Potentiometer Replacement

4/24/2009 1:17:28 PMSteve Bento

I need to replace a 1 Meg linear Pot in my radio. But the problem is that it has a tap. Is there any way to get around this by using a fixed resistor to mimic the tap? I assume the tap is just a fixed resistance from either of the 2 ends of the pot. Anyone else have this problem? Thanks...

4/24/2009 8:08:57 PMMarv Nuce

Steve,
Check the web site for PTOP, think its "old radio parts" I've found several NOS tapped pots in his listing, and actually purchased 2-3 and used them. I did create a unique tapped pot with fixed resistor, and it was within a few % of the schematic value, but without the original to compare sound quality and my tin ear, I guess it worked.

marv

:I need to replace a 1 Meg linear Pot in my radio. But the problem is that it has a tap. Is there any way to get around this by using a fixed resistor to mimic the tap? I assume the tap is just a fixed resistance from either of the 2 ends of the pot. Anyone else have this problem? Thanks...

4/24/2009 8:56:00 PMDave A.

Steve -

Are you talking about multiple taps, because most pots have center taps. The tap is the moving part where the variable resistance is measured.
The 2 other connections are the ends of the resistor.

So, if it is a 1Mohm pot - you will measure 1Mohm across the 2 outer terminals all the time. The center tap will be increasing with reference to one side and decreasing WRT the other terminal when the pot is turned in a certain direction (clockwise vs. counter-clockwise). See example below (for reference only, the directions may be different). It will be the opposite effect when turned in the other direction. Maybe I am oversimplifying your question.

2
|
CW<- | ->CCW
1 ---/\/\/\/\/----3

Dave

:
::I need to replace a 1 Meg linear Pot in my radio. But the problem is that it has a tap. Is there any way to get around this by using a fixed resistor to mimic the tap? I assume the tap is just a fixed resistance from either of the 2 ends of the pot. Anyone else have this problem? Thanks...

4/25/2009 1:25:48 PMSteve Bento

Hi Dave,
This tap is addition to the beginning, end and variable tap. This tap seems to have a fixed resistance to either end. I think this was done for the same reasons that they used multi-capacitor packs, to save on money. I would think there would be a good way to expand that section to accommodate a couple of fixed resistors which would be equivalent to what I have there now...

Steve

:Steve -
:
:Are you talking about multiple taps, because most pots have center taps. The tap is the moving part where the variable resistance is measured.
:The 2 other connections are the ends of the resistor.
:
:So, if it is a 1Mohm pot - you will measure 1Mohm across the 2 outer terminals all the time. The center tap will be increasing with reference to one side and decreasing WRT the other terminal when the pot is turned in a certain direction (clockwise vs. counter-clockwise). See example below (for reference only, the directions may be different). It will be the opposite effect when turned in the other direction. Maybe I am oversimplifying your question.
:
: 2
: |
: CW<- | ->CCW
:1 ---/\/\/\/\/----3
:
:Dave
:
::
:::I need to replace a 1 Meg linear Pot in my radio. But the problem is that it has a tap. Is there any way to get around this by using a fixed resistor to mimic the tap? I assume the tap is just a fixed resistance from either of the 2 ends of the pot. Anyone else have this problem? Thanks...

4/25/2009 5:50:07 PMMarv Nuce

Steve,
Had nothing to do with saving resistors, but only with tayloring the volume control to closely approach the human ears perception to sound, primarily bass. Suposedly enhanced the bass response at low volume levels. Modern hardware uses a loudness control button for the same result. At least thats been my training. If this is a tone control, then all bets are off.

marv

:Hi Dave,
:This tap is addition to the beginning, end and variable tap. This tap seems to have a fixed resistance to either end. I think this was done for the same reasons that they used multi-capacitor packs, to save on money. I would think there would be a good way to expand that section to accommodate a couple of fixed resistors which would be equivalent to what I have there now...
:
:Steve
:
::Steve -
::
::Are you talking about multiple taps, because most pots have center taps. The tap is the moving part where the variable resistance is measured.
::The 2 other connections are the ends of the resistor.
::
::So, if it is a 1Mohm pot - you will measure 1Mohm across the 2 outer terminals all the time. The center tap will be increasing with reference to one side and decreasing WRT the other terminal when the pot is turned in a certain direction (clockwise vs. counter-clockwise). See example below (for reference only, the directions may be different). It will be the opposite effect when turned in the other direction. Maybe I am oversimplifying your question.
::
:: 2
:: |
:: CW<- | ->CCW
::1 ---/\/\/\/\/----3
::
::Dave
::
:::
::::I need to replace a 1 Meg linear Pot in my radio. But the problem is that it has a tap. Is there any way to get around this by using a fixed resistor to mimic the tap? I assume the tap is just a fixed resistance from either of the 2 ends of the pot. Anyone else have this problem? Thanks...

