Help! Faulty Onkyo Integra TX-DS838

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I have an Onkyo Integra TX-DS838 that was brought in for repairs by a client. Just for interest sake there is no Onkyo agent in South Africa so I had to scour the Internet for a service manual. Thank God I found one. The client said that the sound was extremely soft with the volume knob full clockwise. I tested the power amplifiers sections labelled FR, CNT and FL and these checked out just fine when I fed them with a 1kHz test signal. I discovered that the problem seemed to be the muting circuitry on the volume board or the MCU that is responsible for controlling the muting circuitry in addition to a host of other things. The method I used to establish this was I desoldered one of the pins from the connector on the volume board labelled FMUT and thereafter the FR and FL channels played at their proper volumes.

The problem I now have is that I need to to know what could be causing the MCU to keep the muting circuit latched and what steps do I need to take to get it resolved. Any information will be most appreciated.
 
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