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Question about replacing a bias setting variable resistor.

Discussion in 'Solid State' started by saabracer23, Jan 12, 2018.

  1. saabracer23

    saabracer23 AK Subscriber Subscriber

    Messages:
    1,045
    Location:
    Salem, Oregon
    Hello all,
    So I have an amplifier board from an HK A402 sitting in front of me and I’m wanting to replace these old carbon wipers with some Bourne 25 turners. The originals in the board only uses two leads and the third/middle hole is just soldered over.

    The question is, can I just use all three of the leads on the Bournes? As you can see on the back of the board, two of the leads share a trace just as any other amp if replaced these on.

    On a side note, I need to moisturize my hands more.

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    9A58AAA0-9836-4643-AE48-4C5CC41F2335.jpeg
    8DE901E2-2C6D-4E14-BD6F-3282F8EC5E0C.jpeg

    Dan
     

     

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  2. whoaru99

    whoaru99 Epic Member

    Messages:
    40,927
    Location:
    LoTL
    In many cases of using a potentiometer to replace a variable resistor it's fine, or possibly even beneficial, to short one end to the wiper. This provides a complete circuit of the full pot resistance rather than open circuit if the wiper loses contact with the track.

    However, if you want to put it back just as it was you'd snip one of the end legs and use the other end leg and the wiper leg. Putting it back the way it was is a safe bet.

    Which end leg you snip, if you go that way, would depend on which way you insert it relative to which way you'd like to turn it for increase/decrease of bias.
     
    Last edited: Jan 12, 2018
  3. Tom B

    Tom B AK Subscriber Subscriber

    Messages:
    1,494
    Location:
    Bay Ridge, Brooklyn New York
    I like the idea of replacing if you must as a 3 leg pot, rather than 2 leg VR. But here is my 2 cents. I only replace bias or DC balance pots/VR's when needed. When needed is when its almost impossible to get the right setting, or excessive drifting when tpping on the pot/VR.
    As much as I like the 25 turn feature of the bourns trimpots, they are only rated at 1/4 watt to 1/2 watt (The ones you have appear to be 1/4) The ones you will be replacing have much heavier pins, and the copper wiper is much heavier than the bourns trimpot. While you may not need more than 1/4 watt VR, I like the heavier originals. If you ever opened up a bournes pot, you will see how much smaller they are than original vr's. If I can leave the heavier ones in, I do. If you use a bournes and the circuit requires more than 1/4w the trimpot could heat up and open. "Opening" a bias pot/VR usually results in thermal runaway and fried outputs.
    If I do replace originals, I use the trimpot 3296W type, which are rated at 1/2 w
    Has anybody ever seen issues relating to bournes 1/4 watt trimpots?
     
  4. saabracer23

    saabracer23 AK Subscriber Subscriber

    Messages:
    1,045
    Location:
    Salem, Oregon
    Thanks guys, so one vote using 2 pins and other for all three, but would prefer to keep originals in place. The ones I have are are the 3296W series.

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    Dan
     
  5. Superampman

    Superampman AK Member

    Messages:
    3,886
    Location:
    Kitchener, ON.
    In any case, they should be oriented to increase resistance in the same direction as the original spec.
     
  6. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

    Messages:
    1,771
    Location:
    Victoria, Australia
    I have used the 25 turn trim pots in amps where there was no trim pot originally fitted (so replacing a fixed resistor) and l have in each case tied the centre pin (wiper) to one of the outer legs. As whoaru99 said this will provide full resistance in the case of a fault instead of an open circuit.
     

     

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  7. saabracer23

    saabracer23 AK Subscriber Subscriber

    Messages:
    1,045
    Location:
    Salem, Oregon
    Great, thank you. I’ll tie it in.

    Dan.
     
    Oldsansui441 likes this.

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