QRP Labs QCX Operating instructions

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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
Circuit blocks
Equipment
General checks before starting
Techniques for replacing components
Signal tracing with an oscilloscope
Digital section
LCD all black blocks, or all blank
LCD top row is blocks, bottom row is blank
Unreliable processor boot up
Wrong band selected
Buttons / rotary encoder don't work properly
RECEIVE signal path
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
Low Pass Filter fault
T1 transformer
Quadrature Sampling Detector (QSD), IC4
Audio pre-amp, IC5
IC6 and IC7 phase shift circuits
Signal-tracing through the rest of the audio chain
Examples of receiver section faults I have found
TRANSMIT signal path
Current consumption
IC3 the quad NAND gate
Power Amplifier
Low power output
Conclusion
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.

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The two most USEFUL diagrams for fault-finding, are the circuit diagram, the parts layout diagram, and the tracks
diagram. Remember that on the tracks diagram, BLUE tracks are on the bottom side of the board, and RED tracks are on
the top side of the board. These diagrams are in the manual already. But here they are again, just so you have them near
at hand.
Click the diagrams to see the larger version; or even better, right-click "Open in new tab" or "Open in new window" etc.
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Equipment
I use an oscilloscope for detecting the signal at each point through the signal path. Some of this trouble-shooting will be
quite difficult without an oscilloscope. You can still use a DVM for some measurements (or the internal DVM of the QCX
itself, provided you have the digital section working properly). You do not need anything else - the QCX has its own built-
in Signal Generator.
General checks before starting
It is worthwhile to check once again, that all of the IC's are properly orientated, with their dimple matching the one on the
PCB silkscreen; and examine the PCB carefully, using a bright light and optical magnification (for example, Jeweler's
loupe, magnifying glass or USB microscope). Look for any dry joints, solder bridges, any component wires which were
snipped off slightly too long and might touch each other and cause short circuits.
There are a lot of parts to solder and it is surprisingly easy to forget to solder a component wire! Or even forget to install a
component completely! It has happened time and again. A quite nice trick is to hold the PCB up in front of a bright light.
As you look through the board, you will see the sparkle of the light through any holes which have not been soldered. It is a
neat trick that really helps find any forgotten joints and it takes only a moment. Don't panic though - remember that band
pass filter capacitors C5 and C8 are not always installed (depends on the band).

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Also don't panic if you had additional components left over: some capacitors are not used in some band versions of the
kit. And sometimes the kit packers make a mistake and put in one too many of a component (which is better than one too
few, right?).
Also check that the transistors are installed correctly - they are not all BS170's, there is also one MPS751 (or MPS2907).
Make sure that the MPS751 is in the right place (Q6). Check that all the other components are correct too, including:
Component value
Orientation of electrolytic capacitors
Orientation of transistors - the transistor body shape must match the silkscreen; the centre wire of the transistor is
bent forward to fit the centre point of the triangle, in the case of the BS170's
Diode orientation is correct: the stripe on the PCB matches the stripe on the component
IC dimples line up with the PCB
All pins of ICs are properly installed in their holes/socket positions (not bent under the chip)
These are preliminary checks, not very exciting, but they are a lot easier than some of the signal tracing that follows. And
surprisingly often, you can find the fault just visually like this, without any more complex investigation.
Be sure to read the FAQ and the Modifications pages, to see if any of the problematic symptoms you see on your radio
are mentioned there.
Techniques for replacing components
Replacing components should be an absolute last resort! Check EVERYTHING else first! It is easy to suspect a damaged
component... but often, the component is not the problem, something else is! Replacing components is not easy, and
sometimes you can easily damage the delicate traces on the PCB. Removing components from through-hole plated,
double-sided PCBs is not easy. So run through ALL other checks first, to make sure you are certain the component is
broken, before trying it!
Removing a resistor or capacitor is normally not so hard, because they have only two wires. I find that by alternately
heating one joint and gently pulling, then the other, the component can be "rocked" free of the PCB. If the wires are bent
at all on the underside of the board, then start off by straightening them up, so they stand out at right-angles to the PCB.
This makes the job a lot easier.
Transistors can also be removed by the same technique, one wire at a time, gradually pushing them this way and that, to
gently ease them out bit by bit. Don't be tempted to use too much force, which can lift PCB traces, pads, or even pull out
the through-hole plating.
If the component is just installed in the wrong place, and you want to remove it and re-use it, then you need to be very
careful not to damage the component as well as the PCB. If the component is broken, then you don't care about
damaging it. This can make it easier to remove. For example in the case of a transistor, you can just cut off the transistor
wires with a wire cutter. Then remove each wire one at a time, with the soldering iron.
In the case of IC's I don't think you have any choice but to destroy the IC during removal. They have too many legs and
you can't easily heat them all at once, or lever the chip out one leg at a time. Surface Mount ICs are easiest to remove
and can survive the experience. Some people have hot air techniques where they blast the board with hot air until all the
solder joints of the IC are heated, and it just lifts out while everything is molten. This might be Ok if you are salvaging a
chip from an old PCB and you don't care much about what happens to the PCB, which can easily get damaged by all the
heat. In our case, we do care about the PCB. My method for removing chips (destructively) is, use a very sharp wire-
cutter and snip each of the pins right next to the chip body. The chip body can be cut out. Then remove each pin one by
one with the soldering iron.
In order to install new components, you will want to clear the solder out of the holes. The holes are all through-hole plated
and the solder wants to stay in the hole. But you want to get it out, so you can install the fresh component. There are
several ways of doing this. I find that the easiest way is desoldering braid. This is a thin ribbon of copper braid which looks
a bit like the outer shield braid of a coax. You put the braid on the hole and hold the soldering iron to it. When the solder
melts, it gets sucked up into the braid, out of the hole, just like water into a sponge. That's the theory and it doesn't always
work. Don't push too hard with the soldering iron, or hold the soldering iron to the pad too long. Too much heat and it is
easy to lift PCB pads.

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You can also push something into the hole, while holding the soldering iron to it - something that solder doesn't like to
stick to. Some people have reported using sharpened wooden tooth picks for this purpose. I tend to use a thin sewing
needle. It often gets stuck in the hole but with care it is possible to gently pull it out, leaving a cleared hole that is suitable
for installation of the fresh component.
Where you have accidentally lifted a PCB trace, which can EASILY be done - you can repair the trace by running a thin
piece of copper wire along its route. Study the PCB trace diagram in the manual, and make sure that you reconnect the
lifted trace to everywhere it is supposed to be connected to. If a hole's through-hole plating comes out, don't worry - again
just wire up the connection instead. But be very careful, when replacing lifted traces or through-hole plating, be careful to
check on the PCB trace diagram, to see if there are connections on both sides of the PCB. Remember that the job of
through-hole-plating is to join the pads on each side of the board. If the pad at the top is connected to a trace, AND the
pad on the bottom side is connected to a trace, and the through-hole-plating got removed, then you need to make sure
you connect those two sides together. The easiest way in this case is to just apply solder above and below the PCB to
connect the wire of the component to the respective pads. It does NOT happen often, because on the QCX, most of the
traces are on the lower side of the PCB. But it is something to watch out for.
Signal tracing with an oscilloscope
Use a x10 probe when signal tracing. Some parts of the circuit are high impedance and a x1 probe will load them too
much and alter the function.
When using the 'scope probe be very careful that you don't accidentally create short-circuits to nearby component pins or
wiring!
Throughout the tracing, do not get too upset if your oscilloscope screen doesn't look exactly like my examples. There are
a myriad of possible reasons for this:
Your oscilloscope is lower quality than mine
Your oscilloscope is HIGHER quality than mine
Your 'scope probes are differently adjusted to mine
Even where you clip the ground lead, will make a difference in the observations...
Component tolerances and build differences mean amplitudes and waveform shapes may differ
Etc etc
RF is tricky stuff. Just get the general feel for it. Usually when there is a fault, it will be quite obvious - the results will be
VERY different to my examples, not just slightly different.
Digital section
You have GOT to sort this out first, everything depends on the microcontroller operating properly. We cannot go any
further, until it is working.
LCD all black blocks, or all blank
If the LCD looks like this, or it looks like that...

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...then most commonly, the problem is simply that the LCD contrast trimmer R47 has not been adjusted. This is a very
sensitive adjustment (though easier to adjust from PCB Rev 3 onwards). Do not be tempted to assume that the voltage on
the contrast pin of the LCD should match the one shown in the manual - it will vary from LCD to LCD.
If adjusting R47 still doesn't bring visible text to the display, then check with a DVM that you can see the voltage at the
LCD contrast pin (pin 3, i.e. 3rd from the left) does vary from 0 to 5V as you turn R47.
If you see the voltage varying correctly from 0 (full anti-clockwise) to 5V (full clockwise) as you turn R47, then there could
be a DIFFERENT problem.
Check with a DVM that there are no short-circuits between adjacent LCD pins, or between any LCD pin and Gnd or Vcc.
Check for continuity between each LCD pin and the corresponding pin on the processor. You can do this with the board
upside down, and the LCD module plugged in.
If ALL of that checks out fine, then there is one more possibility for a blank screen, in firmware versions up to and
including 1.00e. In these versions the firmware carries out initialization steps which clear the screen. Then it tries to
communicate a command to the Si5351A Synthesizer chip IC1. The commands to IC1 are sent over an I2C serial
communication link. If for some reason, the Si5351A does not respond to the command to acknowledge it, then the I2C
bus will hang up while infinitely waiting for the response of the Si5351A. The screen will remain blank indefinitely and
mislead you into thinking there is a problem with the processor or the screen. So it is worthwhile to check the Si5351A
soldering, preferably with a jeweler's loupe or one of the inexpensive USB microscopes, if you have one. Check for solder
bridges between pins, mis-soldering, a loose pin, or any other sign that the 10 pins of the Si5351A are not properly
connected to the PCB.
Hopefully after all this, you will find the solution to the blank screen.
LCD top row is blocks, bottom row is blank
If the screen looks like this...
...then be aware that this is the default state of the LCD module at power-up, if nobody has told it to do anything. It
indicates that the processor has not issued any commands to the LCD. This is usually because the processor is not
running. If the processor is not present at all, the LCD will look like this too. So though it sounds silly, do check that in your
excitement you didn't forget to install the processor ;-) Also check that the processor is installed correctly - the dimple at
one end of the processor must face to the right side, where it will match up with the dimple indicated on the PCB
silkscreen (white print).

