG Active Member

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    Not much. Hahahaha. I have a nice fluke multimeter. I wish I had a scope.
     

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