B737-200 CAPT OVERSHOT INTERMEDIATE DSCNT ALT DUE TO USING THE WRONG KNOB DURING SETTING THE AUTOPLT ALT SELECTOR PRIOR TO DSCNT.

Narrative

ON DSCNT INTO DALLAS; AFTER PASSING OVER CEDAR CREEK VOR ON THE DUMPY ARR; I WAS DSNDING FROM 10000 FT TO 9000 FT. AS I WAS APCHING LEVELOFF; I WAS ALSO TURNING ONTO THE OUTBOUND RADIAL FROM CQY. I REACHED UP AND ENGAGED WHAT I THOUGHT WAS THE ALT HOLD AT 9000 FT. HOWEVER; I ENGAGED VOR LOC INSTEAD. AS I WAS FOCUSING ON INTERCEPTING THE RADIAL; THE ACFT CONTINUED TO DSND. THE ALT ALERT WENT OFF AS WE PASSED 8700 FT. I APPLIED PWR AND ARRESTED THE RATE OF DSCNT (WE WENT DOWN TO ABOUT 8550 FT) AND BEGAN AN IMMEDIATE CLB BACK TO 9000 FT. SIMULTANEOUSLY; ATC QUERIED US AS TO OUR ALT AND I REPLIED THAT WE OVERSHOT OUR ALT AND WERE IN THE PROCESS OF CORRECTING. I INQUIRED AS TO WHETHER WE HAD CAUSED A PROB (THERE WAS NO OTHER TFC IN THE AREA) AND THE CTLR SAID NO; THAT HE WAS JUST CHKING ON US. THE REMAINDER OF THE FLT WAS NORMAL. CONTRIBUTING FACTORS: 1) I'M NORMALLY AN EVENING FLYER AND WAS FLYING A MORNING TRIP. 2) THE ACFT WAS A NONSTANDARD -200. 3) THE LAYOUT OF THE MCP POS THE VOR LOC APPROX WHERE THE -200 ALT HOLD IS. 4) THE DIFFERENCES IN THE MCP WERE BREAKING MY NORMAL FLOWS AND HABIT PATTERNS. 5) I'VE BEEN LUCKY ENOUGH TO FLY MOSTLY -300'S AND -700'S LATELY AND WAS NOT USED TO LEVELING OFF MYSELF.