Andy,?
I deal with a lot of old circuitry. Mica caps going bad, as you stated, are VERY rare. It does not seem to be a DC problem as I am feeding in a square wave at 25kHz. I doubt over voltage has ever hit the circuit. PCB has conformal coating and, after swapping the opamp set, still functions the same way.
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BTW, the LM121 was not a chopper stabilized version. The ap-note clearly states that the LM121 keeps the noise level down because it is nota chopper-type.?
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I have a scheme to pull the opamps and substitute a different opamp that has low drift and reasonable GBW. I am thinking it may be resistors as well, but initial in-circuit checks show nothing out of the ordinary. I have seen resistors drift to become >+20% out of tolerance due to aging. Imagine expecting to see a set gain and come to find that the 200k resistor measures 300k!
DerekK