Voltage divider network calculation
The circuit can be redrawn (always a good idea) like this:







simulate this circuit &ndash; Schematic created using CircuitLab

There are a couple of approaches to solving the voltage value for pin 3. The one that is likely taught earlier would use a process of applying Thevenin equivalents. Let's do that:







simulate this circuit

Here, \$V_\text{TH}=9\:\text{V}\frac{R_{83}+R_{85}}{R_{83}+R_{85}+R_{113}}\$ and \$R_\text{TH}=\frac{R_{113}\cdot\left(R_{83}+R_{85}\right)}{R_{83}+R_{85}+R_{113}}\$.

At this point, it's a simple voltage divider. Let's pause for a moment and look at it more closely before moving on to the next step. Here, you can see that \$R_\text{TH}\$ is followed by a \$1\:\text{M}\Omega\$ resistor. Since \$R_\text{TH}\$ can't be very large by comparison (a few thousands of Ohms, but nothing close to the value of \$R_{114}\$), we can easily see even at this point that the potentiometer can only vary the pin 3 voltage a little bit. I think that's clear at this point, already.

The above was step 1. But there is another step yet needed to get the pin voltage:

$$\begin{align*}
V_\text{pin 3} &amp;= V_\text{TH}\frac{R_{86}}{R_\text{TH}+R_{114}+R_{86}}\tag{Step 2}\\\\
&amp;=9\:\text{V}\cdot\frac{R_{83}+R_{85}}{R_{83}+R_{85}+R_{113}}\cdot\frac{R_{86}}{\frac{R_{113}\cdot\left(R_{83}+R_{85}\right)}{R_{83}+R_{85}+R_{113}}+R_{114}+R_{86}}\\\\
&amp;=9\:\text{V}\cdot R_{86}\cdot\frac{R_{83}+R_{85}}{R_{113}\cdot\left(R_{83}+R_{85}\right)+\left(R_{114}+R_{86}\right)\cdot\left(R_{83}+R_{85}+R_{113}\right)}\\\\
&amp;=9\:\text{V}\cdot \frac{R_{86}}{R_{113}+\left(R_{114}+R_{86}\right)\cdot\left(1+\frac{R_{113}}{R_{83}+R_{85}}\right)}\tag{Final}
\end{align*}$$

That last step is actually very helpful because it isolates potentiometer \$R_{83}\$ and allows you to see better what impact it has on the whole structure of the answer here.

Another approach would be to use nodal analysis and solve the simultaneous equations (two of them.) But the result would be the same and the above relies on limited knowledge of less powerful tools (if perhaps still some algebra.)



I would still like to hear about what you find in your process of tracking down the problem, too. Please let us know what you find.