BEMI 208 pitch tracking mod?

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QuantumMechanakillWave
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BEMI 208 pitch tracking mod?

Post by QuantumMechanakillWave » Fri Jul 16, 2021 8:19 pm

Anyone modded the BEMI 208e for pitch tracking at the Frequency and Pitch inputs for the Modulation oscillator and Complex oscillator respectively?

I have two 218e’s and would love to control the pitch of each oscillator independently.

Just curious if anyone has done it, how they did it and if they are happy with the results.

Thanks!
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QuantumMechanakillWave
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Re: BEMI 208 pitch tracking mod?

Post by QuantumMechanakillWave » Tue Jul 20, 2021 2:30 pm

:hmm: Anyone?
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Re: BEMI 208 pitch tracking mod?

Post by tarandfeathers » Thu Jul 22, 2021 3:55 am

Assuming the circuit is similar to the original, it won't be trivial to do this. The pitch/frequency inputs of the oscillators are intended to be more sensitive than the input from the keyboard CV, so they will need padding down to match it, and that in turn will mean that the full range of control is no longer available on that input. If you're okey with that, you'll need to cut a trace between the series resistor from the slider and add a trimmer in line which you'll need to figure out an appropriate value for and how to mount it mechanically, which might be annoying - it might be easier to do this on card 6/7 than on the motherboard . You'll then need to match the tracking between the keyboard trimmer and the new trimmer you've added, with the pitch/frequency slider in a known position, likely all the way at the top as this is the only real physical reference possible. Unfortunately, since the pitch input CV goes through the slider, which is not a particularly precision device, it's likely that this will need regular adjustment.

I'd probably just stick to tuning it by ear when you want to use it this way.
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Re: BEMI 208 pitch tracking mod?

Post by Scrog1er » Thu Jul 22, 2021 7:17 am

Doable over midi with the Easel Command….
This is understandably useless to you, sorry 😞

Perhaps a Program Card could be modified, or try lay your mittens on a Meta Expander

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Re: BEMI 208 pitch tracking mod?

Post by QuantumMechanakillWave » Fri Jul 23, 2021 10:33 am

Hi! Thank you for the answer. Actually, the inputs are less sensitive. A 1.2 volt per octave signal only produces a change of slightly less than 8 semitones per 1.2 volt. So, the series resistance will actually have to be lowered. This means that I will have to just reduce the attenuator levels to reproduce the pre-mod ranges.

The mod should be fairly easy. Just reducing the series resistor inline with the attenuator and possibly adding a trimpot for fine adjustments. I am just curious what values folks have landed on and if they had any issues after doing so.
tarandfeathers wrote:
Thu Jul 22, 2021 3:55 am
Assuming the circuit is similar to the original, it won't be trivial to do this. The pitch/frequency inputs of the oscillators are intended to be more sensitive than the input from the keyboard CV, so they will need padding down to match it, and that in turn will mean that the full range of control is no longer available on that input. If you're okey with that, you'll need to cut a trace between the series resistor from the slider and add a trimmer in line which you'll need to figure out an appropriate value for and how to mount it mechanically, which might be annoying - it might be easier to do this on card 6/7 than on the motherboard . You'll then need to match the tracking between the keyboard trimmer and the new trimmer you've added, with the pitch/frequency slider in a known position, likely all the way at the top as this is the only real physical reference possible. Unfortunately, since the pitch input CV goes through the slider, which is not a particularly precision device, it's likely that this will need regular adjustment.

I'd probably just stick to tuning it by ear when you want to use it this way.
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Re: BEMI 208 pitch tracking mod?

Post by CliffordMilk » Sat Jul 24, 2021 4:01 am

Depending on the revision of boards you have on the BEMI 208, it could be that there is already a trimmer in place as the scaling resistor. If you a trimmer hotglued to boards 6 and 7, this is it. I haven’t done this myself but I am thinking you could would need to recalibrate for these inputs. Set the range trimmers on the back of the motherboard to the middle, adjust the trimmers on the card to get the first octave interval and then use the trimmers on the front for the rest.

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