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13 O
PERATION
The following sections describe how to set up and use the MDT.
13.1 S
ETTING UP
1. Connect the antenna. You will require a BNC male plug on the antenna cable. If your
antenna cable has another type of plug, you will need an adaptor.
2. Plug in headphones. The MDT is intended to be used with 32 ohm stereo headphones. It
can drive any type of headphones but they must use a 3.5mm stereo phono plug.
3. Plug in the microphone. The MDT kit comes with a 4 pin socket which is a type
commonly used on radio transceivers. If your microphone does not use this type of
connector, then you will have to either replace the plug or change the socket on the
MDT. If making changes remember that the audio line must be independent of the PTT
line, however a common earth is allowed.
4. Connect the power supply. The MDT is designed to operate from a 13.8V DC power
source. This can be a battery or a regulated DC power supply. Don't be tempted to use an
unregulated plug pack as the AC hum will make its way into the audio sections. The
supply connection is via a 2.1mm DC socket on the rear of the MDT. The positive wire
goes to the centre pin and the negative wire goes to the outer barrel of the connector.
The MDT can operate down to around 10V, but the transmit power output will be
significantly reduced. A supply of 15V DC is the recommended maximum voltage to use.
13.2 R
ECEIVING
The VFO is a free running oscillator and there will be some drift after powering on
as the components warm up. It is best to avoid transmitting until it stabilizes. This
only takes a couple of minutes and any drift after this period is quite small.
13.2.1 SSB
The Tune control is used to tune in stations. The MDT doesn't have a fine tune control so you
need to adjust the control slowly and carefully to tune in correctly. As you move across a SSB
station you will find a spot where the voice becomes clear and sounds natural. This should
coincide with the VFO and the transmit station being on or very near the same frequency.
The AF gain is the only other MDT control. Always start with the control set low when tuning
around so that sudden or strong signals do not overload your ears. The MDT does not have
automatic gain control (AGC) to level out the audio from different stations, so you need to set
the AF gain to match the level of the signal you are tuned.
13.2.2 CW
When tuning CW signals you can tune either side of the transmit frequency to listen in. Tuning
across the signal the tone frequency will decrease as you approach zero beat. Zero beat is where
the VFO frequency is the same as the station transmit frequency. At zero beat you will hear
nothing or at least a very low frequency thumping sound. As you tune away on the other side of
zero beat the tone frequency will increase. Simply adjust the Tune control for a pleasant
sounding tone. Sometimes if there is an interfering signal on one side, you can tune to the other
side and reduce the interference while still receiving the wanted station.
MDT Construction Manual – Issue 2
Page 41
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