Help about potentiometer

Thread Starter

Dzoro

Joined Feb 1, 2019
154
Hi guys
I am repairing an old amplifier which is 23 years old and i repaird everithing but the potentiometer for the volume contorol was bad and aj removed the circuit but i see a potentiometer with 4 leads i was searching in my town in the electronic shops but i couldnt find it anywhere so can i use a normal 3 lead potentiometer and conect the 4th lead with the middle lead beacose i mesure 12 ohms from the middle lead to the 4th pin

I have taken a pic from it, can anyone tell me if these are still selling or i will need to do some twics
Thanks dzoro
 

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peterdeco

Joined Oct 8, 2019
95
What is wrong with the potentiometer? Is it staticy? If it is and being 23 years old it's probably dirty. Spray the insides with contact cleaner. I've also used WD40 and it works.
 

Thread Starter

Dzoro

Joined Feb 1, 2019
154
What is wrong with the potentiometer? Is it staticy? If it is and being 23 years old it's probably dirty. Spray the insides with contact cleaner. I've also used WD40 and it works.
The potentiometer is 250kohm but when i turn it allthe way to the left its infinite ohm and the potentiometer spins infinite same on the right side
 

Delta prime

Joined Nov 15, 2019
164
Water Displacement#40 you're an animal. What what you got there is a ganged logarithmic potentiometer not linear your measurements mean nothing. Replace potentiometer exactly as is.
 

Thread Starter

Dzoro

Joined Feb 1, 2019
154
Water Displacement#40 you're an animal. What what you got there is a ganged logarithmic potentiometer not linear your measurements mean nothing. Replace potentiometer exactly as is.
I can not find anithing like it and the main amp is linear not digital so how the logorithmic pot will work on a linear apm
 

Delta prime

Joined Nov 15, 2019
164
. In linear pots, the amount of resistance changes in a direct pattern. If you turn or slide it halfway, its resistance will be halfway between its minimum and maximum settings. That's ideal for controlling lights or a fan, but not for audio controls. Volume controls have to cater to the human ear, which isn't linear. Instead, logarithmic pots increase their resistance on a curve. At the halfway point volume will still be moderate, but it will increase sharply as you keep turning up the volume. This corresponds to how the human ear hears
 

Thread Starter

Dzoro

Joined Feb 1, 2019
154
. In linear pots, the amount of resistance changes in a direct pattern. If you turn or slide it halfway, its resistance will be halfway between its minimum and maximum settings. That's ideal for controlling lights or a fan, but not for audio controls. Volume controls have to cater to the human ear, which isn't linear. Instead, logarithmic pots increase their resistance on a curve. At the halfway point volume will still be moderate, but it will increase sharply as you keep turning up the volume. This corresponds to how the human ear hears
How can i identefy logorithimic pots how linear and also when i search on the internet i can't find logorithmic pot with 4 leads
 

Delta prime

Joined Nov 15, 2019
164
You seem to know what type of amplifier you have specifically nonlinear or linear and you're very familiar with digital and non-digital slow your roll youngster and all will be revealed.
 

Thread Starter

Dzoro

Joined Feb 1, 2019
154
You seem to know what type of amplifier you have specifically nonlinear or linear and you're very familiar with digital and non-digital slow your roll youngster and all will be revealed.
I didn't understan you what you wanted to say
 

Delta prime

Joined Nov 15, 2019
164
Let's take it step by step I believehttps://forum.allaboutcircuits.com/members/peterdeco.653408/
The second thread is the easiest place to start. Keep us informed and we'll go from there. Clean the potentiometer!
 

Ylli

Joined Nov 13, 2015
893
The 4th terminal is called a 'volume' tap. These type of parts are generally specially ordered by the unit manufacturer for a given production unit and you are not going to find a 'new' part that is an exact replacement. You can try ebay and see what might be available as pull outs.

But, before you go that route I agree that the pot is probably just dirty. Assuming it is still on the board and you haven't mucked it up yet, try giving it a good cleaning with a zero residue electronics contact cleaner. (such as CRC QD) Do your best to get the contact cleaner inside the pot, and work it back and forth a bit.
 

Thread Starter

Dzoro

Joined Feb 1, 2019
154
The 4th terminal is called a 'volume' tap. These type of parts are generally specially ordered by the unit manufacturer for a given production unit and you are not going to find a 'new' part that is an exact replacement. You can try ebay and see what might be available as pull outs.

But, before you go that route I agree that the pot is probably just dirty. Assuming it is still on the board and you haven't mucked it up yet, try giving it a good cleaning with a zero residue electronics contact cleaner. (such as CRC QD) Do your best to get the contact cleaner inside the pot, and work it back and forth a bit.
I tryed that but the problem with it is that the owner of the amplifier broke something on the inside and the pot turns infinitly
 

Ylli

Joined Nov 13, 2015
893
You can replace it with a three terminal device, but the 'volume' switch on the receive/amplifier will no longer work.
 

MrChips

Joined Oct 2, 2009
20,329
How to tell if a pot is non-linear?

Unsolder the wires going to the pot (at least two of the three wires).
Measure the resistance across the two outside terminals. This is the maximum resistance and the value you look for in the replacement pot.
Dial the pot to the halfway position.
Measure the resistance from the middle terminal to one outside terminal.
If the resistance is no way near half the maximum resistance measurement then it is a non-linear pot.
 

Reloadron

Joined Jan 15, 2015
5,140
You can't really measure a pot while it is in circuit. Try the above suggestion. I would also get some contact cleaner and wash it out. Pots seldom fail unless something goes very, very wrong and it goes way over current.

Ron
 

Ylli

Joined Nov 13, 2015
893
Sounds like the part is mechanically defective. Only hope might be to carefully disassemble it and try to fix what is broke.
 

BobTPH

Joined Jun 5, 2013
2,264
Um, why is everyone talking about log pots when the first picture clearly shows the markings “250KB”, which is a linear pot?

Bob
 
The 4th pin on a volume control pot is a tap for the "loudness" switch (not called volume) that boosts the bass when the volume control is set low.

I agree that a pot labelled A is logarithmic, and B is linear.
 
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