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Author Topic: modification to change VCO scaling at highest octaves  (Read 482 times)

azone

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modification to change VCO scaling at highest octaves
« on: August 13, 2013, 08:35:46 am »
I remember at some point on the forum it was mentioned that the high frequency scaling was over compensated in the design of the MB so that first 3 to 4 octave are fairly accurate and the last few octaves are sharp. I think there was a resistor you can change to make better. My MB goes sharp and I'm going to be opening it up and would like to change this part if anyone knows the exact details.   

dexfx69

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Re: modification to change VCO scaling at highest octaves
« Reply #1 on: September 09, 2013, 07:01:48 am »
me too

azone

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Re: modification to change VCO scaling at highest octaves
« Reply #2 on: September 21, 2013, 09:01:55 am »
It's a rather simple fix once you know where to look.
There's a simple HI FREQ TRIM being done by a diode and resistor in series that feeds some of the reference current of the exponential converter back to the input. This is done to compensate for the VCO going flat at higher octaves (even when the tracking is good for the first 3 or 4 octaves). The diode is used so that no current is fed back for the first few octaves (because of the voltage drop of the diode). The resistor is chosen to scale the appropriate amount of current (or voltage if easier to view it that way) back to the reference input of the expo. It's a crude but very effective way of tweaking the HI FREQ octaves. In the case of the Minibrute the value of the resistor is rather low so you can actually get too much compensation so the VCO actually goes sharp.

The resistor in question is R89. I think the stock value in mine was 63.4k. I ended up replacing it with 390k for best case for mine. You could replace it with a trimmer or do some trial and error to get something close.
« Last Edit: September 23, 2013, 10:06:34 pm by azone »

 

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