What was found
(CAN) WENT TO DISCONNECT STAB CHANNEL 1, LOST BOTH STAB TRIM CHANNELS AND MACH TRIM. TRIED TO ENGAGE EITHER STAB CHANNEL WITHOUT SUCCESS. ACTIONED QUICK RESPONSE PROCEDURE AND RETURNED TO AIRPORT USING FLAPS 20 IAW QRH. GREATER THAN NORMAL FORCE ELEVATOR FORCE WAS REQUIRED DURING APPROACH. LANDED WITHOUT FURTHER INCIDENT. THE STAB TRIM DISCONNECT SWITCH WAS CLEANED AND SYSTEM RETESTED SERVICEABLE IAW FUNCTIONAL TEST PROCEDURE P601R4500 PARA 5.6.20, THEN A/C WAS RETURNED TO FLIGHT WITHOUT ANY FURTHER OCCURENCES.