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If the processor is present and orientated correctly, the next thing to check for is that all the pins are properly inserted into
the socket. We did have one or two cases where the 28-pin socket was missing a pin. A manufacturing defect.
Presumably you would have noticed that during soldering - but just check again now too. When the chips arrive from the
factory, the legs are slightly splayed outwards. It is necessary to gently squeeze them inwards so that they fit in the
socket. It is quite easy to push the chip in the socket, and not notice that one of its legs has bent inwards rather than
properly going into the socket. So check that all the pins are nice and straight, and all are properly in their sockets. If any
pins of the processor are not properly connecting with the socket - then this cause the processor to fail to start; it can also
cause many other kinds of failure too, depending on which pin it is, that is not connecting.
Unreliable processor boot up
A common cause of the processor failing to start up, IF your board is a Rev 1 or Rev 2 PCB, is described by this simple
modification described here, to ensure reliable processor startup. On the Rev 3 PCB, this modification is already included
as standard, so this paragraph doesn't apply. Whenever I am fault-finding a QCX, if it is a Rev 1 or 2 PCB, I always
implement this modification first, before anything else.
If that doesn't help, you can also check that the 20MHz crystal is oscillating. This can be done with an oscilloscope with
x10 probe at IC2 pins 9 and 10 (NOT a x1 probe, which will load the crystal oscillator pins of the processor too much).
You could also listen for the signal at around 20MHz, with a general coverage receiver. But bear in mind it will likely be
several kHz off frequency, so tune around for it.
Right at the END, you can suspect that the processor is faulty. This does happen but is rare. My GENERAL RULE is that
we intuitively always want to suspect defective components, rather than our own mistakes in assembly; but when we find
out what is wrong, it usually turns out to be OUR MISTAKE, not a defective component! We want to make this work, not
massage our ego... so always suspect defective components ONLY as a last resort when all other avenues of
investigation are exhausted.
When the processor boots up, the first thing you will see, is the question about which band to select. You have to turn the
rotary encoder to select the band you built the kit for. It is important to select the correct band, NOT to choose the wrong
band then tune to the one you really want, using the rotary encoder. This is because the firmware adjusts the Si5351A
commands for the chosen band, in the configuration which generates 90-degree quadrature oscillator signals; but this
configuration must be altered for very large frequency changes, such as to another band.
Wrong band selected
If you chose the wrong band, the best way to recover the situation is to undertake a factory reset using menu item 7.8,
then turn the power off, and on again, and this time select the correct band.
Buttons / rotary encoder don't work properly
If you discover that you cannot operate the buttons correctly, or the rotary encoder does not work as expected - then this
can occur also due to the lack of the reliable processor start-up modification, on a Rev 1 / Rev 2 PCB. See above. It is
important to realize that even if the processor appears to have booted up properly, and is writing to the LCD etc., in some
cases the unreliable start-up issue can mean that the Analogue-to-Digital Converter (ADC) subsystem does not work
properly. Reading the three buttons depends on the ADC. If the ADC is not working, then the buttons will be mis-read, or
not read at all. So again, it pays to implement the modification.
RECEIVE signal path
Failure to get BPF peak, or a high enough signal strength reading
The best way to check that the receiver is working, is to use menu item "8.7 Peak BPF". If the displayed signal strength
shows at least 8 or 9 in the display top right corner, and if you hear a loud tone in the earphones (do NOT put them in your

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ears while you do this!), then the BPF peaking has probably worked well. This signal strength is independent of volume
control setting. If the signal strength is less than 7, or if there is no peak, then it is necessary to investigate further.
If the peak in BPF occurs with the trimmer capacitor fully open, or fully closed, then it is necessary to add or remove turns
from the T1 transformer. This is described in detail in the manual, so please refer to the relevant section.
I-Q and Phase balance fail to show a minimum
Now check the unwanted sideband alignments:
8.8 I-Q bal.
8.9 Phase Lo
8.10 Phase Hi
Note that the displayed signal strength level unfortunately depends on the volume control setting, because the ADC for
these weak signal (unwanted sideband) measurements is tapped from the signal path AFTER the gain control. It would
have been nice if this was not the case, but it would have required another op-amp.
If the displayed signal strength is 12 or 13, then this is too HIGH, it means the ADC is saturated and it will not show
anything meaningful. So turn down the volume a bit.
If the displayed signal strength is 1 or 2, then the signal strength bar will jump around too much because you will mostly
be measuring noise rather than the signal - so, turn the volume up a bit.
Once you have the signal strength display of a reasonable magnitude, you should be able to make the adjustments as
described in the relevant manual sections. If you do not get the right signal strengths or if you do not manage to see any
minimum, which should be quite easily seen, then it is time to investigate further with some proper signal tracing through
the receive path. If there is a fault in the I or Q channels, then there will be no unwanted sideband cancellation, and no
minimums can be found. So a fault somewhere in the I or Q signal paths is the usual cause of not finding a minimum.
Signal generation by the Si5351A
When in the menu "8.7 Peak BPF", we can use this to trace the signal generator output, which is Clk2 of the Si5351A IC1,
all the way through the Receive path to the earphones. The Clk2 output of IC1 feeds into the IC3c NAND gate, at IC3 pin
10. During signal generation mode, to inject the signal into the receivers input (right at the RF connector of the
transceiver), the NAND gate is enabled by the processor signal "SIG OUT". Then the 5V peak-peak squarewave coming
out of IC3 pin 8 is routed to the RF connector through the 120K resistor R43.
This is what it looks like on my oscilloscope, at IC3 pin 8. Note the horizontal speed 50ns/div, and the signal frequency
measurement on the 'scope, 10.120MHz - this was on a 30m QCX kit. Don't worry too much that is is not a precise-
looking square wave. 'Scope probes, long wires, 'scope bandwidth... long story, out of the scope (pun not intended) of this
article.

