# Pot failure?



## bcmatt (Aug 25, 2007)

What might a pot do when it fails?

I have a DC30 amp that on channel 1, 85% of the time the volume will only work up to a certain point; so it it only goes up to about 2/10 volume and then nothing more is reached as you continue to turn it. However, occasionally it will work fine.

At first, I thought this was a preamp tube problem, but it keeps occurring despite different tubes.

It's possible that it has more to do with something else in the circuit, but I am wondering what sort of characteristic a pot shows when it is failing, and if that is the possibility in this setting.


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## Wild Bill (May 3, 2006)

bcmatt said:


> What might a pot do when it fails?
> 
> I have a DC30 amp that on channel 1, 85% of the time the volume will only work up to a certain point; so it it only goes up to about 2/10 volume and then nothing more is reached as you continue to turn it. However, occasionally it will work fine.
> 
> ...


First off Matt, a pot "fails" about as often as you win the lottery. The Big One! It's just a very simple mechanical gizmo.

I'll bet 3 beer you've got a circuit problem. Is this one of those printed circuit board reissues? Where the pots are soldered directly to the board? Often a bad solder pad on the board will give this sort of problem. The nut holding the pot to the panel gets loose and allows the pot body to get wiggled back and forth. This breaks the solder connection on the board.

The fact that it comes and goes means something else may be going on. You need to have the amp open so you can look at the pot and wiggle wires back and forth to see if it makes the problem come and go. If the pot is dirty inside a zot of contact cleaner may help.

Anyhow, it won't get fixed by sitting around looking at the outside and trying to make a good guess. You're gonna have to open up the amp, roll up your sleeves and get dirty!:smile:

:food-smiley-004:


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## bcmatt (Aug 25, 2007)

Haha, ok thanks Bill.
Good to know. I don't win lotteries, so I will plan to not replace the pot.

No, this is my DC30 that I built and there are no PCBs involved at all.

I think it has to do with when I installed the VVR.
It is my opinion that it has to do with the addition of a .1uf cap between the input and the first grid and another between the volume pot and and 1M resisters from same grids to ground:

_This type installation will turn down all of the voltages in the amp as you adjust the power control on the board. This kind of implementation will require you to move the 1meg resistor on your guitar input jack and the installation of a cap between the input and the grid of the first tube. You also need to install another 1meg resistor and cap between the vol control and the grid of the tube it feeds. See the schematics below. _
http://3.bp.blogspot.com/_4u9l5q2yFBA/SScLgXbUviI/AAAAAAAAArQ/p6eMOTgvmfM/s1600/without+caps.jpg
http://3.bp.blogspot.com/_4u9l5q2yFBA/SScLkql7h2I/AAAAAAAAArY/3imyTKskSDY/s1600/cap+and+resister.jpg
_While you can play with the value of the caps a good rule of thumb is to start out with the same size caps that are feeding the signal from your phase inverter to the grids of the power tubes.This should not cause any change in tone compared to the original tone of the amp. Some people just use .1uf. The reason for the changes are because at low voltages the signal on the grids of the two preamp tubes leaks a little dc voltage onto the guitar pot and vol control when the signal hitting these grids drive them more positive than the bias voltage applied. In simple terms it removes the scratchiness from the guitar and amp vol controls._ -taken from VVR instructions

So, I wonder what needs to be modified for my Channel one of my DC30. Does the fact that it uses a 500K volume pot instead of 1M make a difference?
http://4.bp.blogspot.com/_4u9l5q2yFBA/SScMT4EbG-I/AAAAAAAAArg/SmF9BqJ08mw/s1600/DC30+Channel+1.bmp


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## mhammer (Nov 30, 2007)

Bill's experience with pots, and my own, seem to differ. I suspect this may have to do with the quality of pots that each of us normally deals with.

