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Simple? Mathematical formula for two pots connected in series and parallel

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jimdaniel

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Hi there, this is the first time I have posted on this forum, I have what would appear to be a simple task - I'm trying to work out a formula to represent the relationship between the two pots in this circuit:

**broken link removed**

It is actually the pitch control circuit in a Technics 1210 turntable and I'm trying to work out how it works as an educational exercise. I have tried various combinations of series / parallel sums but can't seem to get the correct formula to calculate the total resistance at points A and B, given each pot setting.

Sorry for posting what is probably really basic but I'm trying to understand the calibration technique.

Thank you
 
Linear or Logarithmic?
 
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There is something wrong with your diagram. The lower pot is totally shorted by the upper wiper to B connection.

Missing a ground?
 
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erm, the diagram is correct I think I took it from this, the service manual for the deck. However I've done some more measurments and I think VR303 is either log or special purpose I'm not sure I think I'll look it up.... no wonder I'm confused!

**broken link removed**
 
The value for the lower pot wont make any change for what you measure between A and B with your ohm meter.

So it's just a waste of pots.

[edit]
Your latest post I didn't see. I was talking abourt your first post, to not making confusion.
 
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The way I see it, in your first drawing, the 50K setting is in parallel with the B portion of the 20K, but floating and the a to b value is the a portion of the 20K, but in the schematic, you missed the ground as suggested by Mike.
 

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