As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
NR 1 TRANSFER BUS OFF LIGHT ILLUMINATED AT CRUISE FL230. ACCOMPLISHED QRH, BUS WOULD NOT RESET, TRANSFERRED TO APU BUS. GROUNDED - YES. REMOVED AND REPLACED #1 GENERATOR CONTROL UNIT REF AMM 24-21-81/401. INSTALLATION TEST PASSED. PERFORMED OPERATIONAL TEST OF AC GENERATION AND CONTROL SYSTEM PER AMM 24-21-00-700-803 - ALL TESTS PASS.