Voltage to PWM Circuit, need to understand frequency
Your circuit will look something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

Help understanding how to use a SG3525

And the PWM frequency will depend on \$C_2\$ and \$R_6\$ values. 

\$Q_1\$ together with \$R_6 , D_2 , D_3\$ forms a constant current source. 

And this current will be around \$I_S = \frac{0.66V}{R_6} = 140\mu\textrm{A} 
\$  

And this current will charge the \$C_2\$ capacitor. And if the voltage across the capacitor reaches the NE555 threshold value (2/3 Vsup). The 555 will quickly discharge the capacitor to 1/3Vsup.   

So, the equation for PWM frequency will look like this   

\$\Large F \approx \frac{3 \cdot I_S}{C_2 \cdot V_+}  \$

So for 5V supply, as you have in your circuit the equation becomes 

\$\Large F \approx 0.6 \cdot\frac{I_S}{C_2}  \$


  I'm trying to simplify all the resistors and jumper leading to U2+
  (R9,R10,R1,R3,R4). Suppose I don't need the option to jumper to a 0-5V
  input and only using 0-10V input. Also would like to eliminate
  adjustability with R1. From testing I know that for best function R1
  is set at 8k Ohm between jumper above R3, and 94k Ohm to ground below
  R3.


In theory, this voltage divider at the input should do the job: