Cessna Citation 525 pilot reported pitch trim malfunction in cruise flight. After the flight maintenance found the nose down cable was jammed.

Narrative

Performed a trip from home base ZZZ to ZZZ1 to pick up passengers and flew to ZZZ2. All flights were uneventful. On the return trip from ZZZ2 to ZZZ; I leveled off in cruise with the autopilot engaged and the aircraft began to speed up; I got an amber caution message to re trim nose down. I canceled the caution and read the appropriate checklist from the QRH; which directed me to disengage the autopilot and re trim the aircraft manually. I attempted to use the electric pitch trim; but it would not move the trim wheel. I then manually adjusted the pitch trim nose down; noticing it was difficult to move in both directions. I landed uneventfully at our home base ZZZ and called my maintenance contact. They requested I fly the aircraft back to their shop for further investigation. I agreed; and returned the next day.On departure from ZZZ; I flew the aircraft without the use of the autopilot and manually tried to adjust the trim; but it would barely move; so I left it in place (in the takeoff range) and managed the control forces by reducing my speed. I again landed uneventfully and maintenance subsequently discovered the pitch trim cables were twisted several times over themselves and the nose down cable was jammed in one of the stop blocks for limiting travel. A plastic guide bushing had fallen out of the block and the nose down cable became wedged in the block. The cables were twisted just aft of the rear pressure vessel bulkhead.Maintenance unjammed the nose down cable and they immediately sprung back to their untangled position. They replaced the plastic bushing and re-rigged the system. The aircraft had just come out of a (36 month) Inspection before the initial trip to ZZZ2. The pitch trim system was tested and checked; but no faults were noted and the system was not worked on.