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Post by mattmayfield on Jan 22, 2008 10:47:43 GMT -5
Hi everyone,
I'm almost done making a Variax/magnetic pickup hybrid guitar, but am having a problem with the regular tone pot. It's a 500 meg pot, and a .047 cap, and I believe I wired it in the standard way (one side to signal in parallel with volume pot, wiper through cap to ground, other side open).
However, it's behaving strangely. When it is turned up to 10, the sound is extremely filtered (as if the tone were on 0). But if the knob is backed off even a tiny bit, it instantly goes back to full treble.
I'm still using the guitar for a couple more days before I take it apart again & finish the body, but has anyone encountered this behavior before, and can you offer some general things to check?
Thanks,
Matt
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zamzara
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Post by zamzara on Jan 22, 2008 12:43:45 GMT -5
It sounds like you have the pot wired backwards. Move the wire on the side lug to the one the other side.
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emin9
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Post by emin9 on Jan 22, 2008 14:06:46 GMT -5
I'd say bad pot if it sounds clear at 9.9 but at 10 it goes to mud. I've had older pots do that after years of abuse. My guess is over the years the knob can go past the stops in the pot and "10" is really shorting you back to 0. Just a noob guess. You could also consider cleaning the buhgeezus out of it with some tuner cleaner spray. Otherwise, I'd replace it.
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Post by JohnH on Jan 22, 2008 14:42:34 GMT -5
It sounds like you have the pot wired backwards. Move the wire on the side lug to the one the other side. I agree, it could be that. It would explain both the tone cut at 10, and the suddenness of return to full treble as you reduce it, assuming it is a log pot. John
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Post by sumgai on Jan 22, 2008 15:24:24 GMT -5
matt, You have a bad pot. It might come back to life with a judicious application of control cleaner ( NOT tuner cleaner - they leave a filmy residue that attracts dirt, thus causing more rapid deterioration), or it could be time for it to go to that big circuit board in the sky. ;D Question to John and zam - if the pot's leads were reversed, how would that account for 'sudden' changes in tonality? Answer: It doesn't. However, reversing the leadsterminals is a valid troubleshooting tool - it'll tell you if the pot is completely bad, or if it's just the one end of rotation that has a problem. (If one likes doing things backwards, one could leave it wired like this...... ) BTW, zam, I've never seen the shaft wear so much that it allows travel beyond the resistance element, but I have seen the carbon picked up off that very end, and taken away by the wiper. The pot in question sat for over 10 years, and the owner "applied a little percussive maintenance" to make the pot move. That was how the wiper, which was stuck to the carbon, got into the distributing business. When I opened it up, I could see why the end of rotation had no connection..... "Hey, what's this little ball of black guck down here in the bottom of the shell?" HTH sumgai ( edited to conform with reversing convention: it's not the leads, that'd be only a polarity swap.... one needs to reverse the terminals. Bit of a difference there. )
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emin9
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Post by emin9 on Jan 22, 2008 16:08:22 GMT -5
( NOT tuner cleaner - they leave a filmy residue that attracts dirt, thus causing more rapid deterioration) did not know that, tx
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Post by JohnH on Jan 22, 2008 17:24:15 GMT -5
Sumgai - My thinking on the sudden tone drop comes from an observation that with normal tone pots, depending on the pickups, nearly all the tone change comes in the last 50 to 100k. At the correct end of a log pot, this happens between 0 and about 6, but if it is wired the wrong way, then it happens quickly on the steep part of the resistance taper, between 9 and 10, and so it feels like it is very sudden.
Also, on my LP, I use the steep end of the tone pot to engage a cap to give a coil-cut bypass at the top of the pot travel as it approaches 10(Red Rhodes scheme). Although the pots are fine and new and theoretically making a gradual transition, audibly the transition happens very suddenly, from bypassed at 10, to full humbucker as soon as I move away from 10. So I think this case is analogous to a tone pot wired reversed.
