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Low power output
The manual contains charts of power output vs supply voltage. Remember to take into account that you will lose 0.4V or
so across the reverse polarity protection diode D3. The chart voltage is the voltage AFTER this diode. The voltage drop
across the diode will be around 0.3-0.4V.
If you see power output, but you think it is too low - then here's what to check.
Firstly, did you calculate the power output correctly? To accurately measure power output on an oscilloscope you MUST
have a 50-ohm dummy load connected. Then the peak-peak voltage of the sinewave can be read from the oscilloscope.
The power in Watts, is the peak-peak voltage squared, divided by 400. For example, if you see a 37V peak-peak
sinewave, this is 37 * 37 / 400 = 3.4W.
You CANNOT reliably measure power in this way, if you have your QCX connected to an antenna. Most power meters will
be measuring the peak RF voltage (or RMS voltage) and converting this to power, using the above formula. These power
meters necessarily assume that you are measuring a pure sinewave. This is also why you CANNOT measure the power
at the drains of the BS170s (Q1, Q2, Q3) in the power amplifier! The signal here is FAR from a clean sinewave. The
calculation is invalid, since the calculation assumes a sinewave. So, ONLY measure power across a 50-ohm dummy load
such as the
, and only measure the power output at the proper RF output connector of the
transceiver, after it has been filtered into a nice sinewave by the Low Pass Filter.
If you are using an external power meter, remember that often power meter accuracy is not great. Due to the Square
function in the formula for conversion from volts to watts, a small difference in peak voltage measurement is translated
into a larger difference in calculated output power.
If you are using the internal RF power meter facility in the QCX, then remember that this too is not very precise. It is a
reasonable indication, for debugging purposes, but it is not a precision instrument.
Note also that to get 5W power output does require a 15 or 16V supply voltage. You can see this from the power vs
supply voltage curves in the manual.
Finally if you really think your power output is a lot lower than you expected - then you can look at the windings on L1, L2
and L3, the Low Pass Filter toroids. There can be variation in component value due to manufacturing tolerances, and
variation in inductance values due to winding style and other factors. First check that you wound the correct number of
turns for your band. Remember that what counts as one "turn" is every time the wire passes through the toroid ring's hole.
Count the turns again to make sure you got the correct number.
If you have the facility to measure inductance then you can do so - but remember that a lot of inductance meters are not
all that accurate either, and this is a topic which always generates a lot of discussion when it arises.
Simply removing a couple of turns from each of L1, L2 and L3 is usually sufficient to raise the filter cut-off, if it was too low
- and you should notice a power output increase.
Conclusion
With a highly methodical approach to the fault-finding, to step through one stage at a time through the receive, then the
transmit, signal paths - it is always possible to find the fault(s).
This page will be updated in future with more examples of faults, as and when they arise.
In my opinion, the signal tracing is a worthy exercise in itself, even if you have a completed and working QCX - because it
is very educational, you really get a deeper understanding (and love) for your QCX when you see all these signals
throughout the transceiver.
