SCA.5
The de-emphasis circuit tends to reduce the amplitude of the SCA signal
since it is at a high frequency (53 to 81 KHz, with carrier at 67 KHz).
However, enough gets through that it can still probably be used even at
the speaker jack, since it is an FM signal and the absolute amplitude
isn't too important.  The best way to recover the SCA audio, however,
would be to go in to the FM receiver and tap off at the "top" of the
volume control -- the hot side where the discriminator signal is brought
in.  Bring this signal out to a jack which you can install on the set
for all such nefarious purposes.  In fact, make sure that the
de-emphasis network is not installed ahead of this point, although in my
experience it usually is not.

Anyway, once you have brought out the full composite signal, the SCA
signal is STILL AN FM SIGNAL that needs an FM discriminator to turn
back into normal audio.  It happens to have a carrier frequency of 67
KHz because that is what was used as the carrier at the studio when the
composite audio signal was made up.

So if you have a VLF receiver capable of covering 67 KHz, you can feed
the composite audio signal into its input, which happens to be the
antenna connection.  You can then tune it to 67 KHz and switch it to FM
mode and you should get perfect SCA audio out.
CONTINUED IN SCA.6