John
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Post by mattmayfield on Jan 23, 2008 11:15:51 GMT -5
Wow, thanks for all the replies. ;D
Maybe I damaged the pot during soldering. It's brand new, and before I arranged the components on it, I checked it with a multimeter to see where it had the most/least resistance. Though now that I think of it, I think I did get it backwards because I wired it like the volume pot without realizing that the highest tone comes with the MOST resistance...hm.
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zamzara
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Post by zamzara on Jan 23, 2008 19:06:54 GMT -5
matt, Question to John and zam - if the pot's leads were reversed, how would that account for 'sudden' changes in tonality? If it's a log pot, the taper will be going the wrong way too, so the resistance will increase very quickly from even a small turn.
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Post by sumgai on Jan 25, 2008 3:09:29 GMT -5
zam (and John), Yeeessss...... but don't you think it'd be kinda obvious that the results are happening in the opposite direction from the expected one? I mean, it is kinda hard to say to one's self "gee, I think I'll reduce some of the high frequencies by twisting the knob clockwise". Except for lefties, the norm is increase by turning clockwise, decrease by turning counter-clockwise. I'm fairly sure the matt would have told us if he had noticed this condition while turning the control in the direction opposite of normal. Matt? sumgai
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Post by mattmayfield on Jan 30, 2008 11:32:14 GMT -5
Hello again!
Well I have not opened up the guitar since I'm waiting for a lull between gigs to paint & finish the body. To clarify, the pot really does behave like a switch - from 0 to 9.9 the tone is wide open, extremely bright (it sounds like the cap & maybe the pot are completely out of the circuit) - and then at 10 it sounds like the cap is fully in the circuit.
I'm puzzled because, again, it's a brand new pot; but I suppose I could have burnt something out while soldering one of the cap wires to the casing. I was quite careful though and did not leave the iron on there for more than a couple seconds.
(A brief aside... when I had a technology class in high school, about 13 years ago, I made a cheesy little LED lighting project. After clumsily soldering it together, only two of four LEDs lit up, and it turned out their resistors were burned out... that was disappointing, but an important lesson learned about heat and components!)
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Post by ChrisK on Jan 30, 2008 12:58:17 GMT -5
It definitely sounds like a pot wired back wards. zamzara and JohnH are spot-on with his analysis. I do hope that "500 meg pot" was a typo since a value of even 5 meg Ohms will indeed function like a switch. I presume that 500K Ohms was the actual value. If one looks at the typical resistance curve here, (the "A" curves), one can see how quickly the resistance changes "at the wrong end" of the pot.
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Post by sumgai on Jan 30, 2008 13:26:49 GMT -5
Chris,
Note Matt's clarification:(emphasis added)
None of your curves show that possibility. Indeed, he is describing the action of a switch, which would lead me to believe that the carbon element is defective. (Although if it truly were a 500MEGΩ pot........)
Let me note, just for the record, that my meds are not working too well this morning. That's as good an excuse as any for me to make note that some members of this board seem to hold the taper of a pot as being very important. In words of one syllable, "that just ain't true". This is an area like pickups, in that one man's gold is another man's iron pyrite. You like this, and I like that, and we're both happy. Which gives us the moral of the story - no one pot taper is correct for every player out there.
If anyone can figure out what I'm saying here, would you please help an old man out, and post it for me? ;D
sumgai
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Post by JohnH on Jan 30, 2008 14:46:03 GMT -5
May we have a sketch or photo of the wiring at the back of the pot? No point in further speculation on forwards/backwards without that!
John
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Post by sumgai on Jan 30, 2008 19:29:22 GMT -5
John,
AMEN!
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Post by mattmayfield on Jan 31, 2008 1:02:45 GMT -5
;D
This is a lot of fun! Much of this discussion is over my head, so now I finally know how other people feel when I start talking about computer or digital audio stuff...
