Doepfer

You shouldn't need a vactrol at all.  There is a current limiting 100K
resistor coming from the wiper (center pin) of the Freq pot.  That's
the only place the wiper goes, this 100K resistor should be easy to
locate. One end of that resistor goes to the wiper, the other end goes
to one of the negative inputs of the two opamps.  That means either
pins 2, 6, 9 or 13.  That's the first thing you have to find. So you
buzz out and find the 100K resistor that goes to the center pin of the
Freq pot, go across the other side of that resistor and buzz out which
IC and which pin (2, 6, 9 or 13) that side shorts to.

Once you find  point where the other side of the 100K goes to either
of the two op amp ICs at either pins 2, 6, 9, or 13...then stop
looking.  That's the one you need.

Add your VC Input jack to the faceplate, run it's output to another
resitor you'll need to add (another current limiter, start at 100K and
work down until you're happy with the response, but try not to go
lower than 47K), and connect the output of that added resistor to the
node of the PCB mentioned above. So you run the output of your
resistor to the other end of the resistor on the Dopefer PCB which
comes from the wiper of their Freq pot.

That should do it.

You can add a attenuation pot if you wish as well (if there's room for
both the jack and the added pot on the faceplate, not sure if there is): 

In that case, the output of the jack you add goes to one of the end
pins of the added pot the other end pin of that pot is connected to
ground. The wiper of the added pot (the center pin)is connected to the
current limiter resistor you just added which I explained in the last
paragraph.

If you attenuator works in inverse (less FM when turning the pot CW),
then reverse the wires coming from it's two end pins.

Not sure how the expo converter works on this particular LFO, so you
may not get linear VC response, and most definately not 1v/oct, but
this mod should work OK.

Yes you could do it by added a vactrol, but that would require cuts
traces on the LFO PCB, a bunch of discrete components to drive the
vactrol LED, and the VAC itself.  Basically another board will be
required. It would be more prudent just to buy a Doepfer VC LFO in my
opinion.

It's worth a skipping all that and trying this first.  

- P


--- In Doepfer_a100@yahoogroups.com, Anthony Rolando <goldenechos@...>
wrote:
>
> 
> I would like to add Voltage control to my A-145 LFO. I would like to
do so with a Vactrol. Would anybody be willing to point me in the
right direction? 
> 
> Would it be possible that I could look at a portion of the schematic
so that I could add this?
> 
> Thanx,
> 
> Tony
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