ACR HAS COM PROB ON DSCNT; CORRECTS INTERCOM SWITCHES.

Narrative

WHILE COMMUNICATING WITH SAN FRANCISCO CTR ON DSCNT INTO SAN JOSE (CAPT ON THE RADIOS AND FO FLYING) I RECEIVED A LOUD SQUEAL THAT TOTALLY NEGATED MY ABILITY TO COMMUNICATE. APPARENTLY THE FO WAS GIVEN INSTRUCTIONS TO CONTINUE DSCNT TO 6000 FT AND A HDG BY CTR AND WAS HANDED OFF TO BAY APCH. I HEARD NONE OF THIS COM; BUT TRUSTED THE FO. FINALLY I DETERMINED THE PROB WITH MY INTERCOM PANEL. I HAD INADVERTENTLY LEFT THE BUTTON DOWN ON THE ADF FOR SAN JOSE. I DID NOT REALIZE WE HAD CHANGED FREQS TO BAY APCH AND WHILE TRYING TO FIND OUT WHAT WAS CAUSING THE LOUD SQUEAL I TURNED THE RADIOS BACK TO THE CTR FREQ. THE FO WAS UNAWARE OF THIS. AFTER BEING LEVEL AT 6000 FT AND ON A 300 DEG HDG; WE WERE SITUATED FOR A PERFECT DOWNWIND TO RWY 12 AT SAN JOSE. I GOT QUIET AND I QUERIED CTR ABOUT A HDOF TO APCH CTL. THEY THEN TOLD ME TO CONTACT BAY APCH. THE PROB WAS SOLVED. NO CONFLICTS CAUSED AND A NORMAL LNDG WAS MADE. SOLUTION: BE CAREFUL USING ACFT INTERCOM WITH PUSH BUTTONS. WHILE LOOKING DIRECTLY DOWN AT THE PANEL IT IS DIFFICULT TO DETERMINE WHETHER THE BUTTONS ARE DEPRESSED.