DURING AN INST TRAINING FLT IN A BEECH 36; THE COM FREQ WAS ACCIDENTALLY KNOCKED OFF SETTING AT THE BEGINNING OF AN ILS APCH CAUSING A BREAK IN 2-WAY COM WITH APCH CTL.

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INST TRAINING FLT FOR INST PROFICIENCY CHK. ON A VECTOR FOR ILS RWY 28; CIRCLE TO LAND RWY 25; HANDED OFF TO ROC APCH. ESTABLISHED COM AND RESPONDED TO SOME TA'S. AFTER A FEW MINS; I GOT CONCERNED BECAUSE OF LACK OF RADIO TFC. THE AIRPLANE HAD PRETTY FANCY RADIOS; INCLUDING ONE THAT STORED UP TO 4 FREQS. APPARENTLY; WHILE SETTING UP RADIOS FOR THE ILS APCH; THE CTLR FREQ GOT KNOCKED OFF. UPON RESTORING IT; THE STUDENT PROMPTLY ANNOUNCED TO THE CTLR THAT WE HAD BEEN OFF THE FREQ FOR A WHILE. THIS RESULTED IN A LECTURE FROM THE CTLR ABOUT TRYING TO CALL US FOR THE LAST 15 MI TO WARN US ABOUT TFC; DON'T EVER LEAVE FREQ WITHOUT PERMISSION. UNFORTUNATELY; MY XMISSION CAPABILITY WOULD NOT WORK; SO THE STUDENT; WHO WAS PRETTY SLOW IN TALKING; WAS HAVING TO DO ALL THE TALKING. THE CTLR WAS GETTING MORE UNHAPPY AND BY THE TIME WE WERE HANDED OFF TO THE TWR; SHE WAS SPEAKING SLOWLY; LOUDLY AND ENUNCIATING HER INSTRUCTIONS VERY CAREFULLY. WE WERE WRONG IN NOT CATCHING THE FACT THAT THE FREQ WAS SET WRONG. MY ERROR WAS NOT HAVING SOME MEANS TO XMIT DIRECTLY AND NOT HAVE TO PROMPT STUDENT ON FIRST IFR PRACTICE FLT.