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Author
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Topic: Tone pot still not working
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Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 23 November 2004 01:56 PM
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I checked the "caps" and they build up on the meter. As they are the wax type, the ends aren't blown, so I assume they are good. I changed the tone pot with a new 250K. Now the sound cuts out when you turn it. With the old one in, the tone remained the same. This is an Epiphone Electar. As I said in the other post, the pot is marked 20 M-Z. Someone responded that a 250K would work. I'm now thinking that way back then they used a pot with a different rating. Any suggestions? The Guldan I also just bought does the same thing, cuts out when you turn the tone knob. The "caps" in this one also test and look ok. Whatever the problem is, they both have it. Any sugestions? Thanks, Ron |
Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 23 November 2004 02:10 PM
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Sorry, should read 200 M-Z |
jerry wallace
Member From: Artesia , NM - 35 mi from Roswell UFO CITY
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posted 23 November 2004 02:55 PM
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Ron how are the caps wired into the pot?..There are several varations that will work.. But its like this, one end of the cap must go to ground threw the tone pot..Thats how the tone is changed by grounding the cap or having the pot resistance between the cap and ground. Again there are several varitaions that will work..One end of the cap can go to the volume pot then the other over to the tone pot.Or you can just have a solid jumper wire from the volume pot over to the tone pot. But both pots must be connected and the cap resisted in and out of ground by the tone pot.. Hpe this makes since!! ------------------
Jerry Wallace/TrueTone pickups-2001 Zum: D-10,8+6, "98 Zum: D-10,8+8,Nashville 1000,Session 500 ,Session 400 head only amp,Tubefex,ProfexII, Artesia, New Mexico
http://www.jerrywallacemusic.com
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Dave Grafe
Member From: Portland, Oregon, USA
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posted 23 November 2004 03:14 PM
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Did you ever measure the resistance of the old pot (outside post to outside post)?Normally 200M-Z would mean "200 MEGA (200,000,000) ohms - Z (audio) taper" - if your volume pot is still a 200M then your 250K tone pot could essentially be passing the bulk of the signal to ground as you open it up and thus causing it to cut out. 250K IS a standard value for most modern pickup harnesses but might not be appropriate for a super high impedance unit. I will look up some old drawings and see if I can find anything that will help you out here. Dave[This message was edited by Dave Grafe on 23 November 2004 at 03:16 PM.] |
Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 23 November 2004 03:24 PM
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The caps are pig-tailed and connected to the volume pot. One cap goes to ground, the other goes to the tone pot. I'm assuming nobody messed with the circuitry. Could there be a cold solder joint even though it looks ok? Would a continuity pick this up?
Thanks, Ron |
Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 23 November 2004 04:05 PM
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I just spoke to a person at the electronics store. When I told him the volume cut out on the 250K pot, he said it's too big and that 200 is 200 ohms and the M-Z has something to do with other specs, maybe temperature. He had a 150 which he said will work. He was very knowledgeable. Does this sound reasonable? |
Bob Leaman
unregistered
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posted 23 November 2004 04:05 PM
It has always been good engineering practice to connect capacitor outer foils to ground (common) so that the outer foil serves as a interference shield. Quality capacitors are usually marked to show which is the outside foil. Any decent engineer knows this practice, particularly where one deals with audio or radio frequency circuits when signals are being by-passed to "Ground". Tone circuits, in guitars, are generally series resistance-capacitor (R-C) circuits where a variable resistor (potentiometer) varies the R-C time constant. However, it seems that sometimes it is forgotten that there must always be some resistance in this series circuit. When the potentiometer is connected so that all resistance is cut out, the capacitor passes vitually all the signal to ground, particularly with high impedance guitar pickups that have very low output signals. The proper connections are: 1. Connect the center potentiometer terminal to one of the outside potentiometer terminals with a wire jumper (Rheostat Operation). Connect the unused potentiometer terminal to the hot output lead of the guitar pickup. Connect a 470 ohm resistor to the two potentiometer terminals that are jumpered together. Connect the other lead of the 470 ohm resistor to the inside foil of the capacitor. Connect the outside foil of the capacitor to the guitar ground (common). Be extremely careful with the use of words "ground" and "common". Isolated commons are used to prevent 50/60Hz signals from entering signal paths via ground loops. When isolated commons are connected to earth ground, common mode rejection qualities are defeated and 50/60 Hz hum results that cannot be eliminated.With the circuit as described, if the signal is lost when the potentiometer is turned to one of its extreme rotational ends, the capacitor is shorted. |