You can take a journey through a pot here: http://www.geofex.com/article_folders/potsecrets/potscret.htm

The wiper on pots can vary widely in its construction. Some, like the one shown in the article, are small perturbations on an otherwise mechanically sound structure that simply "grazes" the resistive strip. Others are more like the cow-catcher on a train that leads the way and scrapes against the resistive element. the one in the article has 3 study spring points that make contact. Others will have more and sometimes less.

If you scroll down a bit in the article, you'll see that the resistive strip wears down, or at least shows evidence of having a pressurized object in contact with it. Depending on the characteristics of that contact, the resistive strip can get worn down over time. This is why even sealed pots can get "dirty". The dirt is not just crap from the outside that seeped in through the openings, it can also be bits of microdust scraped off the resistive strip by the wiper after thousands of rotations back and forth. This is why things like table radios that turn on via the volume control and never had their volume turned up past 11:00 are really crackly at the lowest end of their volume range but work fine if you want to put up with ear-splitting volumes. It's the wearing out of the resistive strip in the most frequently-scraped zone.

So, yeah, depending on the build quality and design of the pot, sometimes one CAN get phenomena like you describe. Myself, what I do is take the pot apart, clean off the resistive strip with a Q-tip and some alchohol, and then lay down a layer of Stabilant on the resistive strip. Stabilant is this rather remarkable contact enhancer that's made in Richmond Hill, of all places. It is not a contact cleaner, but rather forms a stable conductive layer over top of whatever it sits on. http://www.stabilant.com/ I was turned onto it by a buddy with a commercial studio that needs to keep his analog slider pots scrupulously clean and crackle free. To my great delight, an electronics place near my home has these little 0.5ml tester vials (apparently out of production) that they sell for 50 cents (I'll try and bring you a couple, Bill, if we can ever arrange for that coffee. Maybe this Christmas trip.). It is no miracle cure for everything, but if you just need to bridge that last micron or two of surface to make a good contact, it's wonderful. I've taken pots that drove me crazy with crackling and other forms of dysfunctionality, and completely rehabilitated them. Will it restore a 35 year-old wah pot? probably not. At a certain point, the layer it forms is simply not thick enough to provide electrical continuity if the resistive strip is worn down THAT much. But it works on a lot of stuff and also provides prophylaxis by virtue of its viscous properties (i.e., a contact lubricant that can prevent wearing down the resistive strip). I can fix about 30 pots on a 50-cent vial.


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## bcmatt (Aug 25, 2007)

Hey, that's a great article. Interesting reading. I like the pictures and diagrams too.

My previous experience with pots has been that they may eventually get scratchy with lots of use, but so far, it seems that they won't suddenly stop working above a quarter turn as if the quarter turn is as far as it can go.

I did end up swapping out the pot for a new 500K Bournes Audio Taper pot, and it didn't improve anything. I'm still severely lacking gain capabilities on channel one. So, in this case it is something to do with the circuit.
This is what I did:
http://3.bp.blogspot.com/_4u9l5q2yF.../rbZ4xtUYB6w/s1600/DC30+Channel+1VVR+mods.GIF
Can anyone explain why I would be severely lacking gain?

My plate voltages for the first 12ax7 are 158 (wired together), and for the second 12ax7 are 212 and 345. Do these indicate more specifically where the problem is? I bypassed each of those .1 caps and it doesn't fix the gain.


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## nonreverb (Sep 19, 2006)

mhammer said:


> Bill's experience with pots, and my own, seem to differ. I suspect this may have to do with the quality of pots that each of us normally deals with.
> 
> You can take a journey through a pot here: http://www.geofex.com/article_folders/potsecrets/potscret.htm
> 
> ...


I have to agree...anyone who's ever had to service a JCM 800 amp knows this problem....great amp...dirty pots....easy fix...


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## ne1roc (Mar 4, 2006)

I'm sorry. I must have logged onto the incorrect site? I was googling FAQ's about hydroponics?


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## bcmatt (Aug 25, 2007)

ne1roc said:


> I'm sorry. I must have logged onto the incorrect site? I was googling FAQ's about hydroponics?


Haha, I was waiting for the first response of that sort.


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