PW 1986 High Impedance MOSFET Voltmeter

[Nick]

As an aside, with the overall unit at nominal input impedance of around 10Mohms and with the CA3140s having an extremely high input impedance of 1.5Tohms, it might be a good idea not to use sockets for the opamps and to use a conformal layer on both sides of the PCB to prevent moisture ingress and leakage...

Another possibility is to use individual poles from a (turned pin) socket and a guard ring on the PCB to minimise leakage.
See http://www.ti.com/ww/en/bobpease/assets/...om_rap.pdf page 46 (!19.7MB download)

Did you mean http://www.ti.com/lit/an/snoa664/snoa664.pdf (the original app note, AP241 by Bob Widlar, 1980) ?

Guard rings would help if making a new PCB, however as David has already made & populated the PCB, its not an option - if starting again, I'd completely redesign the PCB, for sure

As the board is extant, desiccating the board then conformal coating it as soon as its cooled would be a big help, as would getting rid of the opamp sockets (bad reliability & lots of stray capacitance).

If no suitable spray is available, I suspect that a quick dip (for the board) in Ronseal would do the trick nicely !

Cheap wins !