Dave Grafe
Member From: Portland, Oregon, USA
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posted 23 November 2004 06:49 PM
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200 ohms is definitely not enough. |
Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 23 November 2004 07:03 PM
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When I checked it with the meter, it was still wired in. I forgot the rest of the circuit will affect the reading. I will take it out and then know for sure if it's 200 or 200megs. I did some checking around and couldn't even find anything that big. |
Dave Grafe
Member From: Portland, Oregon, USA
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posted 23 November 2004 07:40 PM
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Here we go, from "The Electric Guitar Handbook" by Jack Darr (1977) - common practice with a single pickup is to use matching 500K audio taper pots for volume and tone (low-pass filter) and wired thusly:  I hope this helps Dave[This message was edited by Dave Grafe on 23 November 2004 at 07:48 PM.] |
Blake Hawkins
Member From: Land O'Lakes, Florida
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posted 23 November 2004 09:25 PM
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Ron and Dave:In some old electronic notation M = 1000. I realize that some use M as "meg" but in the case of this pot, I'm sure that the
200M is 200,000 ohms. The "Z" is either the tolerance or the taper.
Since others have covered the proper connection of the circuit I won't go into that.
However, if the value of the cap is higher than .05, then it will kill the audio signal at one end of the range.
I have a Harmony lap steel that came to me with a switch and a cap in the tone circuit.
The cap (and it looked original) was a .1 mfd which pretty well killed the output of the pickup. I changed that to .022 mfd and had something I could use.
Be sure your replacement pot is an audio taper as has been mentioned in one of the other replys.
Even though the ohm meter shows a charge on the cap, it could still be bad because of a change in the capacity.
What we are dealing with here is a piece of antique electronic gear. It is very different from what the young guys in the electronics stores are used to seeing.
Oddly enough, music stores are more likely to have a correct pot than the corner Radio Shack.
I have bought 250 K volume control pots from Sam Ash Music. I believe Guitar Center also has them.
Blake[This message was edited by Blake Hawkins on 23 November 2004 at 09:56 PM.] |
John Bechtel
Member From: Nashville, Tennessee,U.S.A.
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posted 24 November 2004 12:21 AM
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I found my Ernie Ball 250K Audio-Pots at the same Fender dealer where I bought my ’65 Re-issue Twin–Reverb Custom™ 15” Eminence and I use them along with a .0224 disc-cap. There is no gr. preference on a disc-cap. I wire it, in either direction; between the Center-(Out) Terminal of the tone-pot and the Lead-(In) Terminal of the Vol.-pot. then connect the Ground-terminal of both pots and on to the ground-(output jack) of the guitar. This combination affords a nice gradual transition between Bass and Treble on the Tone-Pot, for me! ------------------
“Big John” Bechtel
’49/’50 Fender T–8 Custom [X2]
’65 Re-issue Fender Twin-Reverb Custom™ 15”
click here click here
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Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 24 November 2004 05:27 PM
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Go figure, the 200 M-Z lettering on the pot must have been a manufacturer code. I took the pot out and it checked 500K on the ends and defective. I picked up a new one at Sam Ash and will solder it in tomorrow. Hopefully, that's the problem. |
Blake Hawkins
Member From: Land O'Lakes, Florida
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posted 25 November 2004 04:19 AM
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Good shot, Ron!At least we all tried. Glad you've solved the mystery of the pot value. Blake |
Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 25 November 2004 06:23 AM
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Well, I put the new pot in and it cuts out when turned the other way. The only thing left is a bad cap. I will forge on. Thanks for all the help.
Ron |
Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 25 November 2004 04:35 PM
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It was a bad cap besides the pot. It works fine now. I am sure I will find the same problem with the Harmony as it's doing the same thing. Ron |
Ron Victoria
Member From: Metuchen, New Jersey, USA
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posted 26 November 2004 05:17 AM
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I got the Harmony working also. Again, it was a bad cap. Thanks for all your help. What a great place for resources!!!Ron |
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