"strange" potentiometer sub

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

 

[Jamie]

frank@notvalid.it wrote:

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

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

 

[Jeff Liebermann]

On 17 Feb 2013 19:22:20 GMT, frank@notvalid.it wrote:

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.

I've fixed pits and burns on pots using conductive silver ink, also
known as PCB repair. The action of the pot will not be perfectly
smooth, but at least it will not be going open circuit over the pits.

If you want to do it perfectly, you can mix flake graphite (available
at the auto supply store as a lubricant) with glue to make something
more like a resistor. Be sure to get flake, not "powdered" graphite
as only flake is conductive.
<http://www.kobakant.at/DIY/?p=634>

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?

I haven't seen one of those since the 1960's.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Gareth Magennis]

wrote in message news:51212deb$0$1581$5fc30a8@news.tiscali.it...

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



I regularly come across 4 pin pots where the 4th pin just isn't connected to
anything. (Korg electric piano just last week as an example)

No idea what that's all about.



Gareth.

 

[Ian Field]

<frank@notvalid.it> wrote in message
news:51212deb$0$1581$5fc30a8@news.tiscali.it...

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

Those are not at all rare on Hi-Fi PAs - there should be component suppliers
around that specialise in parts for consumer goods.

The best I can suggest is search the biggest newsagent you can find for
trade publications, you may find addresses of suitable supply houses.

In the UK, we had Television Magazine & Technology_at_home - sadly, both
went down the gurgler some years ago.

 

[Dave M]

I don't know how criticak the end-to-end resistance value is, but I suggest
that you get a 100K stereo pot from Radio Shack (p/n 271-1732) and give it a
try. Many times, the overall value isn't critical so a 100K unit will work
as well as a 50K (or 47K). The RS part has a 40% tap, as you need.
BTW, these taps are called LOUDNESS taps, and are used to help low- and
mid-frequency response at low volume levels.

Dave M

frank@notvalid.it wrote:

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

 

frank@notvalid.it wrote:

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.

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.

Does anyone know how to source a suitable replacement?

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

 

[William Sommerwerck]

I don't know how critical the end-to-end resistance value is,
but I suggest that you get a 100K stereo pot from Radio Shack
(#271-1732) and give it a try.

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.

 

[gregz]

<frank@notvalid.it> wrote:

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

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

http://www.ebay.com/itm/1pc-Potentiometer-Log-A-50K-ALPS-Audio-Amp-Volume-Control-Pot-Stereo-W-Loudness-/140898232564?pt=LH_DefaultDomain_0&hash=item20ce3068f4

Greg

 

[Sjouke Burry]

frank@notvalid.it wrote in news:51212deb$0$1581$5fc30a8@news.tiscali.it:

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

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

 

[Leif Neland]

mroberds@att.net tastede fřlgende:

frank@notvalid.it wrote:
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.

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.

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



--
Husk křrelys bagpĺ, hvis din bilfabrikant har taget den idiotiske
beslutning at undlade det.

 

[N_Cook]

Sjouke Burry <s@b> wrote in message
news:XnsA16B31FF4BD83sjoukeburrysoesterbe@213.75.12.10...

frank@notvalid.it wrote in news:51212deb$0$1581$5fc30a8@news.tiscali.it:

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


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

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.

 

[Jon Elson]

frank@notvalid.it wrote:

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?
This is called a "loudness compensaton" pot, and it is usually connected to

an RC network to adjust the treble/bass balance as the volume is changed,
to compensate for some subtle effects in the human ear. All good
Hi-Fi gear used to have this type of pot.

Jon

 

[Sjouke Burry]

"N_Cook" <diverse@tcp.co.uk> wrote in news:kfsq0b$g4q$1@dont-email.me:

Sjouke Burry <s@b> wrote in message
news:XnsA16B31FF4BD83sjoukeburrysoesterbe@213.75.12.10...
frank@notvalid.it wrote in
news:51212deb$0$1581$5fc30a8@news.tiscali.it:

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


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

would it ?
cut

Yes.

 

[N_Cook]

Sjouke Burry <s@b> wrote in message
news:XnsA16C5CF930E6sjoukeburrysoesterbe@213.75.12.10...

"N_Cook" <diverse@tcp.co.uk> wrote in news:kfsq0b$g4q$1@dont-email.me:

Sjouke Burry <s@b> wrote in message
news:XnsA16B31FF4BD83sjoukeburrysoesterbe@213.75.12.10...
frank@notvalid.it wrote in
news:51212deb$0$1581$5fc30a8@news.tiscali.it:

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


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

would it ?
cut
Yes.

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. ?

 

[Sjouke Burry]

"N_Cook" <diverse@tcp.co.uk> wrote in news:kfvcs2$c60$1@dont-email.me:

Sjouke Burry <s@b> wrote in message
news:XnsA16C5CF930E6sjoukeburrysoesterbe@213.75.12.10...
"N_Cook" <diverse@tcp.co.uk> wrote in news:kfsq0b$g4q$1@dont-email.me:

Sjouke Burry <s@b> wrote in message
news:XnsA16B31FF4BD83sjoukeburrysoesterbe@213.75.12.10...
frank@notvalid.it wrote in
news:51212deb$0$1581$5fc30a8@news.tiscali.it:

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


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

would it ?
cut
Yes.

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. ?


Yep.

 

<"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 tap goes 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 curve ploted 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 user can add the compensation based on what he hears as well as his taste. Some are additive and others are subtractive. The type that uses the tap on the volume 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 of audio 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.

 

[josephkk]

On Sun, 17 Feb 2013 21:47:55 -0000, "Gareth Magennis"
<gareth.magennis@ntlworld.com> wrote:

wrote in message news:51212deb$0$1581$5fc30a8@news.tiscali.it...

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



I regularly come across 4 pin pots where the 4th pin just isn't connected to
anything. (Korg electric piano just last week as an example)

No idea what that's all about.



Gareth.

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.

?-)

 

[josephkk]

On Sun, 17 Feb 2013 16:11:44 -0800, "William Sommerwerck"
<grizzledgeezer@comcast.net> wrote:

I don't know how critical the end-to-end resistance value is,
but I suggest that you get a 100K stereo pot from Radio Shack
(#271-1732) and give it a try.

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.

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.

?-)

 

[josephkk]

On 18 Feb 2013 23:33:07 GMT, Sjouke Burry <s@b> wrote:

"N_Cook" <diverse@tcp.co.uk> wrote in news:kfsq0b$g4q$1@dont-email.me:

Sjouke Burry <s@b> wrote in message
news:XnsA16B31FF4BD83sjoukeburrysoesterbe@213.75.12.10...
frank@notvalid.it wrote in
news:51212deb$0$1581$5fc30a8@news.tiscali.it:

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


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

would it ?
cut
Yes.

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.

?-)