That was indeed a typo, as it is a 500K pot. (And, embarrassingly, I'm reminded of my older relatives who do not know the difference between kilobytes and megabytes...)
As I mentioned, it will be a week or two before I will take the instrument apart again. I will send a photo then. In the meantime, here is a quick ASCII representation. I did not find an option to set a code block or monospaced font, so you will have to copy & paste into a text editor to see the proper alignment.
---- | +- (==| +-----(cap)---GND | +----(to hot of volume) ----
I don't recall which physical side of the pot is wired to the hot from the volume control, but I always thought that it would simply reverse the direction of the knob, and work either way (like on lefty guitars).
Thanks for all the replies.
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Post by ChrisK on Jan 31, 2008 12:28:52 GMT -5
As in this Fender schematic, on the pot with the 0.022 uF capacitor on it (items 13 & 18), the terminals used are what should be used for a right-handed audio taper pot used for high frequency tone cut. In essence, on this circuit, the center pot terminal goes to the signal, the right terminal goes to one lead of the cap, and the other lead of the cap is connected to the pot shell, which should be connected to the internal signal ground. One does not have to wire them in this order (or even use the shell of the pot), but can have the cap connected to the signal first or the pot (either terminal) can be connected to the signal first. However, for proper operation, the two terminals indicated must be used. In any case, the two terminals with the black ends (and wires) on pot (item 13) are the ones that must be used. Note that the other tone pot (item 14) uses the same two terminals. In this case, it's sharing a common cap with the first pot (item 13).
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Post by JohnH on Jan 31, 2008 21:32:18 GMT -5
I don't recall which physical side of the pot is wired to the hot from the volume control, but I always thought that it would simply reverse the direction of the knob, and work either way (like on lefty guitars). . Thats the info we need, so when you next open up, please take a picture and post it. Also related to this issue, I note from the first post that this guitar has a variax system. This presumably has a low-impedance active output, and so this will also tend to make the tone pot after it work more suddenly, over a lower range of turn. This, combined with reverse wiring of a pot with the usual logarithmic taper, could be highly relevant to your symptoms. John
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Post by jacklthornton on Jan 31, 2008 21:55:28 GMT -5
Reading this thread reminded me of a nagging question for any electronics types out there who can explain this to me. Not that I need to know why this works, but its bugging the snot out of me... Whenever I look at diagrams for RC filters, it shows part of the signal being taken off between the capacitor and the resistor, like this high-pass filter: The low-pass filter is the same, except the capacitor and resistor swap position. However, the way my bass (and apparently most guitars) are wired is like this: obviously with a potentiometer instead of a fixed-value resistor. Now, nowhere that I look can I find what kind of filter a resistor and capacitor in series gives you. Obviously, either highs or lows are being bled off to ground, but again the guitar wiring doesn't match any kind of filter diagram I've found. Also, does it matter which comes first? In most diagrams I see the "hot" signal from the pups being sent to the middle tab on the pot, then the "side" (can never remember #1 or #3) tab connected to the cap and then to ground, which means the resistor comes first and then the capacitor. However, the bass I bought was wired with the cap going between the signal and the middle tab of the tone pot, with the "side" tab being tied directly to ground, which means the capacitor comes first. Does this make a difference? (As you can tell, I know barely enough about electronics to (a) be dangerous, and/or (b) embarrass myself).
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Post by ashcatlt on Jan 31, 2008 23:42:24 GMT -5
Good question. I think the guitar tone scheme actually looks more like this: I don't think the order matters, but I'd love to have somebody with a little more knowledge get more in-depth.
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Post by mattmayfield on Feb 1, 2008 11:04:19 GMT -5
JohnH,
You are correct about the Variax output, but it is on a separate circuit from the passive magnetic pickups (two outputs). The tone control in question is only used for the passive circuit.
