Yamaha MOX8 boot freeze and distorted sound

[maleksa]

My Yamaha MOX8 synthesizer recently developed two issues.

Firstly, after booting up, it operates as normal. However, after about 5 minutes, the sound starts to get distorted (broken up screeching sound) almost like putting it through a guitar distortion pedal. After a while, the distorted sounds will go away for a while and comes back. At times, the synthesizer will also freeze I need to power off and on again.

Secondly, when powering off and on again, there will be times where the synthesizer will freeze during the boot sequence (freezes at the MOX8 Opening Screen), and does not go to the MOX8 Top Screen. I checked the MOX8 Service Manual, the startup sequence screens are as follows:



Since the freezing happens during the MOX8 Opening Screen (and before the TOP Screen), I reckon that the issue could be either at

1. The USB Host initialisation;
2. DAC IC901 (Digital to Analogue Converter IC); or
3. ADC IC911 (Analogue to Digital Converter IC)

Since the freezing occurs around the time the distorted sounds happens, my deduction is that the problem is with the DAC IC (WM8740). The schematics is as follows:





So my questions are

1. Is it likely that the issue is with the DAC IC?
2. If the DAC is the issue, how can it affect the startup sequence? There does not seem to be any feedback to the processor.
3. Any other things/components that I should check which could cause the distortion sounds and the freezing?
4. Any ways to check/confirm that the DAC (is faulty)?

Any views, help, pointers are much appreciated.