CITATION 525 PLT LOST COM ON DEPARTURE AND SUBSEQUENTLY SQUAWKED 7500 RATHER THAN THE 7600 NORDO CODE.

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WE WERE DEPARTING APC ON IFR FLT PLAN. NORCAL DEP CLBED ME TO HIGH ALT SECTOR AND ZOA CLRED ME TO FL350. THEN; THEY STOPPED MY CLB AT FL190 AND SENT US BACK TO NORCAL. AFTER CHKING IN WITH NORCAL DEP AGAIN; THEY LEFT ME AT FL190. AFTER NUMEROUS ATTEMPTS TO GET HIGHER ALT; I REALIZED THAT I HAD LOST COMS. THE OLD FREQ WAS ATTEMPTED; DIFFERENT MIKES AND RADIOS ATTEMPTED TO NO AVAIL. I REVERTED BACK TO BASICS: LOST COM/CLB TO EXPECTED (FILED) ALT AND SQUAWK 7600. I BEGAN CLB (NORDO) TO FL350 STILL ATTEMPTING CONTACT WITH ANY ZOA FREQ. AFTER SEVERAL ATTEMPTS; CONTACT WAS RE-ESTABLISHED. UNFORTUNATELY; I HAD INADVERTENTLY SET 7500 IN THE XPONDER VICE 7600. THE NEW CTLR 'READ ME OUT' FOR THIS ERROR AND THE UNAUTH CLB CONTINUATION. NO CONFLICTS EXISTED. OBVIOUSLY THE CONTRIBUTING FACTORS WERE : THE COMS BTWN NORCAL AND ZOA WHICH PROBABLY RESULTED IN THE LOST COM. I WAS PROBABLY TOO FAR E GEOGRAPHICALLY TO BE AT FL190 STILL TALKING TO DEP. 2) SINGLE PLT OPS ARE AT TIMES HECTIC; RESULTING IN MY INCORRECT XPONDER SETTING.