ozQRP MDT, Руководство по монтажу

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MDT Construction Manual – Issue 2 Page 40
12 M
ODIFICATIONS
12.1 S
ETTING THE
VFO
RANGE
The VFO tuning range is reasonably fixed due to the characteristics of the ceramic resonator and
the available capacitance swing of the varicap but it can be adjusted slightly if required.

The Range link. With the link shorted the two varicap diodes are in parallel and the
tuning range is approximately 7.050MHz to 7.110MHz. With the link open only one
varicap diode is in circuit and the tuning range becomes approximately 7.090MHz to
7.130MHz.
 Resistor R2. Increasing the value of R2 will lower the maximum voltage across the Tune
control and will in turn decrease the tuning range and maximum frequency of the VFO.

Capacitor C3. This isn’t included in the parts list, but adding a small value NPO ceramic
here will lower the overall frequency range.
12.2 R
ECEIVER ALIGNMENT
The tuned circuit at the front end of the receiver containing coil L4 is designed for a bandpass
wide enough to cover the 40M band without adjustment. If it is necessary for some reason to
alter the tuned circuit frequency then this will require a change in the turns on L4.
If, however, only a small adjustment is required this can be done by compressing or spreading
the turns on L4. Nominally the turns should be evenly spaced over about 80% of the
circumference. However if the turns are compressed the inductance will increase slightly and
lower the tuned circuit frequency. Conversely, if the turns are separated the inductance will
decrease slightly and the frequency will increase. To check the frequency of the modified tuned
circuit do the following:

Inject a stable signal into the antenna socket in the middle of the tuning range.

Rotate the Tune control until a clean tone is heard in the headphones, and adjust the AF
gain control for a comfortable level.

Place an audio level meter or oscilloscope across the AF gain control and monitor the
amplitude of the audio tone for maximum while spreading and compressing the turns on
L4.

When a maximum is reached the alignment is complete.
12.3 C
RYSTAL OPERATION
The MDT kit comes supplied with a 7.2MHz ceramic resonator which gives about 60KHz of
tuning range. This gives much better frequency stability than a conventional LC oscillator but is
not rock steady. If very stable frequency operation is required the resonator can be replaced
with a crystal. This is soldered into the outer resonator holes in the PCB. The centre earth hole is
ignored. The Tune control can then be used as a fine tune control with around 1KHz of
adjustment.
12.4 U
SING A LOUDSPEAKER
If you wish to use an external loudspeaker with the MDT, it is necessary to replace the 22 ohm
resistor R43 with a wire link. Otherwise the speaker level will be greatly reduced.