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Entry  Mon Oct 23 20:51:07 2017, Craig, awade, DailyProgress, FSS, North TTFSS Common Path Op-Amp Replaced, works 
    Reply  Thu Oct 26 11:08:05 2017, awade, Craig, DailyProgress, FSS, 50 MHz oscillations in TTFSS rev. C field boxes 
       Reply  Thu Oct 26 11:21:50 2017, Koji, DailyProgress, FSS, 50 MHz oscillations in TTFSS rev. C field boxes 
          Reply  Thu Oct 26 15:57:37 2017, awade, DailyProgress, FSS, 50 MHz oscillations in TTFSS rev. C field boxes 
Message ID: 1960     Entry time: Thu Oct 26 11:21:50 2017     In reply to: 1959     Reply to this: 1961
Author: Koji 
Type: DailyProgress 
Category: FSS 
Subject: 50 MHz oscillations in TTFSS rev. C field boxes 

If you have an oscillation in a circuit, it is no longer linear!

Sounds like a typical AD797 oscillation. GBWs of OP27 and AD797 are 8MHz and 110MHz, respectively. OP27 can't make such high freq oscillation.
I didn't check the circuit diagram. Usually AD797 has ~5pF feedback capacitance to increase the stability. You may be able to stop the oscillation by adding/replacing a mica capacitor  from 10~470pF. But 50% of the time the oscillation is not fixed with this kind of modification. You need a cut-and-try.

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