I do not. BUT, I would suggest first of all measuring the static resistance
of the microphone and comparing that with the resistance of a known-good
microphone.

The static resistance is about 100 ohm, but I do not have another carbon
microphone for comparison

I might also try using a telephone transmitter element (in the US we have
lots of Western Electric T-1 transmitters everywhere) and comparing that
with the test microphone in measured sensitivity.
--scott

I tried to power the microphone with 12 V through a 1200 ohm resistor. Talking
loud into the microphone and with the mouth very close to it, the scope (put
across the microphone leads) shows a peak voltage of about 600mV (or 1200mV
p-to-p). Perhaps it is good enough, but I am not sure on whether the bias
current is too low, and I should then try again using a lower resistance.

73

Tony I0JX