B737-200 CAPT ACCIDENTALLY TURNED THE VOLUME DOWN ON HIS COM RADIO.

Narrative

ZAU FREQ 135.9 OVER GSH VOR ON THE GSH.GHS3 ARR. I WAS PF; FO WAS OFF FREQ ON VHF #2 GETTING MDW ATC. I ASKED ATC FOR A DSCNT CLRNC. I ASKED IF THEY WANTED US TO CROSS AWSUM AT 10000 FT. THERE WAS NO REPLY; SO I CHKED THE RADIO VOLUME AND TURNED IT UP. ATC WAS CALLING US AND WE WERE GIVEN OUR CLRNC TO DSND AND WE EXPEDITED OUR DSCNT. THE B737-200 IS DIFFERENT FROM THE OTHER MODELS WE FLY IN THAT THE RADIO VOLUME CTL IS TOP ON A STACK OF KNOBS THAT ALSO CHANGE THE FREQ. IT IS EASY TO INADVERTENTLY REDUCE THE VOLUME AS IT DOESN'T HAVE DETENT 'STEPS' LIKE THE FREQ CHANGING KNOBS. I HAVE NOT FLOWN A -200 IN A LONG TIME AND FELL VICTIM TO THIS 'CLASSIC' GOTCHA. NO HARM WAS DONE BUT I FELT AN ASAP RPT WAS DUE AS THIS IS A BAD DESIGN FOR THE RADIO. SUPPLEMENTAL INFO FROM ACN 525281: LEG INTO MDW WITH THE CAPT FLYING. I CHKED IN WITH ZAU WITH A GOOD RESPONSE AND SET UP VHF #1 FOR THE NEXT FREQ CHANGE. I TOLD THE CAPT I WOULD BE OFF TO GET THE WX (ATIS); WHICH I DID. I KNEW WE HAD A XING RESTR COMING UP AND WAS MONITORING THE DME (-200) FOR DSCNT. WHEN I CAME BACK; THE CAPT WAS APPARENTLY HAVING DIFFICULTY CONTACTING ATC AS HE ALSO WAS WATCHING THE DSCNT POINT (WHICH HAD COME AND GONE). HE REALIZED THE VOLUME KNOB WAS TURNED DOWN AND CORRECTED IT. ATC GAVE US A VECTOR TO HELP US MAKE THE AWSUM AT 10000 FT RESTR. THE REST WAS UNEVENTFUL.