Tune-up directions and notes for MFJ 1270/74 TNC The directions for calibrating the tones on an MFJ 1270/74 leave a lot to be desired. Here is what MFJ forgot to tell you. The procedure is different for modulator (transmitted) tones than for demodulator (received) tones. ============================================================================ CPU clock Connect a frequency counter to pin 6 of U 10 or loosely couple the antenna of a calibrated communication receiver to this same pin and adjust C 47 for 4.9152 Mhz +/- 100 Hz. This adjustment is not critical for TNC operation but it is for accurate tone calibration. The position of JP 2 (CPU speed select) is irrelivant to the calibration procedure. ============================================================================= Modulator tones 1) Install a jumper from pin 1 to pin 4 on JP 9. These are the two pins on the six pin header nearest to C 44 2) Enter the CALSET number from the list below. "CALSET 439" for 1200 Hz. 3 Put SW 3 in the proper position. (HF or VHF) 4) Enter the calibrate mode by typing "CALIBRA" 5) Type "K" to key the TNC. (you should not have the tranceiver connected) 6) Listen to the output audio to determin if the TNC is sending the high or low tone of the selected pair. Tap the space bar to toggle high/low. 7) Adjust the pot. for that tone given in the list. Note if the tone frequency is low the "CON" LED will be on. if the tone frequency is high the "STA" LED will be on. both LEDs on or alternating between "CON" and "STA" tells you the tone frequency is close enough. 8) Type "Q" to quit the calibrate mode and repeat to step 2 for the next tone to be calibrated. You should calibrate all four modulator tones, but you may wish to calibrate only the VHF or HF pair. You should do both tones of the selected pair. 9) Remove the jumper on JP 9 installed in step 1. This completes the modulator calibration. TONE FREQ. POT. CALSET # SW 3 Pos. HF MARK 1600 Hz R 106 329 IN HF SPACE 1800 Hz R 105 293 IN VHF MARK 1200 Hz R 78 439 OUT VHF SPACE 2200 Hz R 77 240 OUT ============================================================================= Demodulator tones 1) Install a jumper between pin 2 and pin 5 on JP 9. These are the two center pins in each row of the six pin header. 2) Enter "CALSET 155" Do NOT enter the CALIBRA mode! 3) Put SW 3 in the "VHF" (out) position. 4) Adjust R 79 and watch the "CON" and "STA" LEDs. Note the "CON and "STA" LEDs react the same for demodulator tones as for modulator tones. (See step 7 above) 5) Since there is only one adjustment for the demodulator, remove the jumper installed in step 1. This completes the demodulator calibration. =============================================================================== IMPORTANT NOTE FOR NETROM AND OTHER NETWORK USERS You MUST remove the special network ROM chip and insert the original MFJ ROM and be sure that there is no connection to pin 23 of the RS-232 connector. If you have any comments or questions reguarding this tune-up send me a message. Good tuning - 73 - Keith KA4ROS @ KA4ROS