I've just used a logic analyser to look at the signals sent to the volume control. When the volume control responds, the signals are exactly what I would expect according to the data sheet of the TC9459, so 24 bits of data followed, clocked in by a clock signal, followed by a strobe pulse.
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The clock rate isn't consistent but I guess that's just the processor doing something else.
That is followed by more data and 32 clock pulses, and a strobe to something else, so I assume that is data being sent to the signal selectors which share the same clock and data lines but not the strobe line. Edit:- It's actually to another digital potentiometer, the one for the direct analogue output.
The data after the strobe does not change when the volume is changed.
When the input is selected, the volume signal is sent with the minimum volume setting, followed by three frames of 32 bytes each:-
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This is sometimes followed by another set of data about 0.7 seconds later.
So the problem is that when the input is changed, the volume reverts to minimum, and then new volume levels are never sent to the digital potentiometer, even though the display on the front of the amplifier shows the volume level as changing.