I finally got around to looking into the not-working volume wheel (encoder). I put a strong LED flashlight on it, and it worked. This told me that the IR emitter in the encoder pickup must have died. A voltage measurement across the diode confirmed this suspicion. The entire 5V showed on the meter, indicating that no current flows through the limiter resistor.
I replaced the emitter with a "OPTEK TECHNOLOGY - OP240A - IR EMITTER 890NM 0.76MM SIDE LOOKING" from Newark (08F2957). The device has exactly the same package as the original defective unit and pops right in. It has a wavelength of 890 nm and a 1.8V voltage drop. The encoder works now like new.
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