Original Univibe wiring question

Discussion in 'Effects, Pedals, Strings & Things' started by StratLou, Oct 4, 2008.

  1. StratLou

    StratLou Member

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    Hi Guys,

    Could anyone tell me if a Univibe would ever have the blue and yellow wires that are connected to their individual terminals on the foot switch socket in a vibe, connected together and if so for what reason. I know of one where these 2 terminals are connected. Could it cause damage to the unit if they are connected and shouldn't be and does anyone know what the functions of the 2 wires are? Thanks very much if anyone can help.

    Lou
     
  2. amp_surgeon

    amp_surgeon Member

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    Hi Lou.

    The yellow wire normally goes to pin 1 on the pedal jack, and is connected to the wipers on both of the pots in the pedal. The blue wire goes to pin 4 on the pedal jack, and is connected to the end lug on one of the two pots in the pedal. Shorting them together would remove one of the two pots from the circuit, and allow you to get some degree of speed control using only a single pot between pin 1 (the yellow wire) and pin 2 (the white wire), though the frequency range might not be like the original.

    It won't hurt anything to have these two wires connected together, but a standard Uni-Vibe pedal wouldn't work properly with the unit anymore. Somebody probably did this mod because they either didn't have an original pedal, or the dual-gang pot in their pedal broke, and they wanted a single pot solution.
     
  3. StratLou

    StratLou Member

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    How can you check if the the dual gang pots are both functional and what would the original ohm values of each have been originally. Linear or audiolog? Thanks.
     
  4. amp_surgeon

    amp_surgeon Member

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    For one of the pots, measure the resistance between pin 1 and pin 4 of the plug. For the other, measure between pin 1 and pin 2. The resistance should change as you move the pedal. The two pots are mounted on the same shaft (dual gang), and are 100K log taper.

    Note that modern audio taper pots are NOT the same as old log taper pots. On the old log taper pots the taper would gradually change as you turned the pot. Modern audio taper pots are really two different sections of linear taper. The resistance goes from 0% to about 10% in the first half of the rotation, and then from 10% to 100% in the second half.

    EDIT: I had mentioned about using reverse audio taper pots, but this is only necessary if you're making a panel mount speed control.
     
  5. StratLou

    StratLou Member

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    Is the linear pot one which the value changes evenly throughout the sweep range while the audiolog pot is one, such as you mentioned, which has the resistance go from 0% to about 10% in the first half of the rotation, and then from 10% to 100% in the second half? But the overall range would be the same for old and modern? Thanks again.
     
  6. StratLou

    StratLou Member

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    Could anyone comment on whether the original Univibe pedal pots were linear or audiolog. Amp Surgeon mentioned that the old log pots in the original would change gradually (and evenly?) throughout the range and the modern has a 2-stage type curve (gradual for the first half of the rotation, increasing markedly to the end of the rotation). Wouldn't that make the originals linear and the modern log types? Thanks for any help.
     
  7. TAVD

    TAVD Guitar Player Gold Supporting Member

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    IIRC, it's reverse log taper. Try Small Bear for a replacement.
     
  8. amp_surgeon

    amp_surgeon Member

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    It's a normal log taper, but it's mounted in the pedal so that it turns in the proper direction (i.e., backwards) as you press down on the toe of the pedal. You need a reverse log taper if you're going to make a panel mounted speed control, and you want the speed to increase as you turn the dial clockwise.

    Lou, an audio taper and a log taper are similar. The real difference is that the log taper gradually increases the slope, while the modern audio taper has two slopes; shallow in the first half of the rotation, and steep in the second.

    Linear has only one slope which progresses from zero to the maximum resistance in direct proportion to the rotary position of the pot. If you plotted the resistance versus the rotation, it would be a straight line.
     
  9. StratLou

    StratLou Member

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    Is there a link on the web to a site that would show a graph plotting the curve of these 2 different tapers? That would be really helpful. I think I've found and seen a graph of the audio taper curve (not exactly sure now), but not the log taper curve. Thanks for any info on that.

    Lou
     
  10. amp_surgeon

    amp_surgeon Member

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    There's a nice introduction to pots, including a comparative chart of the different types of tapers, here:

    http://sound.westhost.com/pots.htm

    [​IMG]

    He explains in the article why manufacturers switched from making true log taper pots for audio volume (and other applications where a log taper was needed) to using a stepped taper, or what he refers to as "commercial log taper". Most pots called "audio taper" today are of the stepped variety.
     
  11. StratLou

    StratLou Member

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    I measured the pot and it seems to have a log type curve, I think (like the yellow curve in the graph). How can I tell with my measuring if I'm looking at an audio taper or a log taper pot, therefore? How can I tell if I have a commercial log curve? Is it that drastic in the stepped style that I will see a very unsmooth curve like the red line? Or is it just too close to tell the difference between the two. Would an oscilloscope show the pot curve any better? Thanks very much.
     
  12. amp_surgeon

    amp_surgeon Member

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    Well, the absolutely "sure" way is to rig up the pot with a way to accurately measure the angle of rotation, and then measure the resistance at specific angular intervals, and then plot the results.

    Of course, that's a whole lotta work to do just to determine what kind of pot you've got.

    An easier method is to hook up an ohmmeter between the center lug (the "wiper") and the counter clockwise lug. The counterclockwise lug is the one that will read the lowest resistance to the center lug when the pot is turned fully counter clockwise (looking down the shaft of the pot). Now, turn the pot until it's as close to dead center as you can get it. What's the resistance?

    If it's 10% of the full resitance, then it's a stepped taper.
    If it's between 15% and 20% then it's a log taper.
    If it's 50% then it's a linear taper.
    If it's over 80% then it's an anti-log taper.

    That's it! There's enough of a difference in the resistance at the center position that you can tell pretty quickly without having to actually plot the resistance, or try to view it on a meter or scope.
     
  13. StratLou

    StratLou Member

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    Just to be clear, which 2 terminals do I connect the ohmmeter to to check the resistance curve info? I connected terminals 1 & 2 in the picture below. Was this correct? Thanks.

    [​IMG]
     
  14. StratLou

    StratLou Member

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    Well I have determined that my pedal has a reverse log taper pot!
    1) Why would it have this?

    2) How would it differ in the way the pedal functions vs. having a log taper pot?

    3) Can the rest of the missing info be added to the above regarding which wires go to which pins of the DIN cord from the dual gang pot and also how the pots are wired? Many, many thanks if you can help supply that.

    Lou
     

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