STC Resistance Capacity Bridge

[Crackle]

How would you hear the null in the phones with DC?

Yes you are correct I keep forgetting that.
An analogue meter works well with it though, and I think, gives a better defined null than the audio because of the noise from the buzzer.

Thanks very much for the circuit, I agree it does seem to be the same, I will check value of C2 again to see if it is low in value, which would explain the addition of C4.
I wonder why the 2 wires were joined near to S3?
Would it be OK to add that circuit into the Radio Museum?
Thanks again Andy

Mike