What was found
DURING CLIMB-OUT, EN ROUTE, AT APPROXIMATELY 7000 FT, THE CREW REPORTED STAB AND MACH TRIM FAILURE. AFTER COMPLYING WITH THE QRH THE TRIMS STILL COULD NOT BE ENGAGED. AN EMERGENCY WAS DECLARED AND THE AIRCRAFT LANDED SAFELY AFTER RETURNING. MAINTENANCE INTERROGATED THE MDC AND FOUND THE FIRST OFFICERS STAB DISCONNECT SWITCH JAMMED. MAINTENANCE REMOVED AND REPLACED THE JAMMED DISCONNECT SWITCH. OPERATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE.