This photo shows the location of R1 on the audio preamp board ( 80356 ) which has been replaced with a 20K variable potentiometer. A good time to modify the side-tone is when servicing the PTO because the audio preamp board will need to lifted up as shown below to get to one of the screws holding the PTO to the main chassis. The proper value of the R1 replacement will depend on the actual values used in the side-tone oscillator as described earlier in the side-tone oscillator analysis. I found an error in the analysis. Where it says "The time that the LM3900 output is low is determined by how long it takes for C1’s voltage to drop so that v/R3 < 12 microamperes." should be "The time that the LM3900 output is low is determined by how long it takes for C1’s voltage to drop so that v/R2 < 12 microamperes.".