Hi.
You must know that FG90 can't detect fresh gold so don't worry for it.
I am afraid that the missing IC is a 555. If you measure the voltage at pin1 and pin8 and you read a voltage of 9V then it is a 555. If Voltage between pin4 and pin8 is zero but between pin4 and pin7 is 9v then it is a single op.amp as 3130, 3140, 741, TL061 etc try what you have.
Please inform us about the voltage results.
FG80 don't have the U5 and the other IC right of the ion chamber
Also inform me about the IC type (number.... right of chamber), i think that is a 324....
Regards
