[sdiy] Boss PC-2 CT pot replacement

[aankrom]

 This may not be a good solution for everybody, but I take apart a LOT 
 of old electronics. Eventually, you find some CT-pots. I have a whole 
 slew of them. I even have a quad-ganged 50kOhm pot and they're all 
 center-tapped. (It was from a quad receiver that didn't work...). I also 
 have a CT dual-ganged 50kOhm pot that I plan to use in that cool-looking 
 Japaenese "Multiflanger" project... Anyway, getting parts from unused & 
 broken electronics is a great way to fill your parts bin - even with 
 esoteric parts.

 Another thought: I wonder if you could use one of those IC 
 potentiometers?


 On Fri, 9 Sep 2011 12:08:29 +0100, Ben Barwise <clackjunk at gmail.com> 
 wrote:
> Hello,
> For a project I have been working through and re-drawing old simple
> drum synthesizers and making Strip-board layouts / PCB's for them and
> I am going to be publishing it here
> http://clacktronics.co.uk/information/drumlab
>
> Because the PC-2 uses op-amps the envelope doesn't go to the negative
> rail - it floats at about 1 volt- schematic is here
>
> http://img821.imageshack.us/img821/4396/amdekpck100schematic.gif
>
> what I have been trying to do is make a version of it so people can
> build it with more widely available parts (replacing the BA662 with a
> LM13700, I had to use a couple of LED's from its -Ve pin to 0v to 
> make
> sure it properly closes )
>
> My problem is that I have been trying to find an alternative to the 
> CT
> potentiometer for sweep, the best solution I came up with was to use 
> a
> switch here is my progress ... bottom part is were I am at
>
> http://img15.imageshack.us/img15/3672/pc2scem.jpg
>
> but the problem is that when the sweep is off by putting the pot CCW
> the pitch changes when you switch because of this offset on the
> envelope.
> I was wondering if anyone could give me tips on how I could null this
> offset (Is it not possible??)
>
> I have tried to put resistors everywhere but I cant seem to get it to 
> work!
>
> Regards
> -Ben
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