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If you do NOT see a nice 5V peak-peak squarewave here on IC3 pin 8, then you have to trace back further to see why
you do not have a signal from the Si5351A Synthesiser chip (IC1). Check on pin 10 of IC3. You should see a 3.3V peak-
peak squarewave. IC3 pin 10 is connected directly to IC1 pin 6 (Clk2 output of the Si5351A). If you do not see this signal
then check for track discontinuities, check for short-circuits to ground, or to nearby pins.
EXAMPLE: I had one repair case where the IC3 pin 10 was at 3.3V permanently, regardless of what was done with the
processor. Upon inspection of IC1 (Si5351A) with a jeweler's loupe, I could see a very tiny whisker of solder lying between
IC1 pin 6 (Clk2 output) and IC1 pin 7 (+3.3V). This was shorting Clk2 to the Si5351A's 3.3V supply, and explains why
there was no oscillator output signal from Clk2. I could easily remove the solder whisker just by briefly touching IC1 pin 6
with the soldering iron. Problem solved!
Note that this problem of Clk2 shorting to +3.3V would have caused a lot of problems on transmit. On key-down, a high
current would flow from +12V through Q6, through L4, and through the BS170 MOSFETS Q1, Q2 and Q3. Everything
would get very hot and eventually something would fry.
Signal generator injection into the front end
The size of this resistor (120K) has been chosen so that when in the "8.7 Peak BPF" menu, the ADC is not overloaded,
and a meaningful adjustment can be made. However, 120K series resistor into a 50-ohm load, which is approximately
what the impedance of the transceiver's RF in/out is, means the 5V signal is reduced by a factor of 2,400 approx. This is a
2mV signal and it gets further attenuated by losses in the BPF and the mixer... and so it is way WAY too small to be able
to view the signals on an oscilloscope.

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In order to be able to trace an actual signal through the first few stages of the receiver, I first bypass R43 by temporarily
soldering a 330-ohm resistor in parallel. This is easy to do on the bottom side of the PCB. Be careful to find R43, at the
correct place on the PCB, using the layout diagram - it is right next to the BNC connector. Now the attenuation factor is
only about 1/6th. It doesn't have to be 330-ohms. I wouldn't go below 220-ohms which will load IC3 too much and
probably make the signal a bit too big; but 220-ohms, 270-ohms, 330-ohms, 470-ohms, 560-ohms... anything like this
should be good enough. The later stages of the receiver will be massively overloaded. But we don't care - for now we just
want to be able to see the signal as we trace it through the front end of the receiver.
The signal strength will be a bit more or less depending on whether or not you have a dummy load connected. It doesn't
matter. We are only interested at this stage in seeing a signal which is strong enough that we can trace it through the first
few stages. Here's the 0.8V peak-peak signal that I see with the 'scope probe right at the BNC connector.
Low Pass Filter fault
Check with the oscilloscope for a good strong signal at the drains of the BS170 MOSFET's Q1, Q2, Q3. Or alternatively at
the final pad of the LPF inductor L3 (the pad closest to the microprocessor). What you are checking here is that the signal
is coming through the Low Pass Filter (LPF) correctly. If it is not, we have to investigate why.
The most common fault is failure to properly solder to the wire of the toroids in the LPF, which are L1, L2, L3. If the
enamel insulation is not properly burnt or scraped off, the solder will not make an electrical connection between the wire

