Yamaha CS80

Hi, writing here for a friend's CS80.
My friend found  a strange problem: when he does set the ringmod's speed at full with modulation at full , the sound coming from the modulator is quite hearable : the fact is that this sound you can hear even without playing anything on the keyboard; same problem happens with the SubOScillator: when he sets the speed at full, the sound can be heard without any key pressed.
I believe there must be something wrong with some preamps in OE1 board or PRA's muting FETs, my idea is to look at R51/52 to see if is there any faulty resistor concerning the Suboscillator.

Anyway I'm looking for your savvy advices.
Thx for any reply.
M 

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