Korg Mono/Poly

Hi

> Can you explain me a little how this pot works and what should I do
> to test and/or fix it ?
The working principle of the portamento circuit is very simple. There is 
an capacitor between ground and the control voltage line from the 
keyboard for the VCO pitch. If the keyboard control voltage changes, 
then its power will first be used to load the capacitor. This will take 
some time. So the voltage change will not be immediate, but will 
increase slowly. This causes at the VCO what we call portamento.
(The same happens if the voltage drops. Then the unloading will take 
some time).

As said, loading or unloading the capacitor always takes a little time, 
but if the CV comes with enough current (if the power is high enough 
(remember: power = voltage x current), then the loading or unloading of 
the capacitor is that fast, that our ear does not recognize it.

The portamento potentiometer now simply acts as a variable resistor (so 
called rheostat) which is inserted in the CV-line before the capacitor. 
If the poti is turned fully counterclockwise then the resulting resistor 
is zero -> the capacitor will be loaded that fast, that we do not 
recognize it.

If the poti now gets turned clockwise, the resulting resistor is 
increased -> the increased resistor means less current runs into the 
capacitor -> the capacitor will be loaded slowlier -> the pitch changes 
slowlier -> portamento.



> Which is the line for VCO4 to shortcut and
> how can do this ?

I cannot tell you exactly which layer of this four layer potentiometer 
is the one for the VCO4, but I found an article in the web, which 
describes the principle of our problem quite good


Have a look at http://www.allaboutcircuits.com/vol_6/chpt_3/7.html

in the picture
http://sub.allaboutcircuits.com/images/05151.png
http://sub.allaboutcircuits.com/images/05150.png
you have to think the battery as the keyboard CV and the motor as the 
capacitor. That is how the portamento potentiometer is wired.




The picture
http://sub.allaboutcircuits.com/images/05152.png
describes, what might be broken in your portamento potentiometer.

If you connect both the outer connectors of the potentiometer with each 
other, then it is like the potentiometers resistance would not be set to 
zero.





> Does it contain graphite like the sliders on JX3p
> ?
Yes. All potentiometers work like that, but you can't repair it with an 
graphite spray!

Btw: it does not have to be the potentiometer itself. It could be that 
the corresponding solder joint is corrupted.

Florian