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and the PCB. So no current can flow. This is easy to check, using a DVM to check for DC continuity through the
inductors.
The same thing applies equally to L4, and the windings of T1.
T1 transformer
The transformer T1 has multiple functions. It is used for band-pass filtering, and to split the phase of the incoming
received signal into two paths, with 180-degree phase difference. These are fed to the double-balanced quadrature
sampling detector (IC4).
Bear in mind the circuit diagram fragment and the section of the PCB layout diagram:

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Now we can check at the top end of the long winding of the T1 transformer. Since this long winding and the trimmer
capacitor act as a band pass filter, you can see that we see a nice sinewave now. This is labelled "1" in the diagrams
above. It is the point shown by the arrow, in this photograph:
Note that the amplitude of the sinewave seen at this resonant tank circuit does change when you adjust trimmer capacitor
C1. Try it, just to prove it and to check. This is not an accurate way to adjust the band pass filter, because the 'scope
probe itself also loads the resonant circuit. So it is not a substitute for using the configuration menu item. But it's nice to
see that the theory holds true in practice. And it also verifies that the BPF circuit is working properly - which is what this
fault-finding is all about, after all.
If you can't adjust the amplitude with the trimmer capacitor, then check that the trimmer capacitor is correctly installed and
soldered; and check that it was not broken during the installation; or the delicate plastic melted by over-heating.

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Next we can check at the outputs of the secondary windings of T1. These are the windings which feed pin 7 and pin 9 of
the Quadrature Sampling Detector (QSD), IC4. Now although the amplitude is a bit weaker now we can still observe it
quite well, even after the BPF adds some loss, and after the phase-splitter formed by these two transformer windings and
loaded by the Quadrature Sampling Detector's sampling capacitors C43-C46.
This is a bit tricky to see, because there is also an audio frequency component leaking back through the mixer, and other
unpleasantness. If you have a Digital Storage Oscilloscope you can switch on the 20MHz Bandwidth limiting on both
channels, and you can use it in Single-shot rather than continuous Run mode - then you can grab a screenshot like this
one below and see what is going on. The upper trace (Ch 1) is IC4 pin 7, the lower trace (Ch 2) is IC4 pin 9.
The oscilloscope probes for Ch 1 and Ch 2 were connected to the points indicated by the yellow arrows. It is easier to
hold the 'scope probes on these points, than on the pins of IC4 itself, since the IC is Surface Mount and a bit small.

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Note the difference in amplitude, which is not very important, this is a Double-Balanced QSD, and everything cancels itself
out, all sins disappear, and even if they didn't, we still have an amplitude balance adjustment later, which is R27, the I-Q
balance trimmer potentiometer.
Note the 180-degree phase difference between the two channels, which IS important. Critical in fact, to the design of the
mixer, which is a Double-Balanced QSD. If the two input signals to the QSD do not have a 180-degree phase difference,
then it is highly likely that you have mis-wired T1. Remember that the manual prescribes a method for winding T1 which is
designed to ensure that all the windings are in the correct "sense" and the ends of the windings go in the correct PCB
holes, to ensure correct phasing of the T1 secondaries. So if you don't see 180-degree phase difference, go back and
check T1 again.

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Quadrature Sampling Detector (QSD), IC4
Next we can actually observe the audio frequency signals, with 700Hz frequency, at the four outputs of the Quadrature
Sampling Detector circuit around IC4. Note that from now on, I have changed the horizontal speed to 1ms/division, for
suitable observation of 700Hz audio frequency signals. The signals are a bit wide, because superimposed on the audio
frequency, is still a component of the local oscillator frequency. This is filtered out in the pre-amp stages by capacitors C4
and C7.
The following traces are taken from resistor R5 (Ch 1) and resistor R9 (Ch 2), at their junction with the Quadrature
Sampling Detector's sampling capacitors (these are C43-C46, refer to the circuit diagram). If you have assembled the
PCB in the recommended way, putting the body of the resistor on the side where the silkscreen has a circle drawn (refer
to layout diagram in the manual), then the side of the resistors R5, R6, R8 and R9 nearest the top of the board, will be the
long wire of the resistor; it makes a neat hook to attach the x10 'scope probe to!

