Usher R1.5 DC offset and Bias adjustment

Resistor reading below

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200mV range does not produce any readings, change to 200 and I got 25 and 24v reading
 
recently purchased an usher reference 1.5 fibale, I noticed that the right channel has an offset of 175 mv and the right channel 140 mv, since it does not have an offset adjustment trimmer, I kindly ask for advice on where to change it and if you could send me the wiring diagram, so that a friend who is an expert in the sector can handle it.
my address to send the wiring diagram crimadavide@live.it