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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Symptoms of a Broken Pot?

Discussion in 'Tele-Technical' started by ajsmcs, Oct 10, 2014.

  1. ajsmcs

    ajsmcs TDPRI Member

    16
    Oct 8, 2014
    Rockville, MD
    (I mentioned this in another thread, but I recently added a DPDT switch to my AS Tele, and I've been having some weird trouble. I'm worried I may have overheated it.)

    What are the symptoms of a burnt-up/dead potentiometer?
     

  2. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

    Age:
    71
    May 1, 2003
    Jacksonville, FL
    easy to identify, it won't work... :twisted:

    There are several things that indicate a faulty pot..

    Loose solder tabs.. these are crimped, and soldering and re-soldering can cause 'em to become loose, resulting in intermittent failure.

    The carbon filament inside can become worn to the point that the value changes, or there is no longer enough to conduct, result.. failure..

    Improper solder technique can heat the innards to the point plastic parts distort... result failure...

    Pots are so cheep, 'bout 5 bux, that if you suspect they're bad, replace the thing, but note... freezing pots or choosing calibrated "precision" pots does absolutely nothing but, separate YOU from your money.. the passive electric guitar is a circuit that gains nothing from the use of esoteric potentiometers. All I recommend is using quality in the area of durability. Classic CTS are as good as it need be.

    also, note to the vintage freaks, the guys willing to pay big bux for 50 year old pots, old electronics do not get "better" or more mellow, they only get old... the results are the same as when "we" get to old.... Failure.. :eek:

    Ron Kirn
     

  3. ajsmcs

    ajsmcs TDPRI Member

    16
    Oct 8, 2014
    Rockville, MD
    Its a Dimarzio 250k Push-pull (Audio Taper), which I understand is just a rebranded CTS (correct me if I'm wrong.) I bought it to do out of phase switching. It was VERY difficult to solder grounds to, even after sanding/scuffing up the sides to facilitate soldering.

    I also ended up reworking all of the DPDT connections. Everything looks nice and clean now, but I know the pot had gotten pretty hot at various points when I was working on it.

    It does work, but it does funny things when you start turning it down below about 5. It sounds very quiet (duh) but also kind of distorted. Then it goes quiet. Its like a little blip as you turn it down.

    With the switch down, other than the weird blip everything seems pretty normal. When you pull the switch, it gets really noisy in all but the neck position. I'm sure that has something to do with a bad ground somewhere. Not sure where though. Obviously it has something to do with the way the bridge pickup is connected. But I did a continuity and resistance check of all my ground connections and everything checked out. I know the noise gets quieter when I touch the knob of the pot.

    Is it possible its just dirty? What could cause the distorted sound? Could some of the sanding dust have gotten inside, and maybe that's causing the weird crap?

    You're preaching to the choir on old electronics. I'm an engineer. The only thing I care about is "Does it work?" and "Will it work for a long time?" Plus, if you buy new it will be vintage eventually. ;-)

    Too bad it wasn't just a normal volume pot... push-pulls are way more expensive...
     

  4. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

    Age:
    71
    May 1, 2003
    Jacksonville, FL
    Since you had difficulty soldering, I would bet real money that the insides were distorted... ya can't fix 'em, you will have to replace it..

    also, on quality components, there should be no problem soldering to it, the manufacturers know soldering to the case is standard operating practice, thus the cases are Zinc or Tin plated, both very conducive to soldering.


    It's the Cheep made in lord knows where that have cases not particularly easy to solder to... some will have bright, almost Chrome plating, or some weird yellow looking coating.. or some high precision pots will have Stainless steel cases.. none of those can be realistically soldered to without removing the plating. The Stainless cannot be soldered under normal circumstances...

    save yourself the headache.. replace it... and if ya need practice soldering, go rip the power supply out of that old Pentium 1 computer with Windows Millinium still installed... rip 'er open and solder and re-solder until you can do it quick and neat..

    rk
     

  5. Anode100

    Anode100 Friend of Leo's

    May 9, 2014
    Behind my beard.
    The water runs right out...

    In addition to the excellent advice above, I'd recommend 'scuffing' the back / side of any new pot you solder in place.

    The shiny goodness they coat the body with does not accept solder readily - unless you crank your iron up and risk damaging the pot.

    Scuff the plating off with a small file, and use the scuffed area as a grounding point.
     

  6. ajsmcs

    ajsmcs TDPRI Member

    16
    Oct 8, 2014
    Rockville, MD
    That's what I was afraid of...

    You know, when I did the 4-way switch modification and soldered the grounds to the stock Fender vol pot, everything went super smoothly and easily, even using my Portasol butane iron.

    After having so much trouble grounding to the DPDT pot casing, I used that as an excuse to get one of the nice manufacturing-grade electric Wellers. More precise tip, WAY better temperature control, way more power.

    I guess I'll just start over with a new pot and good iron.

