Ramsey Electronics FX Series Manual page 72

ADDITIONAL RECEIVER IC FUNCTIONS AND FEATURES
In addition to audio output at pin 17, a separate high speed data output (up to
35000 baud) is available and may be linked directly to a packet TNC or other
data controller using the jumper wire pad provided on the PC board. The
MC13135 is capable of detecting true FSK (frequency shift keying) in addition
to AFSK.
The receiver has good "hysteresis" characteristics, the ability to hold the
squelch open once it has been broken by a marginal signal, even if the signal
becomes weaker. The IC also has carrier detect circuitry which is put to good
use in the transceiver design by providing very effective squelch action. The
squelch is activated by signal strength, not by noise. When no carrier is
detected, the voltage at pin 16 is high, which causes Q6 to mute the audio
amplifier U2 as explained in Stage B.
When a carrier is detected, the voltage at pin 16 drops low, turning off Q6.
Resistor R13 permits squelch adjustment.
Finally, the carrier detect circuitry affords the same COR ("Carrier Operated
Relay") action as needed in any repeater, which is why the output is also
available on the PC board, designated "COR." It is up to creative users to
determine their own practical implementations for this feature.
Stage CR ASSEMBLY PROCEDURE:
CR1. Install U1, the MC13135 24-pin DIP FM Receiver IC. Be very sure
that the notched end is oriented as illustrated and make sure all 24 pins
are visible through the holes before soldering.
Install the following resistors:
CR2. Install R11, 47K ohm (yellow-violet-orange).
CR3. Install R5, 10K ohm (brown-black-orange).
CR4. Install R4, 68K ohm (blue-gray-orange).
CR5. Install R24, 10K ohm (brown-black-orange).
CR6. Install R3, 1K ohm (brown-black-red).
Install the following capacitors:
CR7. Install capacitors C5, 6, 8, 9, 13, 14, all .1 uf disc capacitors
(marked .1, 10 nf or 104).
CR8. Install C7, 4.7 or 10 uf electrolytic. Watch polarity (or U1's audio
won't reach U2!)
CR9. Select and install FL2, the rectangular plastic block with 3 thin pins.
It fits only one way. This is the 6 pole, ceramic 455 KHz IF filter network.