ozQRP MDT, Construction Manual

Page: 37 / 42

MDT Construction Manual – Issue 2 Page 37
11.4 T
RANSMIT

Remove the RF signal source and reconnect the dummy load. It is suggested you have
some way of measuring the power output. A QRP power meter or oscilloscope would be
ideal.

Turn the Mic gain trimpot VR3 fully counter-clockwise.
 Briefly switch to TX mode by shorting the PTT contacts. Check the power supply current.
If the mixer is already balanced there will be no output and the current and should rise
to about 130mA. If the mixer is not balanced there may be RF output and the current
may be higher. If it is way in excess of 250mA this indicates a problem and should be
investigated.

Place a short across the PTT line. Adjust carrier balance controls TC1 and VR2 for
minimum power output. Initially you can use the LED indicator to monitor this, but as
the mixer reaches balance the output power will decrease significantly and the LED will
not turn on. At this point it’s necessary to use some other way to monitor the power
output to do the final balancing. The following are some suggested methods:
o
Using a second receiver placed nearby listen to the transmitted signal. Ensure
you are not listening to the VFO signal as this is on the same frequency.
o
Connect an oscilloscope across the dummy load.
o
Connect a sensitive RF probe across the dummy load.
o
Connect a spectrum analyser across the dummy load. An attenuator may be
necessary.
In all cases it’s simply a matter of monitoring the transmitted signal for minimum output
while adjusting TC1 and VR2. The carrier null is quite sharp, and there is some
interaction between the controls, so you will need to go back and forwards to obtain
maximum balance.

Check the TX mode DC voltages as shown in Figure 22. If there are any off scale readings
turn off immediately and look for construction errors.

Remove the short across the PTT line. Apply an audio signal generator to the Mic socket
set to 1KHz at around 50mV. Operate the PTT again and slowly rotate the Mic gain
trimpot clockwise while monitoring the power output. The power output should
increase smoothly without any sudden dips or surges and you should be able to easily
achieve 1.5 to 2 Watts output. At this level the front panel LED should be brightly lit.
Remove the signal generator and check that the RF output goes to zero, and the LED
turns off.

If you intend to use an Electret microphone place a short across the EL link. This can be
a soldered link or you can install a 2 pin header and a removable shunt. Leave the link
open for dynamic microphones.

Plug in a microphone and check the relay operates when the PTT is pressed. When you
speak there should be RF output, and the LED should flash. You will now be able to
monitor yourself with a receiver placed nearby and determine an appropriate Mic gain
setting.