[sdiy] TH XR2206 VCO tune pot

[Derek Holzer]

If you look at the schematic:

http://www.bugbrand.co.uk/docs/TH_XR2206VCO_BugPCB.PDF

You'll see that the fine tune and main tune are essentially the same: 
100K pots between V+ and V-. The only difference is that the wiper of 
the fine tune goes through a 3M3 resistor, and the main tune goes 
through a 100K. They are both summed after that. That's the part I'm 
trying to understand.

D.

Scott Gravenhorst wrote:
> Derek Holzer <derek at umatic.nl> wrote:
>> Hi Scott,
>>
>> thanks for your reply. I did think of this, but since the range of 
>> another pot in the schemo is restricted by following it with a larger 
>> resistor, rather than putting resistors between V+/V- and the pot, I 
>> thought that I would try to understand how that worked.
>>
>> best!
>> Derek
> 
> I can understand how that could modify one end of the range, but not both.  I'll be interested to
> hear how that works.
> 
>> Scott Gravenhorst wrote:
>>
>>> You can restrict the range of the pot this way:
>>>
>>>      v+
>>>      |
>>>      |
>>>      Ra
>>>      |                                            
>>>      |                                       
>>>      P
>>>      O<----- ...
>>>      T
>>>      |
>>>      |
>>>      Rb
>>>      |
>>>      |                                  
>>>      V-     
>>>
>>> Where POT is the original tuning pot and Ra and Rb are resistors you will add.  The
>>> values of these resistors will have to be trimmed or calculated by you.  If Ra and Rb
>>> are the same resistance as the pot, then the new range will begin at about a third of
>>> where it was without Ra and Rb and it will end about 1/3 before it gets to the end.  I
>>> can't guess what frequencies this will give...  Ra and Rb can also be trimmers so that
>>> you can adjust the exact range you get.
>> -- 
>> ::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
>> http://www.vimeo.com/macumbista :::
>> ---Oblique Strategy # 101:
>> "It is simply a matter of work"
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> 
> -- ScottG
> ________________________________________________________________________
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-- 
::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
http://www.vimeo.com/macumbista :::
---Oblique Strategy # 152:
"Tape your mouth"