Detailed circuit diagrams are available through the Audio Karma Database or EEVblog forums.
Most LC103 issues stem from a few known physical components rather than complex logic failures. Repair Of Sencore Lc103 Service Manual
Specific step-by-step PDF guides for calibrating the LC103 are hosted on the EEVblog Electronics Community Forum . ⚠️ Critical Safety Warning Sencore TC 162 wiring diagram needed for repair - Facebook Detailed circuit diagrams are available through the Audio
Finding a true "Service Manual" (with full theory of operation) is notoriously difficult for the LC103, but the following resources are your best substitutes: Repair Of Sencore Lc103 Service Manual