Remove the top cover and locate the main PC board (it’s to the rear or the chassis and holds the speech option, see the operators manual). R516, R517 and R518 is located in the right hand corner inside the small metal can. 

There will be 4 adjacent resistor positions with 2 positions populated (R517 & R518)the top position is unknown. The small metal can is located on the right hand side of the main board. R455 is located below and to the left of J12 (the speech module connector). R455 is also unpopulated and it’s to the left of R140.

If you can’t find a 603 size surface mounted resistor, carefully unsolder an exsisting resistor and re-populate it in another position. R455 can be replaced with a zero Ohm resistor by shorting the position with a fine strand of wire that was taken from a ribbon cable.

After the modification, the radio will lose it’s memories and will require reprogramming.