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"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 |
"strange" potentiometer sub
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"strange" potentiometer sub
wrote in message ... 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. |
"strange" potentiometer sub
wrote in message ... 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. |
"strange" potentiometer sub
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/...-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/in...ductId=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 |
"strange" potentiometer sub
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 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 |
"strange" potentiometer sub
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. |
"strange" potentiometer sub
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-Potentio...em20ce30 68f4 Greg |
"strange" potentiometer sub
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"strange" potentiometer sub
tastede følgende:
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/...-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 kørelys bagpå, hvis din bilfabrikant har taget den idiotiske beslutning at undlade det. |
"strange" potentiometer sub
Sjouke Burry s@b wrote in message
2.10... wrote in : 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. |
"strange" potentiometer sub
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"strange" potentiometer sub
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"strange" potentiometer sub
Sjouke Burry s@b wrote in message
.10... "N_Cook" wrote in : Sjouke Burry s@b wrote in message 2.10... wrote in : 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. ? |
"strange" potentiometer sub
"N_Cook" wrote in :
Sjouke Burry s@b wrote in message .10... "N_Cook" wrote in : Sjouke Burry s@b wrote in message 2.10... wrote in : 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. |
"strange" potentiometer sub
"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. |
"strange" potentiometer sub
On Sun, 17 Feb 2013 21:47:55 -0000, "Gareth Magennis"
wrote: wrote in message ... 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. ?-) |
"strange" potentiometer sub
On Sun, 17 Feb 2013 16:11:44 -0800, "William Sommerwerck"
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. ?-) |
"strange" potentiometer sub
On 18 Feb 2013 23:33:07 GMT, Sjouke Burry s@b wrote:
"N_Cook" wrote in : Sjouke Burry s@b wrote in message 2.10... wrote in : 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. ?-) |
"strange" potentiometer sub
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