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I am wiring an electric guitar.

Part of the wiring process is a tone capacitor (0.022 µF) which is attached to a linear taper potentiometer, when the potentiometer is turned, the capacitor is brought into the circuit and takes away the high end treble of the signal, essentially a low pass filter.

I have wired a few guitars and am comfortable with the basic circuitry, I have never had this problem.

I have the guitar wiring hooked up to an amp and I am listening to it, when I heat the solder, the capacitor to one of the arms of the potentiometer I have about 5 seconds of it working as it should, rolling off the treble, being active in the circuit, until it cools down and blam it stops working.

Mind you the cap has two arms, one grounded to the outside of the pot and the other going into one of the arms of the pot, it’s only the one attached to the arm that needs to be warmed.

Other things I have troubleshooted:

  • I have tried four different capacitors, all behave the same way, two of the four are 0.047 µF and the other two are 0.022 µF. One is rated for 400 V the other for 100 V although this doesn’t mean anything in guitar as there is no voltage running through the circuitry.
  • The 400 V one seems to last the longest but my guess is because it can retain the most heat as it is physically the largest and allows the solder to stay warmer longer.
  • I have tried the capacitors in each orientation (if there is an orientation to the capacitor)
  • I have replicated this behaviour of only working when warm dozens of times.

I guess it has to be a cold solder joint problem. Is that suspicion right?
The solder I am using is lead free.

Is it possible the solder is creating an oxidation later as it cools? All other connection in the wiring are working as normal. None of the parts are damaged.

I can provide sound and video and pictures as needed.

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    \$\begingroup\$ Sounds like the pot is broken and works when warm due to thernal expansion. Try another pot, or at least try applying a bit of force to pot leg in different directions, if it works when force applied then you found the problem. \$\endgroup\$ – Justme Jun 2 '19 at 7:26
  • \$\begingroup\$ Thanks, I tried pushing on it but it didnt seem to change anything. The pot looks pretty sound from visual inspection, especially the arm. Its a big old vintage style potentiometer. \$\endgroup\$ – Aaron Carroll Jun 2 '19 at 7:32
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    \$\begingroup\$ @AaronCarroll yeah, all points to the pot; try a different one. \$\endgroup\$ – Marcus Müller Jun 2 '19 at 7:45
  • \$\begingroup\$ Use an ohmmeter/DVM and measure rotor contact (he slider) to either of the ends, as your rotate the pot's wiper. \$\endgroup\$ – analogsystemsrf Jun 2 '19 at 8:09
  • \$\begingroup\$ Thanks @analogsystemsrf Will give that a try and let you know \$\endgroup\$ – Aaron Carroll Jun 2 '19 at 8:13
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If all the caps are showing the same problem, my first guess was the potentiometer is broken. You may have broken it by your inept soldering attempts. It's a mechanical device so it's prone to heat-related problems. The caps aren't.

For the record, how to solder:

  1. remove all the old solder with a soldering iron and solder wick (beginners don't use a pump, and pros seldom do so.)

  2. clean your iron on a wet —wet, not humid— cotton cloth

  3. apply the soldering iron to all the pieces you want to connect for two seconds

  4. apply the solder wire

  5. remove the soldering iron

  6. remove the solder wire

The solder joint must be shiny. If it is dull, you made a mistake. Most likely you moved the parts while the solder was setting. Repeat the whole process. Do not attempt to smear it with more solder.

Oh, and don't use lead-free solder. It's a nuisance for hobbyists. Buy SnPb38Cu2 or SnPb38Ag2. They are very easy to handle. Some SnBiAg solders also are, but much more expensive. Don't buy SnPb40, it's usually without internal flux core, which makes it unfitting for electronics. Buy 0.5mm solder wire. It's easier to handle than thicker ones.

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  • \$\begingroup\$ hey Janka, I appreciate the reply. From inspection im almost certain the potentiometer is not broken ( its a big old school guitar pot and I can look inside and see constant contact between the metal that turns, and the metal that it turns on) The other part is that it does infact work %100 percent perfectly ONLY while the solder is still warm. I also read online that lead free solder does not stay shiny, and all the "dull" looking joints still work correctly in every other part of the guitar. Thansk for the reply \$\endgroup\$ – Aaron Carroll Jun 2 '19 at 7:23
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    \$\begingroup\$ Solder joints get dull over time. They shouldn't be dull from the beginning, that's a cold joint. \$\endgroup\$ – Janka Jun 2 '19 at 7:30
  • \$\begingroup\$ Lead free solder can look duller while leaded solder can look clear and shiny. But the pot might be broken. There might be a crack in the resistive material or the surface it is on. Or just maybe there is some flux residue gotten on the resistive material and it gets sticky when cold. Use a multimeter to measure the pot, if it says it does not work, then it does not. \$\endgroup\$ – Justme Jun 2 '19 at 13:48
  • \$\begingroup\$ @Justme i checked the pots this morning with my multimeter and fortunetly/unfortunetly they are completely fine. So as original expected I thought it was the solder joint itself, the one that allows it to work while warm. I read that a tone capcitor works on a guitar circuit by sending higher frequencies to ground, and out of the final signal, perhaps there is a grounding issue and the higher frequencies are finding a path of least resistance to the output when they should be going to ground? But what would the temperature of the solder have to do with this. \$\endgroup\$ – Aaron Carroll Jun 2 '19 at 17:13

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