Thanks,
Matt
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Post by ashcatlt on Feb 1, 2008 14:09:52 GMT -5
Okay, so this is kind of a hijack, as it doesn't really have anything to do with the OP. It is, however, a question I'd like to see a smart person answer. Since none of them have come along just yet, I'm going to put forth my hypothesis.
I'm thinking the reason it doesn't matter which comes first is that the tone pot is not the only resistance in the circuit. At very least you've got the impedance of the pickups and the amplifier to consider. So the diagram would be more like a combination of yours and mine.
Also seems to me that it's not actually a simple RC filter. The pickup itself throws a big L in there, which i think makes the filter a bit steeper and adds a little resonance at the cutoff.
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Post by JohnH on Feb 1, 2008 14:53:49 GMT -5
Ash - I cant give a full electro-technical explanation. But a resistor and capacitor in series has identical effect on a surrounding circuit, which ever way round they are. That conclusion does not depend on the characteristics of whatever else is connected. Its like A+B = B+A. The same is true in the two dimensional world of complex numbers, with which one can do the math to work out the combined impedance of a resistor and a cap.
The overall effect of the tone control, does however, depend greatly on the pickup inductnace and resistance, and other parts connected.
So I wonder why, in most casess, it is the cap that goes to ground? Maybe it is just convenient for wiring. I thought of one slight, probably negligible point, which woudl apply to guitars which are not screened: The tone pot case is grounded, and so all of the inner workings of the pot are screened from interferance. The cap is not screened however. Hence, putting it on the ground side of the pot may make a tiny reduction in interferance as compared to putting it on the hot side.
John
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Post by ChrisK on Feb 1, 2008 17:23:16 GMT -5
I can, but it takes an awareness equal to the AC circuit analysis courses in a BSEE program to fully understand it. In essence, the effective impedance of the RC network is the reactance of the capacitor Zc = 1/(2PiFC)(F in Hertz, C in Farads, Pi being 3.14159.........) and the resistance of the resistor expressed as a vector sum (sum meaning something well beyond elementary school sums) by the following formulae Z = (R2 + Zc2)^0.5If one finds this kind of stuff interesting; For a simple circuit; with frequency and phase responses like this And more complex (pun intended) stuff like this; with frequency and phase responses like this , one needs to add a little pSpice to their lives................ Yes, aside from the effective taper of the pot (which goes to the effect of the RC effect), when the resistor is analyzed at a particular value, since both the resistor and capacitor make up a two-terminal network and neither are polarized in effect on the overall circuit, order is of no importance. Hmmm, well, a better perspective is that the real number (flat) plane is an infinitely thin 2 dimensional slice thru the 3 dimensions of the complex "plane" (the complex "plane" moniker being a 2 dimensional simplification for use on paper and in most human minds). Absolutely, since guitar (and amp input) wiring comprises one large passive circuit, ALL components interact with ALL other components. Well, the answer (and it ain't anyway even low-tech) is that the original Strats has 3-WAY lever switches. Only the bridge, middle, or neck pickup could be on at one time. There was no tone originally on the bridge pickups since the crappy tube amps of the day lacked high frequency response and distortion was a foul thing to be avoided at any cost. The middle and neck each had their own tone control. By sharing a capacitor between the two (did I mention that there was never two pickups on at one time?), in a product engineered by Leo (who was an engineer and NOT a musician) ..............................wait for it......... ............IT WAS CHEAPER!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! The area (and hence susceptibility) of the cap as an antenna was fairly insignificant. If one had actually connected the outer foil lead (the one indicated by the [generally] black stripe at one end of the NON-POLARIZED wound film axial cap [often mistaken as a polarity band]), one would realize as much shielding as possible. But then, everything capacitor is ceramic these days (cheapest is).
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Post by sumgai on Feb 4, 2008 2:23:54 GMT -5
Chris,
You had me at Zc=1⁄(2πFC)! ;D
sumgai
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Post by gumbo on Feb 4, 2008 6:01:38 GMT -5
I thought it was
E = Fb
;D
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