"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

Reply to
frank
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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

Reply to
Jamie

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.

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

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

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.

Reply to
Gareth Magennis

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.

Reply to
Ian Field

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
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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.

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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:

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This place (California, USA) has a couple of Noble pots with loudness taps - only 10K and 200K though, and $25 (!!). Page 14 of

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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

Reply to
mroberds

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

snipped-for-privacy@notvalid.it wrote:

Reply to
Dave M

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.

Reply to
William Sommerwerck

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

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Greg

Reply to
gregz

snipped-for-privacy@notvalid.it wrote in news:51212deb$0$1581$ snipped-for-privacy@news.tiscali.it:

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

Reply to
Sjouke Burry

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

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Reply to
Leif Neland

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.

Reply to
N_Cook

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

Reply to
Jon Elson

"N_Cook" wrote in news:kfsq0b$g4q$ snipped-for-privacy@dont-email.me:

cut Yes.

Reply to
Sjouke Burry

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. ?

Reply to
N_Cook

"N_Cook" wrote in news:kfvcs2$c60$ snipped-for-privacy@dont-email.me:

and

with

It's

Does

Yep.

Reply to
Sjouke Burry

see

connected to

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.

?-)

Reply to
josephkk

top

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.

?-)

Reply to
josephkk

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.

?-)

Reply to
josephkk

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.

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.

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.

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.

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.

well.

tap before. I have seen amps with two taps, which meant two level of equalization going through the range of the control.

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

?-)

Reply to
josephkk

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