Help needed to fix noise from an old stepped volume pot

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Hi all,

I have a 70s Denon SA-3300 (not my pic) with a 20-step Noble volume pot (see attached pic from my unit). Without taking it out, all I can read from the stamp is NOBLE 0095107

There is no schematic online that I can find for this amp, and the only chatter about it is in a French language forum (only light banteur).

The problem is that the amp gives out varying amounts of white noise with no source plugged in (no difference whether on Aux, Tuner or Phono), depending on the position of the pot. BUT the amount of noise produced is not linear with turning up through the 20 positions - each position leads to a *different* level (and character) of white noise. Unfortunately, in positions 4-8 (loud to very loud on my speakers) the noise becomes v.noticeable in quiet passages.

It's very much "random". For example, position 4 gives off a certain amount of hiss. Position 5 may give out more, position 6 a bit less, position 7 more than position 4, position 8 more again, then position 9 next to none, position 10 none, position 11 some other amount, and so on.

This leads me to think that the problem lies in the pot itself, not the amp - and if I can fix the pot, I can have a super-quiet amp.

I have done all the contact cleaner spraying I think it can take, from multiple angles (some lubricating contact cleaner is now on the way as I will have cleared out every last semblance of lubrication from there already). Wiggling the pot whilst listening does very little to affect the white noise (but maybe a little).

My questions are - should I keep cleaning (maybe unsolder and take the thing apart, but I don't want to really have to go there), try something else or think about replacing the part?

Another thing I'd like to understand is how to even look for a replacement. There are 6 PCB solder points (3 per side) plus the two wires attaching to the pot tracks. If I took the thing out, I'd be looking to find out how to measure it and get an appropriate replacement.


Thanks,


lhalha
 

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This might seem like a dull minded question... but when you clean the pot are you spraying directly into the opening above the wires and spinning it up and down end to end as you spray? The back and forth is needed to wipe the smutz off the carbon trace and wiper.

From the picture, there's nothing special about the pot. It looks like a standard 1" carbon pot. It's the knob that is detented. You can probably replace it with a standard pot of the same resistance and transfer the detent mechanism over to the new pot... and it will work just fine. It would still work just fine without the detents... it just wouldn't be detented. ;)

Also, do you still hear the hiss with the volume turned all the way down?
If so it could be a couple of bad parts that need to be checked and replaced...
 
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That Nobel pot is tapped for the "loudness" circuit, and tapped pots are almost non-available now.
Unless you hear "scratchiness", there is nothing wrong with the pot.
Also, operating the amp with "wide open" inputs WILL show hum and/or hiss, regardless.
Because there is no load to "quiet" the input to normal levels.
Short the input used, and note the "hiss" level, it should ONLY be hearable if at all, at extreme volume setting - something not normal to do.
Otherwise, don't worry about it.
 
This might seem like a dull minded question... but when you clean the pot are you spraying directly into the opening above the wires and spinning it up and down end to end as you spray? The back and forth is needed to wipe the smutz off the carbon trace and wiper.
Yep - done a lot of that! Today I thought that I may not have sprayed with the dial cranked up to full, to allow some fluid to get into the normal listening positions, but this didn't seem to improve things...



From the picture, there's nothing special about the pot. It looks like a standard 1" carbon pot. It's the knob that is detented. You can probably replace it with a standard pot of the same resistance and transfer the detent mechanism over to the new pot... and it will work just fine. It would still work just fine without the detents... it just wouldn't be detented. ;)
Aha! Thank you - That is very interesting - I was trying to get my head around how this this thing is like a stepped attenuator, with just a regular track/wiper pot.


Also, do you still hear the hiss with the volume turned all the way down?
If so it could be a couple of bad parts that need to be checked and replaced...
Nope... only from about position 3 upwards...
 
That Nobel pot is tapped for the "loudness" circuit, and tapped pots are almost non-available now.
Unless you hear "scratchiness", there is nothing wrong with the pot.
Also, operating the amp with "wide open" inputs WILL show hum and/or hiss, regardless.
Because there is no load to "quiet" the input to normal levels.
Short the input used, and note the "hiss" level, it should ONLY be hearable if at all, at extreme volume setting - something not normal to do.
Otherwise, don't worry about it.


No scratchiness - that was all removed by the cleaning.


Re the loudness circuit and tone board, I have replaced the main C51 220uF (see new pic of board I just added) and cleaned the oversized loudness and mute switches, but not done the other smaller Tone caps yet. Anything I could do there to possibly improve things?


Edit - Have not done the Phono circuit either - the caps I've done are the obvious ones where the circuit board markings are larger than the new caps.
 

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It is possible to get scratchiness when moving the volume control if the stem of the shaft has gunk at the point of entry near the nut securing the body to the chassis. That should be the first port of call. Try a small gentle spray into the gap and move the shaft back and forth. Wipe of the excess and repeat the dose - you should not need to do this more than 2-3 times in total. You should also do this with the selector switch, plus the tone and balance controls.
 
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It is possible to get scratchiness when moving the volume control if the stem of the shaft has gunk at the point of entry near the nut securing the body to the chassis. That should be the first port of call. Try a small gentle spray into the gap and move the shaft back and forth. Wipe of the excess and repeat the dose - you should not need to do this more than 2-3 times in total. You should also do this with the selector switch, plus the tone and balance controls.

Incorrect info there, sorry.
Any "dirt" around the outer bushing/shaft is easily cleaned by wiping with a bit of alcohol on a cue tip or a tissue.
Due to the fact that many potentiometers have viscous fluid pre-injected into the shaft collar, using any type of cleaner/spray/lube will destroy the "smooth rotation" of the control that was originally built into it.
Using spray cleaner into the "rear" of the pot is the only way recommended, where it will do the most good.
The "wiper" of the pot is the main thing to have in good mechanical contact with the contact ring and carbon tracks.
 
Alcohol is not the only substance capable of evaporation, there are electronic spray products that have the same properties. These supplied with a narrow tube attachment allow judicious use to fine spray or drizzle a drop or so on targets.

I have a number of old pots in a plastic jar with a silica gel sachet. Some are old new stock Alps and Noble getting on to 50 years old.

Examination shows the exterior of all of the shafts were clean with some small various signs of black residue at the entry point of the shaft into the pot housing.

It may be sufficient to wipe that off with alcohol as you suggest but that was not the case with a repair job I did a few years ago with an old Kenwood amplifier.

There is some lateral movement in a shaft and reflecting on this the build up of gunk could have been the difference between smooth and rough tracking of the carbon.

If it takes a spray product to shift this I will use it.

Doing some comparisons the old pots were not what I would call smooth rotation judging by comparison with modern parts.

There were differences between old style ones by Alps and Noble - due to age there were different signs of oxidation in the eyelet connections.

The properties of whatever lubricants were used in for the shafts would also be affected by airborne contaminants in the home environment.

Some of these were large and clunky and others small but they were all many times stiffer than modern pots. There are varying degrees of stiffness - a couple were very stiff and needed excessive force to move one.

Another was stiff but once freed up by hand was had next to no resistance to pressure.

If an old pot is stiffer to move than a modern one I see no reason to do whatever it takes to emulate the freer action of the modern one.
 
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Unless pots are all manufactured the same way, to the same engineering tolerances and from the same materials, it's unlikely there can be a single, correct way to restore them. I've encountered pots with huge clearances you can see through and others so tight that they lock up at the first sign of corrosion unless used regularly. That said, service policies are another matter. Most do insist that there is a correct way to do things and anything else is Frank's way. YouTube
 
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