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Can't quite figure out the function of the fourth terminal on a potentiometer.

Discussion in 'Troubleshooting and Repair' started by haskellbob, May 14, 2020.

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  1. haskellbob


    Apr 13, 2020
    The attached fragment of a circuit is for the preamplifier board of a Heathkit AR-15 receiver. The volume control potentiometer has a fourth terminal. I can see that it has something to do with the Loudness (bass-enhancement) control, which is either on or off. Something is bypassed when the control is in the "on" position, but I can't quite figure out the logic or the functioning of doing that.

    Could someone explain? Also, I was planning to replace the 25k double-gang potentiometer, but the replacement only has the usual three connecting points. Is there a way I can do this substitution - preferably retaining the "loudness" control, but, if necessary, doing away with it?

    Thank you for your help!


    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    This is seemingly a fixed tap to allow for loudness compensation.
    Assuming loudness is on, part of the input signal to the pot is injected via the R/C divider/filter C127/C125/R161 into the lower end of the pot. Thus this additional signal becomes relecant at low volumes only whereas at high volumes the mainly unchanged input signal is tapped by the wiper.
    With the loudness switch in the off position, this RC divider is short circuited and the pot output depends only on wiper position, not on frequency.
  3. Ylli


    Jun 19, 2018
    And no, without the tap you will not be able to retain the 'Loudness' function. Unless the pot is physically damaged, it is best to clean it up and use the old one.
  4. Bluejets


    Oct 5, 2014
  5. Ylli


    Jun 19, 2018
    The loudness tap is normally not a 'Center tap', it is more commonly between 1/4 and 1/3 of the pots rotation. And then the pot is usually logarithmic which complicates the problem. Again, save the old one if at all possible.
  6. bertus

    bertus Moderator

    Nov 8, 2019
  7. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Can you provide a link of the unit that you propose to use, for our inspection ?
    Considering that replacement dual unit is having the same 1/4 in shaft of the same length and type, plus using PCB twist tab mounting . . . and the pots are being audio taper units.

    BTW . . .you previously short changed us, on not filling us in on the solution of the one channels sound problem that you were experiencing trouble with .

    Last edited: May 16, 2020
  8. haskellbob


    Apr 13, 2020
    Wow am I late. I'm sorry. Would you believe that I have another problem? I don't have the one channel taking time to "warm up" anymore; now I'm wrestling with a volume control that won't go down to zero volume. I've been discouraged; electronic stuff is just a hobby and leads to more disappointment than victory. It feels like victory when I get something to work because the battlefield is so crowded with things I don't understand! So I'm trying to put my AR-15 aside and wait until I can get it to a technician. But I have the constant temptation to give it another try. I just have to ignore that, like an addiction! And maybe take a bunch of electronics classes. Too late though - I'm 65!

    I hadn't looked at this thread until just now. Again, sorry about that (in the words of Maxwell Smart)!
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