Epson Multi-mode Data Controller MFJ-1278B - Manual (Page 346)

MFJ-1278B MULTI-MODE APPENDIX B MODEM CALIBRATION
b. Place a scope probe on the top pin of JMP10, the pin furthest from Q14). The
oscilloscope will be used to indicate 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 <CR>".
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 collector of
Q12.
f.
Turn R115 fully clockwise, until a click can be heard from turning the pot. This
could be a little hard to hear.
g. Now begin to rotate R115 CCW slowly. Watch the scope while rotating R115. A
stable square wave should appear. This signal should appear somewhere in the
middle of the rotation (approx. 10 turns). 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. So turn the pot
CW to where the square wave just appears and is stable. Make note as to where
slot on the screw is at. Then rotate the pot CCW until the square wave just
becomes unstable. Make note as to how many turns you went CCW, and turn the
pot back CW half that many. Now you are in the center of the demodulator.
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
0.59 milliseconds. The square wave will appear as shown below: