Vectronics VEC-422K - Manual (Page 21)

VEC-422K Owner's Manual SCA Decoder Kit
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long run. Remember to turn the SCA Adapter off when not in use to conserve
battery power.
Interconnecting cables: The demodulated SCA signal should be connected to
the VEC-422K using shielded audio cable (jack J1). Radio Shack and most
department stores carry good selections of A/V hookup cables. The FM receiver
or tuner should be equipped with an RCA phono jack for the SCA output. The
line-level output is also an RCA phono jack (J2). Shielded audio cable should
be used for interconnecting the line-level output to an external audio amplifier.
Audio: The SCA Adapter has a line-level audio output to feed hi-fi amplifiers
directly (RCA phono jack J2). The VEC-422K also includes an on-board audio
amplifier to drive a speaker (4 to 32 ohms, 50 mW max.) or headphones at jack
J3. The headphones should be equipped with a 3.5mm monaural jack. Stereo
phones with a 3.5mm jack may be used, but only one earpiece will be active.
Connecting to the receiver: Some FM receivers or FM tuners come equipped
with an SCA adapter jack. If your receiver has a SCA jack, you need only
provide the necessary shielded cable from the receiver SCA jack to the SCA
input (jack J1) on the VEC-422K. Some older FM receivers featured a “MPX”
output jack—this is also an ideal point for connecting to an external SCA
Decoder. The MPX signal is taken from the detector, before the SCA filtering
and de-emphasis circuits. Newer receivers may have a jack for “four-channel
decoders” or “quadrasound decoders”; again, these are also taken directly from
the FM detector and can be used to supply SCA subcarrier signals to the VEC-
422K. You might notice that the VEC-422K reduces the level of FM audio on
some receivers when it is attached; disconnect the VEC-422K when not in use if
this is a problem.
Receiver modifications: If your receiver does not have an SCA adapter jack,
you will have to provide one. FM receivers can use one of several types of FM
detector circuits. The majority use either a Ratio Detector, Discriminator, or
Quadrature Detector to recover audio from the FM carrier. Unless you are an
experienced technician, you may wish to have the jack installed by a service
shop. There is usually room to add an RCA jack on the rear apron of most FM
receivers or tuners. The following schematics are representative of typical FM
detector circuits, and gives an idea of the proper take-off point for the SCA
subcarrier signal. The ratio detector circuit is very similar to the discriminator
circuit, which is not shown.