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"strange" potentiometer sub

Discussion in 'Electronic Repair' started by [email protected], Feb 17, 2013.

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  1. Guest

    Hi all,
    I'm repairing an old italian stereo amplifier and the volume dual
    potentiometer is scratchy beyond any hope. I tried cleaning it and then
    also dismantled it but the carbon part is too much ruined with several
    pits. Also the two channels doesn't really track anymore, having large
    differences in resistances in almost all positions.
    The problem is that these potentiometers (47Kohm) have an extra fourth contact
    which is at fixed position at 10K from one end. It's the first time I see
    such a potentiometer with fixed divider. Does anyone know how to source
    a suitable replacement?
    Thanks in advance and best regards

    Frank IZ8DWF
     
  2. Jamie

    Jamie Guest

    I ran into a problem like that once, only the one I had, the forth lug
    was in the middle of the tap.

    In any case, it's a custom pot and you most likely won't find one new
    any where. On top of that, the pot is most likely a log type.

    You can do as I did and it seem to work. Get the pots you need that
    will fit, remove the tracks and drill a near by hole at the edge of the
    track that fits the value you need. bring in a small wire to be inserted
    into this hole. Use conductive cement, something like used in defroster
    heater element repairs or circuit board repairs to bond and connect the
    edge of the carbon track to this wire point.

    When I did it, I used wire wrap conductor because it was small and
    easy to work with.

    Good luck.

    Jamie
     
  3. Guest

    I'm pretty sure this is called a "loudness tap", at least in English.
    It connects to some extra circuitry in the amplifier. I have seen a
    1960s US stereo amplifier with the usual volume knob plus an on-off
    switch labeled "loudness", which I think was connected to the loudness
    tap. I remember it changed the sound, but I don't remember if it just
    got quieter (like turning the volume down), or if it actually changed
    the tone (like a high-pass or low-pass filter). This post
    http://archive.ampage.org/threads/1/gagd/005162/Whats_a_tapped_pot_-1.html#005183
    says it actually changed the sound.
    If you Google "loudness tap" there are some posts on this same problem
    (people trying to repair old amplifiers).

    The US retailer Radio Shack even has one for $4 (100K ohm, 40% tap) but
    I don't know if they will ship internationally.
    http://www.radioshack.com/product/index.jsp?productId=2062359

    This place (in China) has a listing for ALPS pots with loudness taps,
    but I don't know how easy it is to order one:
    http://www.thlaudio.com/alpsmne.htm

    This place (California, USA) has a couple of Noble pots with loudness
    taps - only 10K and 200K though, and $25 (!!). Page 14 of
    http://www.percyaudio.com/Catalog.pdf

    You can probably do something like this to make the amplifier work -
    it won't have exactly the same sound as the original, but at least
    you can listen to it while you wait for the "correct" part to arrive.
    You might try slightly smaller values (4.7K?) for the 10K fixed
    resistor as well.

    o
    |
    V
    o---/\/\/\---o---/\/\/\---o
    10K 47K

    Matt Roberds
     
  4. I don't know how critical the end-to-end resistance value is,
    Though a 100K pot will almost certainly work electrically, its source
    impedance is twice as a high as a 50K pot. This, combined with the stray
    capacitance of the wiring following the pot, might audibly roll off the top
    end.
     
  5. gregz

    gregz Guest

    You will have to compare mounts and sizes. Others on eBay.

    http://www.ebay.com/itm/1pc-Potenti...564?pt=LH_DefaultDomain_0&hash=item20ce3068f4

    Greg
     
  6. Sjouke Burry

    Sjouke Burry Guest

    Use an ordinairy stereopot of ~100k, and use 2 resistors, 80K and 10k,
    in parallel to simulate the tap. That should do it.
     
  7. Leif Neland

    Leif Neland Guest

    tastede følgende:
    "Loudness" compensates for the ears lower sensitivity for bass and
    treble at low volume, by boosting these frequencies.
     
  8. N_Cook

    N_Cook Guest

    would it ?

    I would go for breaking in and some sliver loaded paint to a thin wire
    through a sub-mm hole drilled through the paxolin and glued, assuming no off
    the shelf pots available (most likely unless you want to buy 100 from HK) -
    on top of the usual pot subbing problem of wrong physical size, pin spacing,
    shaft diameter and cross-section so wrong knob mound etc.
     
  9. Sjouke Burry

    Sjouke Burry Guest

    cut
    Yes.
     
  10. N_Cook

    N_Cook Guest

    Do you mean parallel but the wiper not tied to the join of the Rs, the
    bass-compensation trace only taken to the 20K/80K join. ?
     
  11. Sjouke Burry

    Sjouke Burry Guest

    Yep.
     
  12. Guest

    <"Use an ordinairy stereopot of ~100k, and use 2 resistors, 80K and 10k,
    in parallel to simulate the tap. That should do it. "

    It absolutely will not.

    That tap is for the wiper to come in between it and the bottom and recieve a modified frequency response curve at the lower end of its range. This tapgoes to a resistor to ground usually, and then when the "loudness compensation" is engaged, it provides added low frewquency response, and in better amps, also far high end frequency response based on the human hearing curveploted in detail and called the "Fletcher - Munson" curve.

    With normal human hearing, lows and highs are attentuated more at lower levels, and this curve is supposed to compensate. Many people do not use the "loudness" "feature" because it is oft miscalibrated and results in too boomy a sound.

    High end (pre)amplifiers often have a separate loudness control so the usercan add the compensation based on what he hears as well as his taste. Someare additive and others are subtractive. The type that uses the tap on thevolume pot is almost always additive.

    The tap always goes to a resistor to ground, this gives the pot a slower curve anyway, but when the loudness contour is engaged a capacitor is in series between the resistor and ground providing a bass boost, generally first order, shelving at about 60 Hz but boosting all the way up to about 350 Hz.More refined circuits also use a small capacitance to the top leg of the pot (the fed end) and provide a high treble boost as well when the switch is"on". All this frequency compensation is shorted out when the "loudness" or "conour" is switched off. The amp is then supposed to provide flat frequency response to the best of its abilities.

    There may not be a loudnaes or conour switch in this unit. If not, it cannot be considered a high fidelity unit, but that is true of a hell of alot ofaudio equipment. High fidelity does not mean that it sounds better, it means that it sounds right. Everyone prefers a certain frequency content in their music, and only purists seek that original, uncolored sound.

    If you look up Fletcher Munson on wiki, I bet it can splain more pretty well.

    Actually I ifnd it surprising that osmeone has never seen this kind of tap before. I have seen amps with two taps, which meant two level of equalization going through the range of the control.
     
  13. josephkk

    josephkk Guest

    Bloody kool. So are you seeing them at 47k or 50k with a tap at 10k?
    Duals?

    Long ago, before there were lots of tone controls there used to be a
    compensation tap to somewhat depress the midrange to look more like the
    measured perceived loudness curves versus volume level.

    ?-)
     
  14. josephkk

    josephkk Guest

    That won't be nearly as much trouble as having to replace the caps and
    resistors for the tap. The corner frequencies would be off by nearly 2:1.

    ?-)
     
  15. josephkk

    josephkk Guest

    Rebuttal, NO. There is other circuitry connected to the 4th pin whose
    values are referenced to the 47k/50k whole length and the 10k tap point.

    ?-)
     
  16. josephkk

    josephkk Guest

    I have heard of pots with two taps before but never seen one.

    ?-)
     
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