Scratchy prescence/resonance mod

Discussion in 'The Workbench' started by JM5010, Sep 24, 2019.

  1. JM5010

    JM5010 Member

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    Just as I was getting ready to list this on Reverb, i remembered the nasty scratchy knobs on power section of this amp.

    I converted the PI to a long tailed pair, copied the JCM 800 2203 PI circuit entirely.

    Anyone know what could be causing this?
     
  2. South Park

    South Park Well-Known Member

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    Check for cold solder joints.
     
  3. JM5010

    JM5010 Member

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  4. JM5010

    JM5010 Member

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  5. South Park

    South Park Well-Known Member

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    Replace the pot
     
  6. myersbw

    myersbw Well-Known Member

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    Don't replace the pot. Here's my immediate thought. You say you've converted the PI circuit "entirely"...that would include changes in the Presence circuit, too. From the sound of it, I believe you've introduced a DC component on the pots. In the 2203 circuit, you have a very small cap that DC-isolates the Presence control. In the original circuit, C23 is hanging off that PI cathode. You might think you have DC isolation...but...the question is...what did you do with C23? (Reason I ask is...that large of a cap will charge up significantly in comparison to the very small 0.1uF cap in the 2203 circuit.

    To answer the first issue, measure for DC voltage at P26 on that board or on either end of the pots. If you have DC...you have your answer as to why you're getting noise over the full travel of the pot.

    Now, let's say you are measuring DC voltage. If so, lift R58 and insert a 0.1 to 1uF cap and see if the noise problem is resolved. If the controls are not behaving like you expect...we need a lot more info on what was changed, photos...ideally a marked up schematic showing exactly what changes you made, etc.

    But, I do believe you now have DC voltage on those pots. Replicating the 2203 also means removing (or altering) for the Texture circuit...and changing up the Presence control and the Resonance as, if you intend to have a 2203 PI, those will factor in and changes will need made so they behave as expected.
     
    Last edited: Sep 25, 2019
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    There is DC voltage on the pot.
    This is what causes the scratching sound.
    You have 2 possibilities:
    1. The circuit you modified is wired wrong.
    2. The coupling cap you used has DC leakage.

    2203PI.png
     
    Last edited: Sep 26, 2019
    JM5010 likes this.
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    It's not a solder problem...
    although if there was a bad solder connection, you usually can't tell by looking at it.
     
  9. myersbw

    myersbw Well-Known Member

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    Nah, definitely sounds like DC on the pot. Just curious if that 25uF cap is still in the circuit.
     
  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    So how about some good pictures of your modifications/work?
     
    Last edited: Sep 29, 2019
  11. JM5010

    JM5010 Member

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    I will upload pics soon.
    I have removed the texture circuit. It doesn’t function anymore which was what i wanted. I’ll mark up a schematic in a bit if putting a cap in place of R58 doesn’t do anything.

    That being said, should i also remove the resonance pot + cap? Could that be the issue?

    Also, should the prescence pot have two caps on it? one on the pot and one for the DCV?
     
  12. JM5010

    JM5010 Member

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    Here’s the prescence pot. I put a 4.7k resistor “across it”.
     

    Attached Files:

  13. JM5010

    JM5010 Member

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    OK i gave up with the peavey layout and ended up hand wiring a prescence knob on the back of the amp. It’s fine now, doesn’t make one scratchy noise at all, Thanks!
     
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  14. myersbw

    myersbw Well-Known Member

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    Ah! And, I'll bet you have a small value cap on that top side to isolate the pot, correct? Get rid of that DC voltage and it's amazing how it all jives, eh? :) Congrats!
     
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  15. NewReligion

    NewReligion Well-Known Member VIP Member

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    I arrived late.

    Yes congratulations. I wanted immediately to state wire the Presence like a Plexi but read through the entire thread first. It is indeed amazing how proper DCV & even Foil orientation can affect the DC circuit in addition to abandoning coplanar layouts etc...

    David...♫
     
  16. JM5010

    JM5010 Member

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    Hell yeah man. This thing is a beast now...i might add a choke...is it worth it?
     

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