Good afternoon, a special greeting from Colombia for the members of this forum A soni HCD-GN880 equipment arrives at our workshop with the problem that the client states that the volume suddenly increased, the equipment was turned off and on turning it on it turned on for 2 or 3 seconds and turned off we could corroborate the fault finding that that really happened then we proceeded to check protections according to the service manual and we found two open fuses, we replaced them and when they turned on they blew again, we then started a removal procedure of stk 412-150 and stk442-130 and with these stk removed we perform ignition tests with normal results we proceed s compare the stk and install them done this work we found that the equipment turned on normal and has audio but with a hum in its outputs we made a trace of the signal that enters the stk to verify if it was carrying the buzzer finding that it is heard perfectly without said buzzing the tracking is done with an external amplifier but in the speakers and woffer you hear such a low-pitched buzzing watch instructions and recommendations grateful in advance Francisco Correa