    Are Alpha pots any good? How about Bourns? I'm having a hard time finding any DPDT pots explicitly labled as being made by CTS.

    And what is the standard fender pot shaft diameter? The Dimarzio was definitely much narrower than the hole in the control plate, and it was a butt to get it centered. I wish I'd thought to measure it before I left for work. I'd love to order a new one as soon as possible.

    I used to be into making guitar pedals back when I was in school, so I have a fair amount of PCB-based electronics experience. I made a couple, but got burnt out myself halfway through this frankenstein's monster of a multieffects box I designed..
     

  7. 68Telebass

    68Telebass Tele-Afflicted

    Aug 2, 2014
    Northern Arizona

    In addition to roughening surface with a file to ensure any plating/coating is penetrated, it is helpful to use a paste flux on the area you will be attaching the grounds to- just wipe off the excess with a rag when done-
    The grounding blob of solder will adhere much easier :
     

  8. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

    Age:
    71
    May 1, 2003
    Jacksonville, FL
    How to solder a potentiometer's case..

    on CTS, ALPS, Cornell-Dubblier, alpha (the better ones) nothing other than a rosin core solder (electronics grade) is needed.. and you do NOT need to scrap, sand, or other wise pretreat the metal.. they are designed to facilitate soldering to the case.

    Also the solder... use ONLY solder intended for electronics... it's available in Home Depot, Lowes, True Value and Ace hardwares... but if ya ask for solder and the "helpful hardware man" directs ya to the plumbing department... WRONG... that's for soldering copper tubing together and the flux will eat your electronics... Ya want 60/40 rosin core solder... and the lead free schidt sux... get the
    good stuff with real lead and tin in it.. Radio Shack is fine too..

    next... let the solder tool get good 'n hot... should be at the very least a 20 watt.. more is better... I use a Weller 8200 Solder dual heat solder gun. I use it on high.

    do NOT apply the tip of the solder iron to the case to allow everything to heat up... touch the solder to the case, touch the tip of the tool to that... should melt and flow onto the cae in less than a couple of seconds..

    tin the end of whatever you're soldering to the case... place it on the small blob of solder on the case, , touch it with the solder tool... should again, take a couple of seconds... longer than 5 seconds, you're headed for trouble.


    why?? when you touch the iron to the case, it immediately absorbs the heat... much like if you poured water on a dry towel... the immediate area, where the water is pored, will be soaked, while the outer edges may still be dry... if you continue poring the whole towel becomes soaked..


    Same thing on a pot... touch it with a solder tool and the immediate area will get hot... but leave it in contact, and the whole case will become too hot and the innards will begin melting.. This is because the heat will "flow" to areas cooler than the tip until the whole thing is as hot as the tip... by that time, it's wayyyy too late..

    This is why a seemingly too large solder tool is advantageous.. it can bring the immediate area up to the correct temperature, around 400 degrees F, fast enough so that the solder can flow correctly before the rest of the case gets too hot.

    and . . the above is just one way.. there are others, to be sure... just a little practice and a quality "joint" is easy.


    rk
     
    Last edited: Oct 10, 2014

  9. ajsmcs

    ajsmcs TDPRI Member

    16
    Oct 8, 2014
    Rockville, MD
    Thanks for your help!

    Fortunately, I now have one of these: [​IMG]
    Its a 50W, so next time around I can get in and out before everything gets melty.
     

  10. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

    Age:
    71
    May 1, 2003
    Jacksonville, FL
    that'll work... but don't lick the tip to see if it's hot... :eek:

    r
     

  11. ajsmcs

    ajsmcs TDPRI Member

    16
    Oct 8, 2014
    Rockville, MD
    PROBLEM SOLVED.

    First off, the new Bourns Push-pull is really nice. Its much smoother than the CTS pots the guitar came with, and it just feels a bit more substantial.

    I wired everything up, this time grounding everything to the back of the tone pot. I figured the larger flat surface area would make it easier, and I was right.

    After everything was resoldered, and all connections were checked, I plugged into my AC15, toggled to the Out of Phase setting, and...still noise.

    Pushed the switch back in- no noise. Pulled it out, noise.

    I knew every connection was good. I triple checked everything for continuity and resistance.

    Then I absentmindedly rested my hand on the top of the Vol pot, the hum went away. Same for the Tone pot. But touching the strings or bridge had no effect.

    The electronics weren't being grounded through my hand via the strings and bridge.

    Then it dawned on me - the bridge is grounded through the bridge pickup negative lead. Flipping the out of phase switch puts the bridge ground at a higher electrical potential.

    I pulled everything apart, desoldered the pickups, and put the Neck pickup on the DPDT.

    Voila! No more hum when the switch is pulled.
     

  12. Ronkirn

    Ronkirn Doctor of Teleocity Vendor Member

    Age:
    71
    May 1, 2003
    Jacksonville, FL
    Sweeeet success..

    rk
     

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