Problem adjusting IP pin 1 after "noose" removed

Discussion in 'Fisher' started by thornev, Apr 8, 2018.

  1. thornev

    thornev Active Member

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    I have a problem. With the "noose" resistor removed pin 1 of the IP tubes' pin 1 can only adjust to 60.7 while pin 3 (static) is at 68.7. What do I do to give more headroom to pin 1 or ?? According to the "remove the noose" instructions I have to make pin 1 and pin 3 resistance equal.
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    See my post on the 800c rolling tubes thread.
     
  3. thornev

    thornev Active Member

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    I don't know if there's anyway to predict electronically if one can determine that the IP adjustment can go as high as the pin 3 measurement after removing the noose, but there should be a statement in the "remove the noose" thread that it's a possibility. I cannot reinsert the resistors because I cut them.

    UPDATE: I manager to solder them back in. Ugly soldering job though.
     
    Last edited: Apr 8, 2018
  4. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I seem to remember on one of my 400's that the static went way up into the low 80's after pulling the "noose". Had no problem adjusting the dynamic side up to match it. The pots are supposed to be 25K (end to end). Check the ohms end to end and see what you get (you may have to disconnect it and test out of circuit.).You may have a poorly performing pot. DeOxit 1st and cycle it 30-40 times then hit it with Faderlube and exercise again. This may help. or Maybe a 50K might give you the wiggle room you need.
     
  5. thornev

    thornev Active Member

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    My pots go from 35.6 to 60.7. I'll try the Deoxit. Thanks, Larry. What do you mean a 50K? In place of the noose?

    UPDATE: Faderlube didn't help. I already have the 25K range. Sigh. Just ordered 4 180K resistors. Can use them in inventory anyway.
     
    Last edited: Apr 8, 2018
  6. ncwalz

    ncwalz Member

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  7. thornev

    thornev Active Member

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    Thanks nc ! If I'm interpreting post 10 in that thread correctly ("However, it you want to make the control operate nearly at dead center when balanced, then raise the value of the 39K plate load resistor for the phase inverter section to 43K, same wattage."), upgrading the plate resistor a few K should increase the range of the IP pot. I have some resistors in inventory. Hopefully I have something with which I can experiment. I think I'm talking about R116 and R114 which are 68K and are grounded on one end.
     
    Last edited: Apr 8, 2018
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I just added a note for the 500c in POST #1 on the Phase inverter thread. It references ncwalz thread.
     
  9. thornev

    thornev Active Member

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    In that same thread Dave said: "...and then you would change out the existing 68K plate load resistor for the inverter to 82K, same wattage." My plate load resistor (tied to ground) is 68K. So I guess 82K applies to my situation.
     

     

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  10. RS Steve

    RS Steve Tube Junkie Subscriber

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    I had to solder my resistors back in my 500B, wasn't enough range either.
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Yeah 82K.
     
  12. thornev

    thornev Active Member

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    What would the wattage be of that 82K resistor? 1, 2 or 3W?
     
  13. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Same as before 1/2W.
     
  14. thornev

    thornev Active Member

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    Replacing the V12 pin 3 68K resistor with an 82K turned out to make the problem worse. Now the noose-removed pin 3 measures 80K (it was 68.7K with the 68K), but the pin 1 adjustment still only goes to 60K at max. Did I do something wrong? If I follow this observation with my logic, wouldn't I need a LOWER resistance than 68K to bring the static pin 3 measurement below the 60K max on V12 pin 1? Or is there something I can do to raise the adjustable pin 1 max to a value above the pin 3 static max with the noose removed?
     
    Last edited: Apr 13, 2018 at 6:55 AM
  15. ncwalz

    ncwalz Member

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    Sorry; the point of my post was that if the noose was removed, the value of the resistor might have to be changed to allow the pot to be adjusted so that the desired value would be about in the middle of the pot’s range. What the necessary resistor value required might vary with a particular model.
     
    Last edited: Apr 13, 2018 at 11:29 PM
  16. thornev

    thornev Active Member

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    247
    Location:
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    No problem. I plan to experiment tomorrow with different resistors to see what brings the static pin 3 measurement below 60K. It seems that the pin 3 noose-removed measurement is always about the same value as the grounded pin 3 resistor. So if I want something under 60K, I need to go with a resistor that's under 60K.
     

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