Since we are close to getting our receiver finished, let's
adjust our VFO for operation in the amateur bands.
First, remove the temporary jumper that we installed before from Q6 to
C36. This will remove our "cheat" and stop feeding xmit signal directly
into the receiver.
You can use a frequency counter for this or a calibrated receiver. Using
a freq counter, connect the probe to the top of R29 or the cathode of
D6. Turn on and key the rig, note the frequency. It should be around 7
Mhz. With a receiver, connect a clip lead to the top of R29 or the
cathode of D6 as a small antenna. turn on the rig and your receiver.
Start at 7Mhz on your main rig and key the SW-40+. Tune the main rig up
until you hear the transmitted tone. It helps to key it on and off so
the carrier will be distinctive. Zero beat the receiver (tune it down
until the receive tone gets so low you can't hear it, 0 Hz). The
receiver is reading your transmit frequency.
Now, go to the manual page 16. Look up your frequency on the chart and
find what capacitor that you will need for C7. Make sure you use the
correct chart for Novice operation. Mine required a 68pF capacitor for
C7. Install C7.
Now remove the temporary jumper at J2.
Install the off board tuning pot (not included with kit) as per page 15
of the instructions.
Now you can tune your xceiver. I built mine for general+ segment. My
freqency coverage is from 7.015 Mhz to 7.050 Mhz, perfect for me.
Without C7 mine tunes from 7.111 to 7.149. Maybe I can install a switch
on C7 to switch between General and Novice operation.
I don't remember if we covered this in the VFO section, but you can
change the range of tuning by changing C8. Increasing C8 widens the
tuning range and decreasing it decreases the tuning range. Does anyone
want to take a shot at why it works this way? (post to the list please).
What limits the tuning range of this design?
The manual states that the practical upper limit of C8 is 1000pf, and at
the larger values capacitor and varicap thermal stability become
Use NPO/C0G caps if you replace C8. Why?
Now that our VFO is set we are ready to finish the receiver and listen
for some stations!