The output of the 50W Power Amplifier connects directly to the antenna system – which could
include matching units or tuners. Do not transmit at full power into a mismatched (high SWR)
antenna system.
It is strongly recommended to connect a 10K resistor in the QCX from IC3 pin 13 to IC3 pin 14.
This modification is detailed further, here:
http://www.qrp-labs.com/qcx/qcxmods/ptt.html
7.4 Adjustment
There is only one adjustment required in the 50W PA kit, which is the R5 trimmer potentiometer.
Initially,
BEFORE POWER UP, MAKE SURE R5 IS FULLY ANTI-CLOCKWISE
.
To make the adjustment, use a power supply with a current measurement readout, or use a DVM
in current measurement mode in series with the 50W PA.
Set up the QCX as follows:
1) Enable Practice mode (set Practice mode to ON in the Keyer menu)
2) Set Keyer mode to “Straight”.
This will allow you to use the microswitch key the QCX, which in turn will key the 50W PA
(generate a “high” +5V signal on the PTT line) which puts the 50W PA kit into the Transmit state.
However since the QCX is in Practice mode, no RF is generated – this is very important.
With R5 turned all the way fully anti-clockwise, press the microswitch on the QCX to key the 50W
PA. Observe the current consumption of the 50W PA kit. For example, suppose the current
consumption is 85mA. Then turn the R5 trimmer potentiometer very slightly clockwise, a tiny little
bit at a time, until you see the current consumption just rise above 85mA. This is a very fine
adjustment – going to far beyond this point will cause a very high current to flow. Once you have
found this point where the current just started to increase above 85mA, then turn it a tiny bit anti-
clockwise again to just put it back at 85mA again.
NOTE: 85mA is only my example; the value depends on supply voltage and you should always
refer to your observation of the current consumption when R5 is fully anti-clockwise.
Now your 50W HF PA kit is ready to use!
7.5 Supply voltage value and power output
Note that the output power depends on the supply voltage. Full 50W power output is obtained at a
supply voltage of around 19-22V. The exact value also depends on other things like the setting of
the R5 trimmer potentiometer, the band, and the drive power from the QCX.
Refer to the power output vs supply voltage charts in the Measurements section of this manual for
a good guideline.
It is recommended NOT to operate this power amplifier at power outputs in excess of 50W. This
will cause too high voltage for the IRF510 transistors and too high power dissipation and will likely
lead to failure of these transistors.
50W QCX PA kit assembly
1.00q
66
