NAD C370 protect mode because right channel

If you would lift the leg of R428 (the side that is connected to the pre-driver) then take this leg and jumper it
to chassis ground (as per @floriant suggestion) this should simulate a "good" pre-driver stage with 0V DC noise (offset).

Then measure VOP R to see if the value is in the <100mV range rather than 2.5V [2500mV]

If <100mV then the driver/poweramp stage is clean for offset and you can concentrate on the pre-driver (comparator) stage.
 
Now its problem of idling current.
Its 100mV... normally...5mV.
At first I managed to adjust 5mV, but after three days it became 100mV.
DC offset OK....0V
Right channel is very hot.

Impossible adjust idling current via VR402.

But Amplifier works fine.
Thanks
 
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Your initial checking into the protect mode you said that Q415 thru Q417 checked out OK.
Did you verify with a diode test or transistor tester? Could be a weak transistor failing after hours of heat /on-time.

Might be the VR402 is failing?

Also would not pass up checking C411 + C412 (or just replacing if these are crappy caps).

Is the left channel bias maintaining around the 5mV range?