APPENDIX F MFJ-1278 CW MODEM TUNEUP PROCEDURE The CW modem in the 1278 is fundamentally different from the other modems. It is NOT an AFSK modem. It sends and receives only 1 tone. For this reason, it requires a different alignment procedure than the others. Depending on which of several modes of operation is most important to you, a choice will have to be made as to what frequency will be used for the modem Fc, and whether or not the modulator will be aligned to the same frequency as the demodulator. NOTE! If the modulator AND demodulator are not EXACTLY on the same frequency for BOTH stations involved in VHF FM MCW operation, CW receive will be degraded if not eliminated. NOTE! If the DEMODULATOR is not EXACTLY centered in the receiver's CW filter passband, CW receive performance through a narrow filter will be degraded. Also, the demodulator will likely not be receiving on the transmitted signal frequency. This note is for the HF direct keyed radio case (NOT MCW via SSB transmitter microphone audio which is discouraged). NOTE! Since the modulator tone used for MCW is also one of the SSTV transmit tones, it will have to be set for 850 Hz if SSTV mode is important to you. This will mean that MCW receive will not work if the demodulator is aligned to a different frequency for HF CW work through a narrow filter. Code practice transmissions on VHF FM MCW with an 850 Hz tone will still be possible. As can be seen from the above notes, there are definitely some conflicting requirements. Since MCW operation is primarily intended to provide a means to transmit code practice and not for CW reception, the demodulator center frequency may be set to a different frequency than the modulator without penalty in most cases. Typically, the modulator will be set to 850 Hz, and the demodulator will be set to whatever frequency is dictated by the HF radio with which it is to be used. The CW demodulator can be used without a narrow filter in the radio. It will of course be subject to more noise and QRM in this case. The tuning range for the demodulator will be VERY restricted even though the wide filter is used in the radio since the demodulator itself acts as a very narrow filter. An oscilloscope will be required to adjust the demodulator to a 50 percent duty cycle square wave output just as for the AFSK demodulators. The difference is that there will be only 1 continuous tone presented to the demodulator for this adjustment. Tuning this modem is basically a 4 step procedure. The steps should be followed in the order presented. The steps (in broad outline) are: 1. Determine the required demodulator Fc for the radio you intend using in the direct keyed CW mode. 2. Set the CW modulator tone to this frequency. 3. Set the demodulator center frequency using the modulator tone as reference. 4. If you care about 2 tone SSTV, set the modulator tone to 850 Hz. DETAILED ALIGNMENT PROCEDURE 1. Determine the required Fc for the demodulator. If the radio has a narrow filter for CW, one of the 2 methods presented in the AFSK modem alignment procedure should be used to determine the filter center frequency. This frequency should be used for Fc. If the radio has no narrow CW filter, use the CW carrier offset frequency of the radio. This information should be available in the radio manuals. 2. Set the modulator tone to the required Fc using the following procedure. a. Command the 1278 into MCW mode by typing "MODE MC ". b. Install push on jumper at JMP-4. This is the watchdog timer defeat jumper. c. Connect the counter input to JMP 9, pin 1. d. Command the TNC into calibrate mode by typing "CAL ". e. Command the TNC to key the modem by typing "K". f. Select the MCW tone by pressing the space bar until a voltmeter connected to U16, pin 9 reads + 5 volts. This signal is more conveniently available at the junction of Q12 collector and R62. g. Adjust R129 until the counter indicates the intended tone frequency as determined above. h. Return the 1278 to command mode by typing "Q". NOTE! The internal calibration routine can be used instead of the counter to set this tone as described in the AFSK modem alignment procedure. 3. Align the demodulator to Fc using the MCW tone. Use the following procedure. a. Place a push on jumper at JMP 7. This connects the modulator output to the demodulator input. b. Place a 'scope probe on left pin of JMP 10 (pin closest to Q 14). The oscilloscope will be used as described in appendix A to inticate when the square wave that will appear at this point during the adjustment procedure approaches 50 percent duty cycle. c. Command the TNC into calibrate mode by typing "CAL ". d. Command the TNC to key the modem by typing "K". e. Select the MCW tone by pressing the space bar until a voltmeter connected to U16, pin 9 reads + 5 volts. This signal is more conveniently available at the junction of Q12 collector and R62. f. Turn R115 fully clockwise, until a click can be heard from turning the pot. g. Now begin to rotate R115 CCW slowly. Watch the 'scope while rotating R115. A stable square wave should appear between about turn 4 and turn 9. Which turn on the pot where you find the spot that produces a stable square wave may vary from unit to unit. There are only about 1 to 2 turns where the square ware will appear on the scope. h. Adjust R115 so the square wave displayed has a 50 percent duty cycle. Note that when R115 is adjusted properly, the period of the square wave should be about 0.66 milliseconds. NOTE! If you have a square wave on the 'scope face but the tuning indicator is not near center, you are on a false setting. adjust R115 so that the tuning indicator is near center and THEN set the square wave duty cycle. 4A. If 2 tone SSTV is of importance to you, use the above procedure to reset the MCW modulator tone to 850 Hz. 4B. If MCW transcieve operation with another station is desired, do the above procedure to BOTH 1278s using an agreed upon frequency for Fc. Then leave the modulator tone on the agreed on frequency with the demodulator Fc. This completes the CW modem alignment procedure.