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You should check the waveform on each of R5, R6, R8, R9, which is equivalent to saying, across each of the capacitors
C43-C46. You are expecting to see 700Hz audio frequency signals. Yes they all have a slightly different amplitude, but
you want them to be approximately equal; and yes, the waveforms are not really nice sinewaves... just ignore that too, for
now. Just make sure that all four signals are present and look very approximately similar.
If any or all of these four QSD outputs do not have a signal, then we have to check the circuit around the QSD.
The first thing to do is to check the Clk0 and Clk1 outputs of the Si5351A Synthesiser. These have a test pin soldered in
the PCB (if you followed the assembly manual accurately). You can easily check there with an oscilloscope and should
see the 10.120MHz squarewave (for 30m; for other bands, of course it will be the selected alignment frequency for that
band). If you do not see either of Clk0 or Clk1 then you have to examine the Si5351A for any issues with soldering, such
as short-circuits, solder blobs (too much solder), poor connections (too little solder). Many of these defects will be possible
to resolve, with careful soldering.
If you see Clk0 and Clk1, but you are still missing one of these four phase outputs, or if one looks very wrong, then you
should inspect carefully the FST3253 multiplexer chip, IC4. Again inspect under a bright light and optical magnification,
look for solder bridges or other defects, as for the Si5351A. The Si5351A and the FST3253 were soldered by SMD
machines in the factory, but just like humans, even these machines are not 100% perfect.
Also check that the capacitor and resistance values are correct, and all these components are correctly installed and
soldered. I once made the mistake of soldering the four resistors where the four capacitors should go, and the four
capacitors where the four resistors should go. I have no idea why I am even admitting this in public...

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Audio pre-amp, IC5
Now we will check the audio pre-amp, IC5. The following trace shows IC5 pin 1 (upper trace, Ch 1) and IC5 pin 7 (upper
trace, Ch 2). These are the famous "I" and "Q" channels. Notice that I have changed the vertical scale to 5V/division. The
waveforms are horrendously distorted. Why? Simply because these pre-amps have 36dB of gain. The small input signal
results in a large output signal, after 36dB of gain... and the output amplitude cannot be represented accurately by the
operational amplifier, whose outputs cannot stray beyond its supply voltage rails (0V and 12V in my case). So you get this
nasty clipping.
I'm using 500us/horizontal division now - but either 500us or 1ms is fine for looking at 700Hz audio frequencies.
Since we have now come to a stage of the receiver where we have some gain applied, we need to now reduce the
amplitude of the signal injected into the input. I removed the 330-ohm resistor in parallel with R43 and instead, soldered in
a 10K resistor.
Now the vertical scale on the 'scope is 500mV/division and you can see the 1.1V peak-peak amplitude is a nice 700Hz
sinewave without any visibly evident distortion. Again the I channel (IC5 pin 1) is top ('scope channel 1) and the Q channel
(IC5 pin 7) is bottom ('scope channel 2).

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Now if I shift the Channel 2 trace ("Q" channel) to overlay the Channel 1 trace ("I" channel) you can very clearly see the
90-degree phase offset between the I and Q channels. The amplitude is also very similar.

