Potentiometer noise tester

I am thinking of making a simple tester for potentiometers to detect if they have become noisy due to moisture or dirt.
I am thinking of a small AF amplifier and a speaker and the potentiometer for testing to be connected in its input, but no signal fed at the input of the potentiometer. When I turn the potentiometer I should hear in the speaker any noises caused by the moisture.
Will this work?
Any other propositions?

Probably not, at least as you describe it.
You can amplify and measure noise as it is turned but simply changing the resistance to ground alone will not create an input voltage. To make it work you should pass a small current between the track and wiper so that changes in resistance (what we perceive as noise) are turned into a voltage change.

Brian.

betwixt said:

Probably not, at least as you describe it.
You can amplify and measure noise as it is turned but simply changing the resistance to ground alone will not create an input voltage. To make it work you should pass a small current between the track and wiper so that changes in resistance (what we perceive as noise) are turned into a voltage change.

Brian.

Click to expand...

So just connect the input of the potential divider to VCC through a 1Meg resistor will suffice?

Yes, but ensure leakage into the amplifier is very low, preferably by coupling through a non-electrolytic capacitor. 1M is fine if you do that but consider it amounts to only 1uA per volt so natural leakage through an electrolytic might upset the measurement.

Brian.

betwixt said:

Yes, but ensure leakage into the amplifier is very low, preferably by coupling through a non-electrolytic capacitor. 1M is fine if you do that but consider it amounts to only 1uA per volt so natural leakage through an electrolytic might upset the measurement.

Brian.

Click to expand...

Ok I will reduce the resistor to something like 100k and see how it goes.
Then couple the pot wiper to the amplifier through a non-electrolytic capacitor (1uF or so?).
No measurements, will be done, I will just notice the sounds on the amplifier speaker as I turn the wiper, I believe this should be sufficient to show a dirty potentiometer.

Do I have to account for any DC on the wiper if I connect it that way? (DC on the wiper causes noise as it is moved, even if the pot is ok).
Or there will no DC on the wiper?

Do I have to account for any DC on the wiper if I connect it that way? (DC on the wiper causes noise as it is moved, even if the pot is ok).
Or there will no DC on the wiper?

Click to expand...

No, it is essential there is some DC there otherwise the sudden changes in current that cause the noise cannot exist.

What you have to remember is that a potentiometer, no matter how bad it is, does not create noise by itself, it is currents from the signals, coupling capacitor leakage and any intentional DC that cause it. Essentially, the voltage caused sudden changes in resistance is made evident within the audio range and that's what we hear as crackle.
Note that it would be advisable to do your test twice, once with each end of the track grounded as the increasing resistance as the control is 'turned up' will result in less current and therefore less obvious noise.

Unless a potentiometer has been physically damaged or had enough current through it to burn the track, my experience is that most of the noise doesn't come from the wiper to carbon track contact, it comes from the metal to metal contact to the center pin.

Brian.

betwixt said:

No, it is essential there is some DC there otherwise the sudden changes in current that cause the noise cannot exist.

What you have to remember is that a potentiometer, no matter how bad it is, does not create noise by itself, it is currents from the signals, coupling capacitor leakage and any intentional DC that cause it. Essentially, the voltage caused sudden changes in resistance is made evident within the audio range and that's what we hear as crackle.
Note that it would be advisable to do your test twice, once with each end of the track grounded as the increasing resistance as the control is 'turned up' will result in less current and therefore less obvious noise.

Unless a potentiometer has been physically damaged or had enough current through it to burn the track, my experience is that most of the noise doesn't come from the wiper to carbon track contact, it comes from the metal to metal contact to the center pin.

Brian.

Click to expand...

Very useful info!
Yes, I have noticed also in some of my regen designs, that the regen pot produced more noise "crackle" when the resistance was set to low values. However this might have been because of the greater gain of the regen stage.
The metal to metal contact thing you mention is very informative. I have tried cleaning some of them at the carbon-wiper and I still got too much noise, I never thought it would be another point. This reminds me some air variable caps I had, where the metal-metal contacts were dirty and the cap had backlash. Not the same issue but related to dirty contacts.

I am not sure if I must really have to reverse the connections of the pot, the noise should present when the resistance is low either side.

*These days there is a very good propagation at 40m, we should try a sked.

Potentiometer scratchiness bothers me most at soft volume, rather than at high volume. Several of my older radios no longer increase volume gradually. Instead they produce intermittent music and crackle. Frequently I can't get the precise loudness I want no matter how carefully I dial the knob. It jumps from too soft to too loud.

There's little crackling at high volumes. The music competes with the scratchiness.
Furthermore I seldom turn it all the way up so the pot contacts are pretty clean in that range.

At low volume the music is attenuated
but the scratchiness is not. It goes into the amplifier and receives entire gain with little music competing with it.

True Brad, but the point I was making is that if you ignore the uV noise produced by the resistance alone, the intermittency and crackle does not come directly from the poor connection but from basic Ohms Law converting resistance to voltage and that implies a current must be present.

Neazoi is attempting to quantify the noise, or at least grade the quality of the control by rotating it and noting and noise in the result. To do that, I suggested deliberately introducing a small current (microamps) through the wiper to ground so it resulted in a voltage change for subsequent amplification. As the current will diminish as the resistance increases and hence less noise noticed, I advised that the ends of the potentiometer were swapped and the test repeated. It isn't the ideal solution but it does test it over more of the track length. It should also be possible to ground both ends of the track simultaneously and achieve the same thing.

It isn't an ideal test but then it isn't something we normally quantify anyway. I suppose a constant current source would make it better but I wonder if it is worth the extra effort.

Brian.

betwixt said:

Neazoi is attempting to quantify the noise, or at least grade the quality of the control by rotating it and noting and noise in the result. To do that, I suggested deliberately introducing a small current (microamps) through the wiper to ground so it resulted in a voltage change for subsequent amplification. As the current will diminish as the resistance increases and hence less noise noticed, I advised that the ends of the potentiometer were swapped and the test repeated. It isn't the ideal solution but it does test it over more of the track length. It should also be possible to ground both ends of the track simultaneously and achieve the same thing.
Brian.

Click to expand...

I thought the connections were:

one end of the pot to VCC through a resistor eg 100k
the other end of the pot to gnd
the wiper of the pot to the amplifier through a 1uf or so non-electrolytic

Now you say to do it that way?:

one end of the pot to the gnd
the other end to the amplifier through a 1uf or so non-electrolytic
the wiper to VCC through a resistor eg 100k

I am confused

A bit of both...

The intention is to feed a small current through the wiper to ground and monitor the signal on the wiper as the shaft is turned. So the connection is:
one (or both) ends of the track grounded.
100K from the wiper to a DC supply.
Low leakage cap coupling the wiper/resistor junction to an audio amplifier.

If the pot was perfect, rotating the shaft would change the voltage on the wiper and a low frequency signal would be amplified although it would probably be too low in frequency to be audible. If there was any discontinuity or sudden resistance change, the frequency would be higher and heard as a crackle.

Brian.