FK1T - VCF build notes (NonLinearCircuits)

These are my build notes for the NLC FK1T VCF.
This VCF is based on the extremely rare Korg FK-1 pedal.
They are hard to find and often sell on Ebay in the $1000AUD range.

The FK1 is a similar VCF to the ones found in the 800dv, 770 and 700 synthesizers.

These early Korg filters consisted of high pass and low pass stages in series.

The virgin faceplate & PCB.
Labels are based on 70s Korg nomenclature as per the original module.

 Korg 800dv

 ..
Mainly surface mount passive components.  ... just one TL074 opamp.
 ...

  No NLC build would be complete without some words of wisdom

 Contemporary Art Hates You - John Waters.

The point of art is to "wreck whatever came before it," he believes. "That something is pretty and beautiful is probably the worst thing you could say today in contemporary art...unless it's so pretty it's nauseating."

This 074 is hopefully the only tricky part of this build.

Solder all the passive SMD components next then the 5 trannies.
The BC857 are PNP & are labeled "p" on the PCB.
The BC847 are NPN & are labeled "n".

Finally the through hole stuff : Resistors, caps, headers.

I'm using Silonex NSL-32 vactrols. They are cheap as chips. The white dot marks the cathode of the LED part of the vactrol. Andrew has labeled this as "K" on the PCB.

The pots are 100k linear (B).

Initial tests were good except for CV input 1 which was dead.
The connection between the jack and the PCB may be dodgy ???

This spot on the PCB responded to CV and it looks like its one of the feedback 100k resistors for the op-amp.

I'll probably build this module again using different vactrols and then see if the same issue with
my CV1 input occurs. It might just be a poor solder connection somewhere. ???
For the moment, I'll probably leave this mod as is. It sounds great.
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Edit ... From Andrew F himself.
"the 220k resistor between cv1 jack and the 100k feedback resistor connects the two. There are definitely traces there. It is not really good to connect the jack directly to the feedback resistor with a jumper, way too sensitive for a start and can damage the op amp.

The two pots ( Travel 1 & 2) may seem to affect each other as it will be a bandpass filter at some (most) settings and each pot will affect one side each (hp & lp) but the overall sound will change.
Inputs 1 & 2 are summed together at the 1st stage."
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 So I'll remove my mod.:)

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To Be Continued. JP.