68 Custom Princeton Reverb Reissue with noisy volume pot

[MSVguitar]

I am working on a 68 Custom Princeton and am having an issue with a noisy volume pot.  When I increase the volume from 1 I hear a 'rubbing' noise starting at 3 all the way up to 10.  I checked for DC on the pot (didn't have any DC), cleaned the pot, changed the tube and then replaced the pot and still have the noise.  It only happens on the volume pot, not the treble or bass. 

[Platefire]

Unless your wanting to keep the amp all original parts, might be time to try a new volume pot

[Latole]

There may be several causes, as I'm not there and don't know your electronics skills.

1-Do you have a good cleaner contact? Deoxit it is the best ( and sometime the only one do the job) 
    MG Chemical is my second choice.
2-Do you spray well in the access holes?
3-Do you turn the knob at least 20 times after cleaning?
4- Are you sure you don't have any DC on the pot?

If everything has been done properly, AND with what you've written, the pot should be replaced.
I've had to clean dozens of pots for dirt or oxidation problems (various noises) and I may have only had to replace 2 of them.

It's a mistake to think that a tube is the problem. 

[Latole]

I'm helping a guy on the forum whose pedal has a potentiometer problem.
He says he has cleaned the pots with no results.

After several days of discussions I learned that he had sprayed the pots without removing the cap covering the access hole or where the 3 lugs were !!!!!

So he didn't clean anything!
And we don't know what contact cleaner he uses!

See arrows on picture.

[Dave]

Sometimes your meter won't lock it on a specific DC voltage. It will just jump around. Does it jump around zero or does it jump around 2 to 3 volts (as an example). If it jumps around, but zero volts is not part of the dance, you may still have DC voltage on the pot.


Probably going to get some push back on this one. Try WD-40. Its not the cleaner everyone else uses, in fact its not a cleaner at all, but it's non-conductive and it lubricates and leaves an oily film on the resistive surface inside the pot. Works real good sometimes.


Dave

[bmccowan]

I am working on a 68 Custom Princeton and am having an issue with a noisy volume pot.  When I increase the volume from 1 I hear a 'rubbing' noise starting at 3 all the way up to 10.  I checked for DC on the pot (didn't have any DC), cleaned the pot, changed the tube and then replaced the pot and still have the noise.  It only happens on the volume pot, not the treble or bass. 

Given that you actually did replace the pot - I think its DC on the pot despite what you metered - I'd replace the coupling cap.

[Latole]

Sometimes your meter won't lock it on a specific DC voltage. It will just jump around. Does it jump around zero or does it jump around 2 to 3 volts (as an example). If it jumps around, but zero volts is not part of the dance, you may still have DC voltage on the pot.


Probably going to get some push back on this one. Try WD-40. Its not the cleaner everyone else uses, in fact its not a cleaner at all, but it's non-conductive and it lubricates and leaves an oily film on the resistive surface inside the pot. Works real good sometimes.


Dave

WD40 brand make also a contact cleaner in spray, not only lubricant . I don't know how is good or not

[MSVguitar]

Thanks everyone for the feedback.  Just to clarify what I am working on and what was done to the amp, I am including some pictures.  This is a Fender Reissue with PCB mounted components.  I replaced all the electrolytic caps (filters and bypass), replaced all dropping resistors with higher watt resistors, moved the heater wires, HT connection and all the screen and grid resistors off the PCB.

The volume pot had the issue as I received it. The PCB that contains the pots and has to be removed in order to have access to apply the cleaner, I use Deoxit D5.

Given that you actually did replace the pot - I think its DC on the pot despite what you metered - I'd replace the coupling cap.

I replaced the 250 pF cap (C19) and still have the issue.  I checked for DC again and found that there is 10 mV on the volume pot wiper when the volume pot is set at '7'.  So I measured all the points in the tone stack with the volume at '1' and '7'.  See the attached file.

What I found was that the largest voltage I measured after the coupling cap was 10 mVDC on the volume pot wiper (vol @'7') which is connected to the grid (pin 7) of the next preamp stage.  Could the grid be causing the problem?  I checked the voltages on all of the tube pins and they all look good.
I would think if the coupling cap was leaking I would see a larger voltage from the cap and a smaller voltage on the wiper.

[sluckey]

What I found was that the largest voltage I measured after the coupling cap was 10 mVDC on the volume pot wiper (vol @'7') which is connected to the grid (pin 7) of the next preamp stage.  Could the grid be causing the problem?

Pull V1 and see if the tiny voltage disappears.

[MSVguitar]

Pull V1 and see if the tiny voltage disappears.

I pulled the tube and the voltage increased to 20 mV.  I checked all the caps in the tone stack and no leakage. 3 mV or less.

The voltage is tiny.  So maybe not causing the noise.  No leaking caps.  The tube is not the source of the voltage.  I replaced the pot with a 'Fender Genuine Part'.  Maybe Fender has a batch of bad pots?

[PRR]

...there is 10 mV on the volume pot wiper when the volume pot is set at '7'.

That's fairly normal for tubes. (Some large part of a Volt is bad.)

The sensitivity at the next grid is 16mV (0.78V at plate divided by gain of ~~50).

So passing through 10mV when 16mV is MAX is like 2/3rd of MAX. LOUD.

Does it crackle all the time, when not touching? Or only when moving the knob?

[Lauri]

Does the rubbing noise disappear if you touch the chassis with your other hand when you turn the pot?

[Latole]

To check a cap for leaking, you must disconnect the lead of the capacitor where you read the smallest voltage.

With this lead on the air, you must read nothing.

[MSVguitar]

Does it crackle all the time, when not touching? Or only when moving the knob?

No crackle only a rubbing sound and it only occurs when moving the knob past 3.

Does the rubbing noise disappear if you touch the chassis with your other hand when you turn the pot?

No

To check a cap for leaking, you must disconnect the lead of the capacitor where you read the smallest voltage.

With this lead on the air, you must read nothing.

I understand.  The caps are mounted to the controls PCB and are not accessible when the board is installed.  I will look at it to see what I need to do to lift one end of the cap and check for leakage. 

I much prefer a turret board and hardwired components for troubleshooting.