A C210 pilot landed gear up because he was distracted on a test flight while attempting to resolve an intermittent alternator fault by cycling the landing gear and forgot it had been raised on the previous circuit.

Narrative

The aircraft had just come out of repair of a failed alternator. We were trying to leave on vacation. Upon starting the engine; the alternator again failed; but after shut down and restart; it worked till just after takeoff; then failed again. So I landed uneventfully and returned to the FBO. During taxi it restarted then briefly failed again. The Mechanic then offered to come with me for a test flight to see if an over-voltage transient could be triggering the problem. At his suggestion; we stayed in the traffic pattern; cycling the gear up and down a few times; but could not duplicate the problem. After the first touch and go we still hadn't seen a problem. On downwind; I lowered the gear as usual and did a GUMPS check. But the Mechanic then suggested cycling the gear once more. I raised it; but failed to lower it. Just before touchdown; the Tower noticed the lack of gear and called for a go-around. But it was too late; and we landed gear up; with full power. Lessons: 1) Do maintenance work at altitude away from the airport. 2) Sterile cockpit from downwind on. And 3) report GUMPS check on final.