This is what we suspected earlier but couldn't prove. Those capacitors, plus C54, need to be replaced with film capacitors similar to C23, C24, C52 and C53.
73,
John KD6OZH
----- Original Message ----- From: "Juan Rivera" juan-rivera@sbcglobal.net To: "John B. Stephensen" kd6ozh@comcast.net Cc: eagle@amsat.org; "Bill Ress" bill@hsmicrowave.com; "Dave Black (Home)" dblack1054@yahoo.com; "Dave Black (Work)" dblack@mail.arc.nasa.gov; "Dave hartzell" hartzell@gmail.com; "David Smith" w6te@msn.com; "Don Ferguson" kd6ire@sbcglobal.net; "Juan. Rivera (Home)" juan-rivera@sbcglobal.net; "Juan.Rivera (Work)" Juan.Rivera@gd-ais.com; "Samsonoff@Mac. Com" samsonoff@mac.com Sent: Saturday, July 14, 2007 18:18 UTC Subject: Source of 70 cm Microphoncs Found
John,
The microphonics is caused by C19, C27 and C28 (.047uF 25V) and C50 and C51 (.022uF 25V).
I found the best way to isolate this is to inject a CW signal into the receiver, tune the signal in at the IF output using my TS-2000 in AM mode, and then listen to the audio output while lightly tapping each component one at a time.
According to my research, barium titanate, the base ceramic material for most ceramic SMD capacitors is inherently microphonic.
Juan