Installed new pots in 2203, amp doesn't pass guitar sound now

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Janus Alfador, Jun 24, 2019.

  1. Janus Alfador

    Janus Alfador Member

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    I'm hoping for an assessment of what's the problem here. I've just replaced the pots in my 1982 2203, and after soldering in the new pots and turning the amp on to test things, there's no guitar sound being passed by the amp.

    I can hear normal background static hiss which I can manipulate it with the knobs. The Master, Treble, Middle, Bass, and Presence knobs all have the effect of the static that I expect them to when I turn them.

    The Preamp knob, though, doesn't produce a sonic effect on the static hiss that is what I'd expect it to. Instead, it sounds quiet at the start of its sweep, from 0 - 2 the sound gets quieter, from 4 - 7 a low buzz increases in volume, from 7.5 - 9 it gets quieter again. The Preamp knob is also scratchy when it's turned.

    I measured the pots before installing them, and the Preamp knob seemed to read fine.

    I took lots of photos of everything before I desoldered anything, and I think everything looks soldered just as it was before. Here are photos of my soldering compared to the original.


    Original pots:
    [​IMG]


    Original pots closeup:
    [​IMG]



    New pots:
    [​IMG]


    New pots closeup:
    [​IMG]



    By the way, all the original pots are significantly under spec. They read as follows:

    Presence: 20k
    Bass: 798k
    Middle: 20k
    Treble: 160k
    Master: 937k
    Preamp: 789k


    The new Alpha pots I've installed are all above spec, except for the Presence knob, which is slightly under its 25k speck, at 23k:

    Presence: 23k
    Bass: 1.04M
    Middle: 25k
    Treble: 256k
    Master: 1.05M
    Preamp: 1.04M

    I would expect old pots to increase in value as they age from maybe the material on their elements wearing off.
     
  2. HH1978

    HH1978 Member

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    As the wiring looks identical, I would suspect a bad solder joint or a short somewhere. Is it normal that the legs of the 100pf cap on the preamp pot touch the pot casing? I suppose there's some kind of isolation on them, but if some of that melted while soldering, you might actually have a short to ground.
     
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  3. HH1978

    HH1978 Member

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    I meant 1n cap.
     
  4. Tron Pesto

    Tron Pesto Member

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    You're definitely on to something here - looks like that is where the signal is coming in from the Low input jack. If the leg from that cap is touching the pot, basically the entire input (or cascade signal from first stage of the High Input channel) is being dumped to ground. (It's basically is a closed Mute switch.)

    Lifting that cap away from the pot should solve the problem.
     
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  5. Janus Alfador

    Janus Alfador Member

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    You two are correct. I pulled the cap away from the pot housing and the signal passes fine now.

    Thanks!
     
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  6. Bob Arbogast

    Bob Arbogast Supporting Member

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    Excellent!

    Next, shorten the leads on that cap so it won't even be possible for them to touch the pot case.
     
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  7. gldtp99

    gldtp99 Member

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    Janus Alfador and Tron Pesto like this.
  8. Rob s

    Rob s Member

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  9. Tron Pesto

    Tron Pesto Member

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    Another option in a pinch is simply a stripped section of larger gauge connection wire. I keep a lot of my small wire trimmings so I can use them as insulators on legs. On heavier legged components, the inside diameter of the insulation may be too narrow, but with this particular cap, just about any wire insulator should go over the legs nicely.
     
  10. pdf64

    pdf64 Member

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    For such pots, the spec for the track's end to end resistance value's tolerance to nominal is typically +/-20%; so on that basis, all pots reported on are within spec.
    UK / European made audio taper is typically 10%, whereas USA Alpha is typically 15%; so you may find a slight change in the 'user interface' due to that, eg you may need to reduce vol and bass control settings a little.
     
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  11. Dave M

    Dave M Supporting Member

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    In this application don't bother, because ....

    ..... This ^^^
     
  12. jthomas666

    jthomas666 Member

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    File this one under: "Shortest path to ground."
     
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