PLEASE READ THE BASIC ASSEMBLY AND USE INSTRUCTIONS IN
THIS MANUAL VERY CAREFULLY BEFORE APPLYING POWER TO
THE BOARD!
IMPORTANT!
QMX+ can be built for 9V or 12V operation! You need to decide NOW!
A 9V QMX+ produces 5 W power output from a supply voltage of 9V or a little over. At 12V, a
QMX+ built for 9V could be producing 8 W power output which is likely to cause over-heating and
perhaps failure of the BS170 final transistors. Do not power a QMX+ with a higher voltage than
you built it for.
Operation of QMX+ at more than 5W power output is NOT RECOMMENDED.
If you wish to operate using a 12 V supply, you may use a two turn secondary winding on the
output transformer T501, so a 3:2 ratio instead of the 3:3 turns ratio documented in this manual.
Remember this when you come to the assembly step for preparing and installing the output
transformer T501. The “primary” is still 3 turns, with a tap half way at 1.5 turns. The secondary (no
tap) will now be only two turns.
The chart below shows the measured power output vs supply voltage for the standard 3:3 winding
(Red line); at 12 V supply the output power of around 8 W is too high and likely to cause over-
heating or failure of the power amplifier transistors. If you wish to use a supply of 12 V the 3:2
winding style is more suitable and will produce 4 – 5 W output for 12V supply. The graph shows
40m but other bands are very similar.
Operation at more than 12V will reduce the protection safety margin available in the event of very
long key-downs, hot environment, antenna mismatch, cable faults etc. I don’t recommend more
than 12V. It isn’t worth pushing more just for a fraction of a dB. It is acceptable to use diodes in
series on the power line, each one will drop the voltage 0.6V or so. Beware that a 12V nominal
battery will potentially be several volts higher when fully charged.
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