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QCX Troubleshooting
I have fixed several QCX kits. The following describes my fault-finding procedure. You can follow these steps too.
Even if your QCX radio works perfectly, signal tracing through the circuit as described on this page, is a very educational
experience! You will find you get a greater and deeper understanding of how the QCX works and why. Even *I* find it
highly rewarding, and I'm the guy who designed the thing! So - get inside your QCX! Be at one with the circuit! Live the
QCX!
Contents
General checks before starting
Techniques for replacing components
Signal tracing with an oscilloscope
LCD all black blocks, or all blank
LCD top row is blocks, bottom row is blank
Buttons / rotary encoder don't work properly
Failure to get BPF peak, or a high enough signal strength reading
I-Q and Phase balance fail to how a minimum
Signal generation by the Si5351A
Signal generator injection into the front end
Quadrature Sampling Detector (QSD), IC4
IC6 and IC7 phase shift circuits
Signal-tracing through the rest of the audio chain
Examples of receiver section faults I have found
Circuit blocks
The circuit breaks down into three main blocks, for trouble-shooting purposes: Digital section, Receive signal chain, and
transmit signal chain. I handle them one by one. Often you only have a problem in one of these blocks. Throughout the
trouble-shooting, you need to keep referring to the circuit diagram, AND the component placement diagram.
This diagram shows the digital section (processor, LCD, buttons, rotary encoder, Si5351A Synthesiser) in the green
square at bottom right; the red line shows the transmit signal path, and the blue line shows the receive signal path.
Generally speaking, the receive and transmit paths can be debugged by following the signal through from the start to
finish of the signal path. Sooner or later we will find something wrong. An incorrect signal, or an absent signal, etc. This
will tell us where the fault lies. This is a very methodical approach to the fault-finding and it always works.
