[sdiy] Deciphering Roland 100 filter schematic

[mark verbos]

On Apr 21, 2008, at 4:15 PM, Aaron Lanterman wrote:

> Howdy,
>
> I'm meeting right now with my student Billy, who is working on  
> making a version of the Roland 100 VCF:
>
> http://fa.utfs.org/diy/roland100/101_op27.jpg
>
> See the bottom half, 3rd "block" from the left.
>
> 1) There's a row of pots at the top of the filter (on the  
> schematic), that is that have notations like "100kA" and "100kB".  
> VR305 is like that too, it's marked "100 kB". Any ideas of what the  
> A and B notations mean?

it's the pot taper. A is audio, B is linear and C is reverse audio


>
> 2) Several of the caps, such as C321, C322, C323 have notations on  
> them like "10/16 N.p" and "100/16." I'm guessing N.p means "not  
> polarized," but I'm not sure about the slash notation. What do  
> those notations mean? They look like something from the mad hatter  
> in Alice in wonderland. (Most of the other caps have microfarad  
> markings.)

value in farads/voltage rating    n.p. is non-polarized

>
> We're also trying to decipher the resonance pot. There's two  
> variable resistors ganged together.

The resonance control compensates for the gain loss as you add  
feedback to the filter.
>
> 3) What do the circles that are tied to ground just below the  
> variable resistors indicate?

those join up with the A and B in circles near the FET on the other  
side of the ladder.

>
> 4) Related to 3 - should we interpret the variable resistors as one  
> end tied to a pot, another end tied to the wiper, and the other end  
> left floating? Or is that ground marking somehow important?

I think they mean to indicate that the wire to the pot is shielded. I  
would wire the pot as you said.

>
> 5) I see what the "right" resonance pot is doing - it looks like it  
> controls the amount of feedback to the negative part of the diff  
> pair at the bottom of the diode ladder. But what is the other pot,  
> the one that connects "A" and "B" (just to the upper right of FET  
> Q314, which looks to be a buffer) doing?

changing the gain going into the filter


>
> 5) Is VR305 a trim to adjust the DC bias point on the left side of  
> the ladder to make it match that on the right? That's my intuition.
>
> - Aaron
>
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