PW 1986 High Impedance MOSFET Voltmeter

[Nick]

.. an AC probe, which only goes up to 10V RMS (28V P-P) as it uses a BAT85 Schottky diode which won't stand any more than that.

You could always use a higher voltage barrier diode, like a BAS70 - VR(max) = 70V (would give 24VAC RMS), IF = 70mA (lower, but not a problem in this design), Cd = 2pF (20% of a BAT85), Trr = 100pS (40 times faster than a BAT85) - since that PW design, a lot more devices are available. Though the BAS70 is typically an SMD component, it comes in some quite big packages (in SMD terms) and easy to solder wires to.

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...

If you have an electric (but not gas) oven, baking the PCB for an hour or so on about 70C to remove any incumbent moisture BEFORE spraying with the conformal coating would be a good idea too...

Edit: Actually, I feel a thread about usage of the kitchen for electronics coming on! I've often used the oven to de-humidify items before spraying, and the dishwasher to clean keyboards and wash 100s of storage boxes etc...