Date: 201703
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Horizontal Stabilizer Control
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: Our inbound flight to ZZZZ arrived a few minutes late. My First Officer (FO) and I walked down to aircraft as passengers finished deplaning to get a head start on our preflight duties.The inbound captain told us about a write up dealing with a discrepancy in the altimeters and the effect it had on VNAV in the FMC not reaching top of climb.As FO performed the exterior preflight I spoke with maintenance as he started doing some BITE tests in the CDU. I asked if he would like me to hold off boarding; he said no it wouldn't be necessary; he then took the logbook and left the aircraft; I asked if it would be OK for us to set up cockpit; or he if he would need to get back in; he said yes we could do what we needed to do.FO and I completed the interior through flight check; set up the FMC; contacted dispatch to get a valid Takeoff Performance System (TPS) that we were not able to pull up by ACARs. We had completed all our checklist items up to the 10 minutes prior to departure.Passengers were now boarding; another maintenance person came about 15 minutes prior to departure and asked were the logbook was; I said you (as in maintenance had it). Appropriately 10 minutes prior to departure two maintenance men and a maintenance supervisor came to the cockpit. They needed to sit in my seat as they did some last minute things in the CDU. I stood in the galley area and observed their work on the CDU. Maintenance then said they were done and quickly left the cockpit; I asked if everything was good; they said yes everything checked out OK. No mention of any flight deck switches they had turned off or on. As I read the logbook sign off I noticed it was a radio altimeter system checked.As per our 737 Operations Manual when maintenance has been performed the preflight items associated with that system needed to be checked. Both the FO and I reviewed the CDU; and verified all pages after load close out. This was the system item we rechecked as this is what maintenance had worked on with the bite check; and they never mentioned any other systems components; nor did I observe any other.We pushed back late; I had not been in ZZZ in a while; very congested and busy. FO and I even brought up the Threat and Error Management (TEM) model saying we were in the green but not to rush anything as we had been up since [before sunrise] and had already flown [one leg] that morning. We were cleared for Takeoff and FO was flying; out of about 3000 feet the master caution light illuminated. Speed trim light on the overhead panel was illuminated. I had a FO continue to fly and work with ATC; I ran the QRH checklist. The Checklist stated to continue the flight. At about 6000 feet FO tried to engage the auto pilot. It would not engage; we tried recycling the flight directors; verified the aircraft was in trim and checked appropriate circuit breakers.As we continued to climb I contacted ZZZ maintenance to see if they had any ideas to troubleshoot this; they had none other than what we had already tried. We discussed continuing to ZZZZ without an autopilot in RVSM was not an option. At this point our weight was 149.0. Knowing we were going to return to ZZZ I took this opportunity to contact operations; advise the flight attendants [and] passengers of our situation and then run the emergency landing checklist in an effort to make sure all items were accomplished and allow us to continue to burn down toward our max landing weight. Finally we [advised ATC of emergency condition] as we would be overweight landing back in ZZZ we received vectors to XYL; final touch down weight was about 3;000 LBS over max landing. In the descent back into ZZZ we received an ACARS message from tech advising us to check the auto pilot stab trim. Indeed it was off; once selected on the auto pilot engaged. A quick discussion ensued whether we should continue back to ZZZZ; however having [advised ATC] and having burned fuel in flight we electedto return to ZZZ. The landing was normal. At the gate we waited for maintenance to show up and discussed how this could've been missed. It was a unique situation which slipped through some barriers.1. Maintenance and never advised us they had switched the auto pilot stab trim off; we completed all of our preflight checklist items before they had turned the switch off; and following SOP when maintenance had worked on a component we focused on the CDU; as no other items were mentioned by maintenance.2. We still had electric trim usage.3. I thought the auto pilot not engaging may have been more of an electronics issue after receiving the speed trim light. 4. From my vantage point in the left seat the stab trim cut out switch to the auto pilot is not very visible and can be easy to look over if not specifically looking for it.One take away from this event that I will implement in the future is regardless what maintenance is working on in the cockpit I will do a complete interior preflight.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737-800 flight crew reported during climb the Master Caution light came on and a Speed Trim light on the overhead panel illuminated.