This fixed mine.
The problem lies with the mylar ribbon cable that connects the Keypad to the circuit board. The Mylar ribbon cable folds back onto itself and are touching. Over time the mylar 'breaks down' electrically and the wire traces in the mylar will 'arc' onto the other mylar ribbon cable. The solution is to separate the two mylar ribbon cables using paper and electrical tape. I've done this on several microwaves (not just Samsung) and it has worked every time. Patience and about 20 minutes is all it should take to fix it. I've described below how I did it for the SMH9187ST Microwave Good luck to all.
1. UNPLUG THE MICROWAVE !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
2. 1. Remove the top front cover by removing the (2) screws that are along the top edge (phillips).
3. 2. Slide the cover to the left about a half inch (there are tabs holding the cover in place) and remove.
4. 3. Remove (1) screw from the top of the Keypad Housing (phillips). Lift the Keypad Housing upwards about half an inch (there are tabs holding the Keypad Housing in place) and remove. Be careful of the metal grille that is just above the Keypad Housing as this may fall out since it is being held in place by the Housing.
5. 4. Be careful not to pull on the wiring harnesses that are attached to the Housing. Carefully unplug the wiring harness that has the RED connector on it from the circuit board. Carefully unplug the small white connector that has the mylar ribbon cable on it (this cable is see-through and you can see thin, flat wires in it).
6. 5. Remove (4) screws that are holding the circuit board onto the Housing.
7. The Housing should now be free of the circuit board and wiring harnesses. It's safe to let the wiring harness/circuit board hang free, but caution should be used anyway.
8. 6. Remove the thin metal frame from the Housing. There are (4) plastic tabs holding the frame in place. Using a small screw driver pry one side of the frame away from the housing (try to do this to the side that has smaller plastic tabs). When this side of the frame is free of the Housing you have to lift the frame out at an angle to remove. It might put up a fuss, but be patient and be gentle with it since the frame is thin and fragile.
9. 7. With the frame out of the way you will see the mylar ribbon cable that is coming out from underneath the Keypad assembly. You will notice that the mylar ribbon cable folds back onto itself as it re-enters the Keypad assembly. This 'touching of the two ribbon cables' is where the trouble lies. The ribbon cables may be stuck together. Carefully separate these two and then insert a small piece of paper between them (paper can be 1/2 x 1/2 ) and work the paper all the way down into the Keypad assembly as far as it will go.
10. 8. Using electrical tape, apply a small piece between the ribbon cables to hold the paper in place. Next, use more electrical tape and put it between the mylar ribbon cables enough so as to keep the ribbon cables separated.
11. 9. Re-attached the thin metal frame making sure to press it into the Keypad assembly enough so that the (4) tabs click into place.
12. 10. Use electrical tape and apply it to the raw metal edge of the thin metal frame to prevent the mylar ribbon cables from rubbing on the metal edge.
13. 11. That's it. A final word of caution. Be careful when inserting the ribbon cable connector back on to the circuit board. The connector is loose and you need to gently place the ribbon cable into the circuit board connector.
14. 12. Reverse the order of assembly from above.
15. 13. Plug in the microwave and then it should reset and ask you to set the time.
16. Hope this works for you !!