MFJ-8621 Data Radio
10
CRYSTAL OSCILLATOR CALIBRATION:
Transmit Oscillator:
To measure the exact operating frequency of the MFJ-8621 transmitter, monitor the
radio's RF output signal with a digital frequency counter. To obtain a usable signal
sample, place the counter's pickup antenna near the MFJ-8621 PA section--or install a
sampling tap in the coax line leading to your dummy load. Use the CALIBRATE function
on your TNC or short pin-3 of the data jack (PTT Line) to ground to key your
transmitter's PTT line. Refer to Figure 3 on page 9 for specific locations on the pc
board.
[ ] Key the PTT line and check for a stable counter reading.
[ ] Using an insulated tuning wand, adjust L6 to the exact operating frequency.
[ ] Key and unkey the transmitter a couple of times to confirm setting.
Alternatively, if you don't have access to a good VHF counter, you may use a
synthesized HF receiver for approximate calibration. To pick up the transmitter
crystals fundamental signal, place a short antenna pickup wire near the MFJ-8621
and tune your HF receiver to the exact frequency stamped on the transmit crystal
case (in the 18-MHz range). Next, key the transmitter PTT line and adjust L6 for zero
beat. You must be precise; any frequency error appearing at 18 MHz will be multiplied
8 times at 145 MHz!
Receiver Oscillator:
Your MFJ-8621 has a unique built-in feature that allows you adjust the receiver
oscillator by using the calibrated MFJ-8621 transmitter signal as a reference. You'll
need a accurate DC voltmeter (or DVM) and an insulated tuning wand for this
adjustment. Refer to Figure-3 for locations on the pc board:
[ ] Locate DISC-TP (discriminator test point) in front of the radio's data jack.
[ ] Set up voltmeter to measure DC readings in the 3-volt range.
[ ] Connect the voltmeter to DISC-TP.
[ ] Key the transmitter on.
[ ] Adjust L4 for a DISC-TP reading of 2.5 volts.
The MFJ-8621 is now calibrated to transmit and receive on the same frequency.
To calibrate your MFJ-8621 receiver for a frequency different than the transmitter (i.e.
for split-channel operation), you can substitute a 2-Meter HT for your signal source.
First, connect a dummy load to the HT antenna jack (a 47-Ohm 1/4-W resistor is
okay). Set your HT for low-power simplex operation and dial in the desired calibration
frequency. Now, follow the procedure outlined above, substituting the HT as your
signal source.
