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This is a followup to Is it possible to double the resistance of a potentiometer?, which solved my AB problem of adding a half-tempo switch to an existing beat generator by suggesting that I add a switch + capacitor in parallel with the circuit's capacitor.

I've got the answer from that question and am stuck on the implementation.

Here's the schematic of the audio board I'm trying to alter (area of interest boxed in red): Original schematic

Here's what I'm trying to do with it according to my understanding of the answer I got for the linked question:

schematic

simulate this circuit – Schematic created using CircuitLab

However, when I touch the new capacitor's legs to the existing capacitor while the beat generator is running (to test the effect) nothing happens.

I'm pretty sure

  • I've identified the right capacitor on the board
  • The replacement capacitor value is correct
  • Both capacitors are unpolarized

I don't know

  • If there's a mismatch in the kind of capacitor on the board vs what I've bought, preventing it from affecting the RC frequency properly because of differing slew rate, hysteresis, etc
  • How to troubleshoot this further, i.e. what some smart next steps would be :/

Edit: Here's a photo of the board, with the capacitor in question highlighted: Picture of full board

And here's the full schematic for the rhythm board:Full schematic

buildsucceeded
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  • Can we see rest of the schematics? – smajli Oct 18 '17 at 21:11
  • @smajli added them! – buildsucceeded Oct 19 '17 at 08:32
  • Capacitor marked 330K on your schematic won't affect much of generated frequency. What will do is a resistance of potentiometer and its branch. Try 470K, or try to increase value of 33K resistors which is in series with the pot. – smajli Oct 19 '17 at 11:01
  • @smajli good call. A 5.8-Ohm resistor in parallel with the 33k seems to double the tempo; I can just wire the switch upside-down to make it a 'half-tempo' switch. (It seems a strange value as I'd expect it to be closer to the existing resistor's value though.) – buildsucceeded Oct 19 '17 at 15:09
  • nice to hear. I wouldn't expect you have to go this far with lowering the value. You can also try to experiment with 220K resistor (not potentiometer). – smajli Oct 20 '17 at 07:45

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