Technician reported concerns after discovering a failed Trim Motor when diagnosing a Horizontal Stabilizer Trim Failure during a gate return. The motor was replaced and the aircraft was returned to service.

Narrative

On taxi out stab trim locked up and did not move electrically from either yoke; returned to gate. Initial inspection verified neither yoke switches would activate horizontal stab movement. Tried stab manual operation with stab wheels on center pedestal; would not turn more than one revolution in either direction. Checked forward nose compartment for any damage or debris in stab chain wheel mechanism and cables; none found. Checked in stab compartment to look for damage or debris; none found. Removed stabilizer trim motor ref AMM 27-XX-XX-XXX-XXX; and manually operated stab wheels full nose up and full nose down and back to green band with no binding or issues; operated normally. Installed new stabilizer trim motor ref AMM 27-XX-XX-XXY-XXX; accomplished stab trim motor test ref AMM 27-XX-XX-XXZ-XXX. Operations checked normal. I am expressing concern about this issue; due to what I found with the horizontal stabilizer not being able to be operated manually with a failed stab trim motor installed. I have never come across this issue before. My educated guess would be if the stab trim electric motor fails to operate electrically; the failed motor would not affect the manual operation of the horizontal stabilizer.