How Does a Volume Pot Fail?

[JJCalvillo]

Have a Pioneer QX-949A quad receiver with a volume pot that doesn't completely shut down the right channels. I've used regular Faderlube and F100 to clean it and it isn't noisy, it just doesn't cut the volume all the way.

So a few questions:
• Is this a case where the wipers aren't coming back fully counter-clockwise or could it be a problem with the trace?
• Would dirt cause this or is it a defect/mechanical problem with the pot?

Don't want to yank and disassemble the pot because: A) it's a nochtmare and B) these pots are hard to find if things go south.
I can kill the volume with the balance pots next to the levels scope so this is more of an inconvenience than anything else. Still I'd like to fix it.

 

[Mr Boochie]

The unit may be experiencing coupling-cap or tone-shaping issues between the volume pot and the signal. That can cause odd control problems.

That is one complicated box of components!

It might be best to seek a good tech.

 

[rickl]

Have a Pioneer QX-949A quad receiver with a volume pot that doesn't completely shut down the right channels.

I think I'd try to live with it.

I've worked on one and now I turn those quads away.

 

[dr*audio]

I would add a resistor in series with the center tap for both channels. Measure what the minimum resistance is to ground from the center tap. I would select a resistor at least 10X the minimum value to start and increase it if necessary. Also measure the resistance between the bottom terminal to ground. Maybe you have a bad ground.

 

[JJCalvillo]

I think I'd try to live with it.

I've worked on one and now I turn those quads away.

I usually feel the same way, but the 949 is laid out pretty well. The only board that's a PITA is the one with the switches under the lamps and above the Control Amp. Everything else is accessible.

 

[JJCalvillo]

I would add a resistor in series with the center tap for both channels. Measure what the minimum resistance is to ground from the center tap. I would select a resistor at least 10X the minimum value to start and increase it if necessary. Also measure the resistance between the bottom terminal to ground. Maybe you have a bad ground.

So each of the 2 right channels/pots would have a resistor on the center tap, correct?

May be a dopey question, but not sure what you mean by "bottom terminal" regarding ground.

 

[gadget73]

+1 on confirming the ground. Also worth an ohm check from center to the grounded end to verify it goes to 0 ohms.


The "bottom" of the pot would be the end going to ground. Normally you have 3 or 4 terminals per-section. The "top" is where the signal comes in, the center is the moving wiper, "bottom" is ground, and if there is a fourth its a tap usually for the loudness circuit.

Often on schematics pots are drawn vertically with the grounded end towards the bottom of the page and the signal input at the top.



as an example.

 

[dr*audio]

So each of the 2 right channels/pots would have a resistor on the center tap, correct?

May be a dopey question, but not sure what you mean by "bottom terminal" regarding ground.

In a volume pot, looking at a schematic, the input goes to the top terminal, the output is the middle terminal and the bottom terminal goes to ground. Looking at the pot itself, one of the outer terminals goes to the input, the other one goes to ground. If you connect a DMM to the center terminal and turn the pot all the way down, then set the DMM to Ohms and connect the other lead to the outer terminals, one at a time, the one that measures lower resistance should connect to ground.
After thinking about this a bit I realized that putting the resistor on the center pin won't work. Putting it in series won't do anything and putting it to ground will cut down the volume but would limit how loud it can go. A better solution is to add resistors in series with the outer terminals that do not connect to ground. Some value between 47K - 100K should work. It will probably still not be silent with the volume all the way down but it will be lower.

 

[JJCalvillo]

@gadget73 Muchas gracias!

Ground appears to be good, it varied from .2 to .6 ohms on the 4 channels.

None of the center/wiper to bottom/ground goes to zero from back pot to front I got 3.8K, 259, 2.7k, and 52 ohms. Measured twice, w/volume off, to confirm.

I'm guessing the 3.8K pot (back) and the 2.7K pot (2nd from front) are the right/problem channels since the other two pots get closer to 0.

@dr*audio 's revised formula would mean:
• Starting with a 38K and 27K resistors on the top connection of those pots with the wires currently connected on the other end of the resistor?
• Would those be .25W resistors?
• Is the best method to test the resistors with gator clips to the pots and wires before soldering anything in place?

 

[JJCalvillo]

What conclusion/s do we get from the resistance not going to 0? Mechanical deterioration of the pots/worn carbon, dirt?

 

[gadget73]

0.2 to 0.6 isn't bad, what does your meter read if you short the leads together? You may just be showing lead resistance.

 

[JJCalvillo]

0.2 to 0.6 isn't bad, what does your meter read if you short the leads together? You may just be showing lead resistance.

.1 shorted, it's a pretty good meter.

 

[Mr Boochie]

.1 shorted, it's a pretty good meter.

If you move the pot or the wires, does the resistance change?

 

[JJCalvillo]

Not much play in the wires, but if I move them I can get the back pot ground to go from .2 to .3 and the 2nd from the front to go from .3 to 4, then both return to the original reading, .2 and .3.

 

[gadget73]

I'd consider that to be pretty good honestly. Sounds like you're at worst about 0.3 ohms resistance once the meter leads are subtracted out.

I wonder if its maybe just how these are by design. Some things won't shut up completely because of lead routing or grounding design issues.

 

[JJCalvillo]

So would you try testing resistors on the tops using clips?

 

[Dave1027]

A workaround may be simpler like switching off the speakers with the speaker selector. Or switching the Input selector to an unused input. On my preamp, I partially depress any other input button and that makes the selected input button pop out, immediately and completely disabling all sound.

 

[dr*audio]

@gadget73 Muchas gracias!

Ground appears to be good, it varied from .2 to .6 ohms on the 4 channels.

None of the center/wiper to bottom/ground goes to zero from back pot to front I got 3.8K, 259, 2.7k, and 52 ohms. Measured twice, w/volume off, to confirm.

I'm guessing the 3.8K pot (back) and the 2.7K pot (2nd from front) are the right/problem channels since the other two pots get closer to 0.

@dr*audio 's revised formula would mean:
• Starting with a 38K and 27K resistors on the top connection of those pots with the wires currently connected on the other end of the resistor?
• Would those be .25W resistors?
• Is the best method to test the resistors with gator clips to the pots and wires before soldering anything in place?

"I got 3.8K, 259, 2.7k, and 52 ohms."
Is this from the center pins to ground with the volume all the way down?

 

[gadget73]

Missed this, but yeah it should be the basically zero ohms that I thought you were getting. I guess i misread the post and thought the 0.3 ohms was wiper to ground. If its hundreds to thousands of ohms, something is unhappy

 

[JJCalvillo]

"I got 3.8K, 259, 2.7k, and 52 ohms."
Is this from the center pins to ground with the volume all the way down?

Yes.