DC Nanovolt Amplifier Model A10 Reverse Engineering

[Mickle T.]

Perhaps someone may find it useful for researching or repairing an amplifier.

I would be very grateful if you could point out errors in the schematic and PCB.

[TimFox]

What do you mean by "" ?
For example, do you not understand why the first block is called "modulator"?

The post changed my three question marks into an emoticon.

[Mickle T.]

All reverse engineering was carried out using photographs of the A10 circuit board, schematics of EM benchtop nanovoltmeters, and long-standing correspondence with the founder of EM.
I am not sure of the values of some capacitors, the types of transistors of the modulator and the input stage of the AC preamplifier are unknown. The modulator circuit is drawn according to a sketch from a forum member LTZ2000.

[Kleinstein]

The modulator block still looks a bit odd. The 2 transformers are partially wired in parallel. In the phase when M2 is on this would be an odd configuration with only counter-acting turns in the path. So I think there may still be something wrong with the transformer connections.

The amplifiers are 2 seprate parts, though they a drawn a little like a differential amplifier.
J1 seems to be a relatively low gain direkt path and J2 the path through the modulator.

This would suggest that the lower transformer used for J1 should get a non modulated signal and only the signal to the upper transformer should get the modulation. So maybe the pin2 of the lower transformer is not connected.

The JFET choice should not be that critical, most likely some low noise types that work well with some 100 µA. Similar the FETs in the demodulator should not be that critical. There are not that many types with a seprate substrate anyway.