Accuphase DC-81L D/A conv - strange distortion on right ch. (service manual anyone?)

[Spyhunter]

Hi, fellow DIYers!

Does anyone happen to have a service manual available for Accuphase DC-81L D/A -converter? It's a bit old model by now
(about 10 years), but I decided to give this a shot.

My problem is that there has been some strange distortion audible lately on the right channel when playing back CDs or even DVDs encoded with PCM. This distortion seems to be heard only on low-level parts of the music and it is particularly easy to hear on dynamic noises, like piano. It sounds a little like digital distortion, because it reminds me of poor sample-rate playback on the computer.

Has anyone had any similar experience with D/A converter? I'd like to get it fixed because excluding this little annoying feature, it's otherwise a nice-soundind converter.

Thanks in advance.