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EXAMPLE
: I had one QCX radio for repair, where the current consumption, in receive, was 260mA. IC3 felt very warm to
touch. I measured the voltages at the pins, in receive mode... and I saw some strange voltages e.g. 1.4V which were not
near to 0V or 5V. In transmit mode, the 3.3V peak-peak squarewave was present at pins 4 and 10, but there was no 5V
peak-peak signal at the relevant pins. Reluctantly, and ever-mindful of my own rule about not assuming a component is
defective until the last possible moment, and wary of the fact this IC3 has 14 pins and is going to be a bear to remove - I
decided I had to replace it. It wasn't as hard to remove it as I had expected (see above section on techniques for replacing
broken components), and desoldering braid sucked most of the solder out of the holes. One trace got damaged and I
repaired it. The current consumption after the change returned to normal, around 110-120mA, and the voltages looked
normal again.
Power Amplifier
The output Low Pass Filter (LPF) continuity has already been covered further up this page. If your receiver is working
properly, you will have verified that the signal is coming correctly through the LPF. The only thing in between IC3 pin 3
and the LPF is the Power Amplifier! IC3 pin 3 is connected directly to the gates of Q1, Q2 and Q3 which are the BS170
transistors of the Power Amplifier.
Make sure a 50-ohm dummy load is connected to the QCX transceiver RF output, when testing the Power
Amplifier!
Now check the drains of Q1, Q2 and Q3. Refer to the parts layout diagram and the track diagram. You can see a big fat
trace on the bottom of the board, which connects each of the drains of the BS170s and connects them to C29. From
there, the signal goes to the Low Pass Filter. On key-down you should see a huge ugly signal, that looks like the upper
