Receive; Carrier Balance; Receive Bpf; Transmit - ozQRP MST3 Construction And Operation Manual

13.3 R
ECEIVE
13.3.1 C B
ARRIER ALANCE

Connect a frequency counter at the emitter of transistor Q3. Open the U/L switch.

Adjust trimmer capacitor TC2 until the frequency is 9.999MHz. This is the upper carrier
(BFO) frequency and is placed on the upper skirt of the crystal filter.

Close the U/L switch. Adjust trimmer capacitor TC1 until the frequency is 9.997MHz.
This is the lower carrier (BFO) frequency and is placed on the lower skirt of the crystal
filter.

Switch the U/L switch back and forwards noting the frequency. There is some
interaction between the settings, so you may need to adjust them a little each time until
they are correct.
These are the suggested carrier frequencies and may need adjusting slightly
depending on the precise crystal filter characteristics. See section 13.6 for more
details.
13.3.2 R BPF
ECEIVE

Remove the dummy load and apply a moderate RF signal (100uV) to the antenna
connector and tune the VFO until a clear tone is heard in the speaker. Carefully adjust
the trimmer capacitors in the receive BPF for maximum volume. As you approach
maximum you may need to reduce the AF gain control to avoid overload. You can
experiment with the position of the two trimmer capacitors to obtain a specific
bandwidth, but in general, peaking at the middle of the band will be satisfactory.

If fitted, set up the LED S meter display as per the instructions in the LED S meter
construction manual.
13.4 T
RANSMIT

Remove the RF signal source and reconnect the dummy load. It will be an advantage to
have either a QRP power meter or oscilloscope to obtain an indication of power output.

Turn the Mic gain control fully counter-clockwise.

Un-plug the VFO power connector.

Briefly switch to TX state by shorting the PTT contacts. Check the power supply current.
This is the idle transmit current of the MST3 board only and should be about 210mA.
Anything far from this indicates a problem and should be investigated.

Plug in the VFO power connector. Check that the current draw now is the sum of the
MST3 idle transmit current plus the specified VFO current. If not check your VFO and
connections.

Write the new idle transmit current value down so that it can be used to measure and
set the PA bias current.
13.4.1 PA
BIAS SETUP

Operate the PTT and using a screwdriver slowly rotate the bias trimpot clockwise. At
about half rotation the power supply current should start to rise gradually and
smoothly. Any excessive spikes in current indicate instability and need to be
investigated.
MST3 Construction and Operation Manual – Issue 1 Page 51