You might not need a guru for this problem. I have a Gryrodec as well and experienced a similar issue. A few questions:
1) Is this an intermittent problem?
2) If you swap the right/left phono leads into the preamp, does the problem move to the other channel?
3) Did you make any changes prior to this problem surfacing?
In my situation the techno arm wire had frayed right at the RCA connector. But losing a channel could be caused by several things.