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Re: dodgy pot on electric mandolin?

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surfguy13surfguy13 Frets: 133
in Making & Modding
Just wondering if anyone might be able to shed any light on this?  I have recently bought an old Chris Eccleshall electric mandolin and there is a small issue with the volume pot.  Chris installed a new 250K volume pot when the mandolin was in his workshop to have a new pickup installed.  The pot works fine and graduates up from zero without any issues but as it gets to the last cm of travel the signal cuts out.  Step it back a touch and the signal returns.  The volume pot appears to be correctly wired so my only thoughts are that it is a dodgy pot?  It is a Jap pot not USA.

I will include a photo of the electrics.  The new volume pot is the full sized pot and the tone is the original small pot:

https://i.imgur.com/6oL9pBJ.jpg

https://i.imgur.com/RtLyNF3.jpg
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Comments

  • FunkfingersFunkfingers Frets: 14423
    Does the volume pot feel stiffer in the portion of rotation when the signal is lost? This would suggest a physical problem. It might be worth slackening off the hex nut, checking that the pot is seated correctly, then, re-fastening the pot.*

    If the volume pot has taken a physical impact, it is possible that the wiper loses contact against the resistance track. If pressing down very gently on the pot shaft restores the signal, this is the explanation. 

    It is possible that the innards of the pot have suffered heat damage during soldering. 

    I notice that the grey insulation on the pickup output cable has a melted patch right next to the ground lug of the volume pot. It is possible that enough of the insulation has been lost to expose the signal carrying inner conductor wire. Physical contact between the strands of the pickup conductor wire and the ground lug on the pot would short out the signal.


    * I find it hard to believe that Chris Eccleshall would get this wrong.
    You say, atom bomb. I say, tin of corned beef.
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  • surfguy13surfguy13 Frets: 133


    OK, on further investigation it turns out that Chris gave the previous owner of the mandolin a pot to install rather than doing it himself.  Sorry for the confusion.  However, the guy who installed it seemed quite competent.

    Thanks so much for taking the time to list all the possible issues, really appreciate it.  OK, volume pot feels very smooth throughout the rotation and not at all stiff at all when the pot is full on and the signal is lost.  I have removed the pot to check it over and it looks 100% OK, can't see any damage or anything to suggest that there was any type of impact.  However, I will try gently pressing down on the shaft to see whether this restores the signal.

    Yes, I wondered about heat damage from soldering but not sure how you would identify this?  If it is heat damage, is the only option replacing the pot?

    Yeah, I saw the insulation was melted a little in one spot.  Checked the wire and there is no damage through the grey outer housing and definitely no wires exposed.  However, is it possible for the wires inside the housing to be damaged if they are not exposed?  Also, I guess if it was shorting out wouldn't that mean it would short all the time rather than just when the volume pot was full on? 
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  • ICBMICBM Frets: 72308
    It sounds like a bad pot to me, either with a poor contact between the wiper and the track at that point, or possibly a contact between the wiper and the inside of the casing causing a short to ground - this is rarer but I've come across it twice in the last few months.

    In either case it *may* be fixable by dismantling the pot, but it's easier and probably better to replace it, preferably with a better quality pot. While you're at it, replace the low-quality jack with a Switchcraft as well.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein

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  • surfguy13surfguy13 Frets: 133
    ICBM said:
    It sounds like a bad pot to me, either with a poor contact between the wiper and the track at that point, or possibly a contact between the wiper and the inside of the casing causing a short to ground - this is rarer but I've come across it twice in the last few months.

    In either case it *may* be fixable by dismantling the pot, but it's easier and probably better to replace it, preferably with a better quality pot. While you're at it, replace the low-quality jack with a Switchcraft as well.
    Many thanks @ICBM, appreciate your thoughts.  I think it would be far easier to swap the pot out as I really wouldn't have a clue how to dismantle the pot.  I have a switchcraft jack so that sounds like a plan too.  I will do as @Funkfingers suggested and try gently pressing down on the shaft to see if that resolves the problem.
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  • FunkfingersFunkfingers Frets: 14423
    surfguy13 said:
    I will ... try gently pressing down on the shaft to see if that resolves the problem.
    The pressing down test may identify a physical damage problem. It will not resolve it.

    ICBM said:
    it's easier and probably better to replace it, preferably with a better quality pot. While you're at it, replace the low-quality jack with a Switchcraft as well.
    +1 for combat-tested electronic components. 
    You say, atom bomb. I say, tin of corned beef.
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  • surfguy13surfguy13 Frets: 133
    Funkfingers said:
    surfguy13 said:
    I will ... try gently pressing down on the shaft to see if that resolves the problem.
    The pressing down test may identify a physical damage problem. It will not resolve it.

    ICBM said:
    it's easier and probably better to replace it, preferably with a better quality pot. While you're at it, replace the low-quality jack with a Switchcraft as well.
    +1 for combat-tested electronic components. 
    OK, tried pressing down on the shaft with the mandolin hooked to the amp but no joy, doesn't resolve it sadly.  Definitely worth a try though!  :) 
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  • surfguy13surfguy13 Frets: 133
    I had come to the conclusion that I would need to install a new CTS pot with a wider diameter bushing so was just about to widen the hole by 1.5mm this afternoon and thought I'd plug it in and give it one last try and..........the volume pot is working perfectly!  All I can think is that the problem was gunk in the pot, probably quite a lot, and the Servisol I sprayed into the pot a few days ago has taken its time to dissolve everything.  Anyway, working perfectly with no blank spots and smooth as you like!!

    Thanks for all the advice guys, particularly @ICBM and @Funkfingers.
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  • FunkfingersFunkfingers Frets: 14423
    Gunk happens! ;)
    You say, atom bomb. I say, tin of corned beef.
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  • DartmoorHedgehogDartmoorHedgehog Frets: 891
    I'd have thought a good proportion of folk instruments have at least traces of dodgy pot on them ;)
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  • surfguy13surfguy13 Frets: 133
    DartmoorHedgehog said:
    I'd have thought a good proportion of folk instruments have at least traces of dodgy pot on them ;)
    Yup, and that's what I was aiming for but after a forensic style search I have to report the only type of pot, dodgy or otherwise is electronic!  :( 
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