4/25/2009 6:47:37 PMLewis L

:Steve,
:Had nothing to do with saving resistors, but only with tayloring the volume control to closely approach the human ears perception to sound, primarily bass. Suposedly enhanced the bass response at low volume levels. Modern hardware uses a loudness control button for the same result. At least thats been my training. If this is a tone control, then all bets are off.
:
:marv
:
::Hi Dave,
::This tap is addition to the beginning, end and variable tap. This tap seems to have a fixed resistance to either end. I think this was done for the same reasons that they used multi-capacitor packs, to save on money. I would think there would be a good way to expand that section to accommodate a couple of fixed resistors which would be equivalent to what I have there now...
::
::Steve
::
:::Steve -
:::
:::Are you talking about multiple taps, because most pots have center taps. The tap is the moving part where the variable resistance is measured.
:::The 2 other connections are the ends of the resistor.
:::
:::So, if it is a 1Mohm pot - you will measure 1Mohm across the 2 outer terminals all the time. The center tap will be increasing with reference to one side and decreasing WRT the other terminal when the pot is turned in a certain direction (clockwise vs. counter-clockwise). See example below (for reference only, the directions may be different). It will be the opposite effect when turned in the other direction. Maybe I am oversimplifying your question.
:::
::: 2
::: |
::: CW<- | ->CCW
:::1 ---/\/\/\/\/----3
:::
:::Dave
:::
::::
:::::I need to replace a 1 Meg linear Pot in my radio. But the problem is that it has a tap. Is there any way to get around this by using a fixed resistor to mimic the tap? I assume the tap is just a fixed resistance from either of the 2 ends of the pot. Anyone else have this problem? Thanks...

If I knew the make and model of the radio, I could make a much better judgement.
Lewis

4/25/2009 7:27:45 PMDave A.

I see Marv, that makes sense. The extra resistance then may be getting used as an offset / voltage divider to the main pot. I found 40 and 50% "loudness tap" pots after searching the net.

Steve - If you just connect an ohmmeter btwn each of the different terminals as you adjust the pot, you will know what they are doing and be able to replace that functionality. If part of it remains fixed, then it could probably be replaced by a fixed resistor in series.

As Lewis noted, knowing the model number would help. Then, we should be able to check the schematic and see what the circuit looks like.

Dave

4/25/2009 7:34:00 PMDave A.

Also, when replacing note that volume pots should be of the linear type. Check this page - "pot types" section @ 1/2 way down. It also mentions (criticizes) the 'loudness' feature in the last paragraph:

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

4/25/2009 10:48:54 PMplanigan

:
:Also, when replacing note that volume pots should be of the linear type. Check this page - "pot types" section @ 1/2 way down. It also mentions (criticizes) the 'loudness' feature in the last paragraph:
:
:http://sound.westhost.com/pots.htm

Steve, that is most likely for tone control as mentioned and you can not add a fixed resister in parallel or series as the effect will vary the resistance of the over all circuit. You will have to find and old pot. of if your handy you can try what I did. Get new pot same resistance, open it up, find position on resistance path to get the center tap resistance, use conductive paint and bring out "lead to edge of pot,be carful not to get paint on resistance strip in path of wiper, hardest part was making terminal lug, I used #2 nut and bolt from hobby store, also file out a notch in cover so when you re-assemble pot it won't short out paint to cover. This was a PITA but so far its working. Ideally, if you could find those terminal pins that were used on early circuit boards it would be easy. Those pins were about 1/16" in dia and you would put thenm in hole and use a dies to "rivit" them into board. PL

4/26/2009 1:03:20 AMMarv Nuce

Steve,
Planagins suggestion has merit if you're handy with a Dremel, files, conductive epoxy etc.

marv

4/27/2009 2:15:34 PMEdd

I also would typically question your pots type, in respect to it being a LINEAR type, if being used in a
conventional volume control application towards the rear of the set in an audio amp grid section.

NEVER-NO-HOW-SOME-EVER . . .IF its being used in an RF gain type situation up in the FRONTAL section of
an old TRF receiver or even some first generation superhets, it could be a linear unit. . . .but taking
issue with its ohmic value even being anywhere near the high value of a I meg unit.

In that situation, the “volume” control pot was utilized in a combinational action, with it shunting across (shorting out)the sets RF input the first portion of its rotation, and then at some transitional point, it then kicked in and became a lower resistance, variable cathode control /resistor in a frontal RF tube(s) stage , thereby , I can see a
linear unit being used in that particular application, BUT with the resistance values typically being
waaaay down in a typical 5-10 K ohm value.

