Need help buliding an individual voltage pot board Fisher 400

Discussion in 'Fisher' started by daveyh, Aug 9, 2017.

  1. daveyh

    daveyh AK Subscriber Subscriber

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    Can someone guide me through the building of a board to individually adjust the the voltage on my 7868 tubes. I have seen this done before but not sure hows its done. I have recapped the 400 but still need to adjust the voltage on the tubes. I would like to do this individually. One pot for each output tube. I would buy some new matched ones but they are expensive. I have already installed some cathode to ground 1/4 w 10 ohm resistors on the outputs.
     
  2. Patrice B

    Patrice B AK Subscriber Subscriber

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    Search this forum with the term IBAM...
     
  3. daveyh

    daveyh AK Subscriber Subscriber

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    It gives some information but now how it all connects. I will keeps reading and searching. Thanks
     
  4. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Daveyh. Also install the 100ohm 1/4w screen stability resistors on the 7868's. This is also noted in the COMMON PARTS FOR FISHER's STICKY.


    http://audiokarma.org/forums/index.php?threads/most-common-parts-needed-for-fishers.327561/page-2 Page 2 of the COMMON PARTS FOR FISHER STICKY

    Post 39 of the above thread. Schematic of the IBAM, and 2 installation pics of a 400 and 800c.

    http://audiokarma.org/forums/index....needed-for-fishers.327561/page-2#post-6364459

    6 wires.
    1.)Hot goes to BIAS POWER off DUAL electrolytic cap
    2.) Ground goes to Chassis ground as close to board as possible.

    3-4-5-6.) 4 leads from the board go to each of the 220K resistors (4 leads/4 resistors).

    Mount it on standoff's close to the output section. Replace the single terminal strips with 3 terminal (2 isolated, 1 ground lug) strips. Tie each of the 220K resistors to a isolated terminal and bring one of the IBAM LEADS to each of the resistors.
     
  5. The Fuxtor

    The Fuxtor AK Subscriber Subscriber

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  6. daveyh

    daveyh AK Subscriber Subscriber

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    Thanks for the links and the post Larry. That helps a lot!
     
  7. daveyh

    daveyh AK Subscriber Subscriber

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  8. daveyh

    daveyh AK Subscriber Subscriber

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    IBAM. So that's what it's called.

    Thanks Patrice B!
     
  9. daveyh

    daveyh AK Subscriber Subscriber

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    I may get this thing built out after all. Been wanting to build one for quite sometime.

    I will keep you posted of my final outcome.

    :)
     
  10. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    Individual
    Bias
    Adjustment
    Mod

    And Dave's IBBA board is
    Individual
    Bias
    Balance
    Adjustment
     
  11. daveyh

    daveyh AK Subscriber Subscriber

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    Larry: I got the board built. Now I don't see 220Ω resistors unless I am looking in the wrong place. On this unit I see 10kΩ tied to number 2 pin. Am I looking in the wrong place? They are only two 220Ω resistors coming off of pin 4 to ground.
     

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  12. daveyh

    daveyh AK Subscriber Subscriber

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    Correct me if I am wrong but I think what I was looking for is 220Ω where there are 330Ω original resistors. The picture I was looking at had 220Ω resistors but I do not think there were original. I think someone may have changed them to run 7868EH tubes. So, if I want to use 7868EH output tubes I should replace the 330Ω resistors with 220Ω and then solder my IBAM wires to them.
     
  13. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    You got the numbers almost right but you're on the right track.
    1.) Remove the 330K ohm resistors (one at time) and replace with 220K ohm resistors. Leave the one end (that's being used as a common tie point) up in the air for now.
    2.) Replace the single terminal strip with a dual terminal strip. The white wire attached to them comes out and gets coiled up next to the DUAL BIAS CAP FOR NOW. This will end up as your HOT WIRE for the IBAM BOARD, or remove it and solder in your IBAM HOT (POS +) lead to that point. You'll have to loosen one side of the transformers and undo completely the other to get 4-40 x 3/8" screws in the old rivet holes to attach the new terminal strip.
    3.) With TWO NEW Terminal strips installed, place the free end of each of the 220K ohm resistors into a separate terminal and run the IBAM LEADS for each tube up to the terminal strip. Attach the IBAM LEADS to the 220k ohm resistors (one per terminal).
    4.) Tighten everything up (transformers), and make all solder connections.

    You're done. Adjust as needed. remember each adjustment on one tube is somewhat interactive on the other tubes. So keep at it until they settle down. Should be about 3 series of adjustments.

    When you change to the 220K resistors, also change the coupling caps to .068uf. That's the ideal value to keep the timing constant of the R/C network. But if you already have .1uf caps in you'll be ok. The changes aren't audible. This upgrade is also warranted for old tubes as the 330K value is too high even for them.
     

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