Darko,
The schematic you have is probably the one I scanned from my Owner's manual, and have uploaded on a few sites. Sorry but I don't have the service manual.
The only adjustment trim pots in the CR-420 are for the most important Bias adjustments. They are identified as VR401, for the left channel, and VR402 for the right channel.
When you look at the board from the front of the receiver, VR401 is close to the heat sink, on the left side of the board, and VR402 is on the right side of the board.
For the Bias adjustments, measure the voltage across any one of the 0.47 Ohm power resistors, close to the power transistors. Adjust the corresponding Bias trim pots for a reading of 20mV ±3mV
For the Offset adjustments, it is a modification of my own and it is amply described in my previous thread. You will never see this in the service manual.
In short, the problem is that the collector's currents in both transistors of each differential input stage are not equal and the best way to correct this is by triming the value of resistors R407, for the left channel, and R408, for the right channel.
You can replace R407 and R408 by a multiturn 1.5k trim pot, preferably with the adjustment screw on the top, and pre-adjusted to 1kOhms before soldering in place.
In my experience, and other's experience too, the original offset is negative. This implies that you should reduce the value of the original resistors R407 and R408 in order to bring the offset close to 0mV.
The method I proposed (with the jumpers) is cost effective. It cost nothing in fact because we all have a large assortment of resistors, no? I simply bypass the original resistor with another resistor, and change the value of this resistor (you may use a 10k external potentiometer if you prefer) until the offset is 0mV. Then, I solder this resistor in parallel with the original resistor.