Come forth with the maker and model of your set and we can research and see what the true situation
is.

Quickee-Qiuckee Testee-Testee:

To analyze that pot which you have, out of circuit, or else with center terminal and one side terminal
disconnected from radio circuitry, place a larger knob on it, preferably with a pointer or add on a
wire to act as a “pseudo” pointer and then play with it and find its rotational capability to typically be
limited to ~270 degrees of rotation. . . .mechanical end to end

Use your pointer index referencing to then to find MID / CENTER rotation position of the pot.

Now, take ohmmeter in hand and measure across the units extreme end terminals to find the resistance being in the order of ~ your specified 1 meg unit . If your unit truly was a linear unit, expect ~ ½ of that 1 meg (500k)as your
ohmic reading when doing a re measuring from its center terminal to either side terminal.

Next, further that 135 degree positioning of the shaft in half and then expect the then position of ~67.5 degrees to be creating a new reading of ~250k ohms for one legs reading and ~750k ohms on the other leg.

In that testing manner, your readings would have now fully identified a linear pots resistive response.

NOW, should you have a LOGARITHMIC, or audio taper. . . .the common type used for a volume control. . . .
now let’s take a look at it:

On the log pot, the resistance change from max CCW position is so condensed, that it seems to take for-ev-ah to make a pronounced change of the resistance value of the portion that is deposited upon the pots resistance element in the portion between the max CCW rotor /wiper position and its mid
position.

Specifically. . . .a 1 meg pot would have only transitioned to half of its resistive value until the pot shaft
had been fully rotated ~ 90 percent of its rotational range.

Reference the supplied chart below and interpolate for different mechanical rotational positions of the
rotor positioning, versus the expected resistance value and something becomes readily apparent.

The idea being, for the potentially most used portion of the control, then to be able to give a slow
gradual adjustment of the sets volume, with a very precise and exacting degree of control.

Now, compare the insertion of a LINEAR type of control into that volume control application and see the
FAST transition of value within that audio “sweet spot ” area of adjustment. . . .expect it to have one
extreme “hair trigger” effect on your, now very limited, primary volume adjustment range.

REFERENCING. . . . .Linear Versus Log Pot. . . . . . Resistance Characteristics:

73's de Edd

4/27/2009 3:29:53 PMSteve Bento

Sorry for the lack of info - it's a Grunow 660. This is for the volume control and not the tone because the radio already has a separate tone control. Ed's right - I need to determine if this is indeed a linear taper or an audio taper. I assumed it was linear but could very well be wrong. But it sounds like planigan's idea would be worth trying. I am getting a fixed resistance from that tap of about 230k from the 0 ohm terminal.

4/27/2009 4:54:43 PMLewis L

:Sorry for the lack of info - it's a Grunow 660. This is for the volume control and not the tone because the radio already has a separate tone control. Ed's right - I need to determine if this is indeed a linear taper or an audio taper. I assumed it was linear but could very well be wrong. But it sounds like planigan's idea would be worth trying. I am getting a fixed resistance from that tap of about 230k from the 0 ohm terminal.

Now that I've seen the schematic, I can tell you that the tap is for loudness control, that is, the ear hears frequencies at different volume levels. You will need a log pot, and if there is not a tap, you probably will not ever notice the difference. But if you are like me, you will want a one meg pot with a loudness tap. Not an uncommon animal.
Lewis

4/27/2009 4:56:30 PMLewis L

::Sorry for the lack of info - it's a Grunow 660. This is for the volume control and not the tone because the radio already has a separate tone control. Ed's right - I need to determine if this is indeed a linear taper or an audio taper. I assumed it was linear but could very well be wrong. But it sounds like planigan's idea would be worth trying. I am getting a fixed resistance from that tap of about 230k from the 0 ohm terminal.
:
:
:Now that I've seen the schematic, I can tell you that the tap is for loudness control, that is, the ear hears frequencies at different volume levels. You will need a log pot, and if there is not a tap, you probably will not ever notice the difference. But if you are like me, you will want a one meg pot with a loudness tap. Not an uncommon animal.
Lewis

OOps, meant to say: the ear hears frequencies differently at different audio levels.

4/27/2009 6:05:42 PMplanigan

Steve, the Achille's heel to my method is the solder terminal, after you soldered lead to it in circuit make sure you didn't burn away the conductive paint. You may have to touch up. I used the automotive product for rear window defrostors. The printed circuit board stuff is most likely a better product but you need so little and I believe that stuff is expensive. PL