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Along the top of the circuit board of my bass are three small, black rectangular switches. It's an active bass, meaning it contains a 9V block battery in another, smaller compartment (that's where the black/red cables on the right come from). There aren't any marks on the switches except the tiny letters "MKK" on each one (hard to read, not sure if I got it right, if I had to guess I'd say it looks like a brand name).

The rightmost switch seems to turn the sound on and off completely, as in the off position the bass doesn't output any audible sound. The other two seem to only increase/decrease the volume by a little bit (might affect tone, but I didn't notice much difference except the slight volume change). When the first switch is off, the other two have no noticeable effect at all (bass still doesn't output any sound).

What are these switches called, if anything, and what are they for?

Circuit board

Front view

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  • If those two "soap bars" towards the neck are humbuckers, then the modest change in volume may indicate the switches are coil tapping (single-coil emulation)
    – Yorik
    Oct 9 '20 at 16:35
  • @Yorik it's fair that you take guesses, but this is frankly a pretty bogus one. That's a P-bass pickup; using only one of the coils would mute two of the strings so that would be stupid and obvious. (In principle it would be possible to wire the coils in parallel instead of series; on a passive bass that would reduce the output level whilst notably brightening the sound, but on an active bass it wouldn't really make any sense.) Oct 9 '20 at 17:01
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    Can you tell us any brand or model for the bass, pickups, and/or preamp? Oct 9 '20 at 17:04
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    The question, I'm afraid, isn't aswerable unless you can paste a schematic (or somebody happens to know it; it would certainly become less unlikely if you mentioned what model it actually is). Oct 9 '20 at 17:05
  • I mean, there are humbucking P-style pickups. Dimarzio makes one.
    – Yorik
    Oct 9 '20 at 17:22
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One possible explanation (but I'm sure there can be many others) is that this circuit board was designed to be configurable for different bass models and different pickups.

Why? Probably because that'd be cheaper and easier than having to design, produce, inventory, etc. multiple similar boards for different instrument models.

This way, instead, you have a multi-purpose board that can be easily configured in the right way when assembling the instrument, depending on pickups, model, etc.

For the same reason, while I could be wrong, I don't think it's a single coil/humbucker switch, because if it were, there would be no point in placing it there -- you'd want to make that switch accessible to the user, which is something that is commonly done e.g. by pressing the potentiometer toward the body, or with a regular switch on the body.

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  • Any idea which setting could be the intended one? The louder one or the quieter one?
    – MaxD
    Oct 10 '20 at 11:15
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    If the quality of sound is otherwise the same, I'd go for the louder one. Another reasonable approach would be to check for noise levels: plug it into an amp, with high volume, but without playing anything, and check the background noise that either positions produce. Whichever position ultimately produces the best signal to noise ratio, is probably the best.
    – MMazzon
    Oct 10 '20 at 11:46

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