1. Remove the 750pf caps from the selector switch on both channels.
2. Remove the 27K resistors from the selector switch on both channels.
3. Replace the 68pf cap on the PC-6 pc board with a 820pf silver mica cap.
4. Parallel the 100K resistor with a 2 Meg metal film resistor
5. Replace the 2750pf cap with a parallel combination of 2200pf and 470pf.
6. Replace the 4.7Meg resistor with a 2 Meg metal film resistor
7. Replace the two .1uF caps connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel) with a larger .33 or .47 cap at 400 volts or higher.
If you do the changes above your RIAA equalization should now be off no more than .3dB (3/10 of a dB) of the ideal equalization.