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Now, if you see signals that are wildly different from this, or not with the right phase offset, or not approximately equal
amplitude - then worry about IC5 and its surrounding circuits. This is the area of the circuit to examine carefully for faults,
dry joints, incorrect component values, solder bridges, etc.
IC6 and IC7 phase shift circuits
Now it is time to check the 90-degree phase shift circuits, the four op-amps made by IC7 (I-channel) and IC6 (Q-channel).
My simple technique is to look at pin 1 and pin 7 of each of the chips IC6 and IC7. At each of these outputs I am expecting
to see a clean 700Hz sinewave and the amplitude should be practically the same as in the former 'scope screenshot. The
audio phase shift circuits are supposed to have a gain of 1.
NOTE: that the only exception to this, is the case where 1K and 3.3K are swapped. This was the case in some early batch
kits, due to an error on the silkscreen. The silkscreen wasn't changed in later batches, but the assembly manual was
altered to match the PCB silkscreen. If you built your kit using a manual version 1.00 to 1.07, then you have the swapped
1K and 3.3K resistors, R19 and R25. If you built your kit using manual version 1.08 or above (published 23-Oct-2017)
then R19 and R25 are not swapped. These two resistors R19 and R25 are next to each other on the board. R25 is closest
to the IC7 chip body near pin 8 and it is supposed to be 3.3K. R19 is supposed to be 1K and is next to the R27 I-Q
balance trimmer potentiometer. If these two are swapped, it is nothing to worry about. It means that the gain of IC7a will
be increased, so it no longer has unity gain. By good fortune (or otherwise, since it masked the problem for some time), if
R25 is 1K and the IC7a gain is increased, then R19 being 3.3K largely compensates for this unexpected gain, cancelling
out the problem. Remaining amplitude imbalances can be adjusted out using R27. So the swapped resistors are NO

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PROBLEM, I am just mentioning it because if you saw the sinewave at IC7A pin 1 looking 3x as big as the one at IC6A
pin 1, then this is normal IF you have the swapped resistors. No panic.
This screenshot below shows the signals at IC7a pin 1, and IC6a pin 1. These are the outputs of the I and Q channels,
after the 90-degree phase shift has been applied. Remember that the kit is in menu item "8.7 Peak BPF" where the signal
generator is injecting a tone 700Hz higher than the receiver oscillator, resulting in 700Hz beat note. This is on the
WANTED sideband, so we expect that after the 90-degree phase shift, these two signals will be IN-phase. In the
screenshot below, you can see that the signals have similar amplitude but are displaced - this is a phase error.
Now you can easily adjust the phase shift adjustments R17 and R24, to bring the two channels into phase. As you turn
R17 and R24 you will see the two sinewaves line up on top of each other. This is only a rough adjustment, the
oscilloscope won't show tiny phase errors closely enough to get really good unwanted sideband cancellation. This should
be done only using the menu items 8.8, 8.9, 8.10. I only mention it here because it is useful to check these things during
fault-finding.

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Signal-tracing through the rest of the audio chain
I didn't take any more screenshots of the process. You get the idea, hopefully. Just keep going, from left to right across
the circuit diagram, checking the output of each op-amp - which is either pin 1 or pin 7 of the dual-op-amp chips IC5, 6, 7,
8, 9 and 10. Use a 330-ohm (or similar) resistor temporarily soldered across R43 in the first few stages, where the signal
level is low. Change it to 10K to check IC5, IC6, IC7 outputs (since 36dB of gain has been applied by the pre-amps made
from IC5 op-amps). Then remove the 10K resistor completely for checking IC8, IC9 and IC10 outputs. If the volume
control is too high (too far clockwise) then you will see the output of IC10b and IC10a is a distorted sinewave or clipped
(reaching near the voltage rails). This is normal, it just means that you are over-driving the amplifiers. Even with a 120K
signal, the 2mV input injection is a MASSIVE signal compared to the weak signals well below 1uV that the radio can
detect. Just reduce the volume.
As you move from left to right, if there is a fault in the Receiver, then at some point you will find the error. The signal will
disappear, or the amplitude will be wrong, or the signal distorted, or some other problem. In these cases, check the
relevant components near that op-amp carefully, for soldering errors: dry joints, solder bridges, solder blobs, etc. Some
thin solder whiskers can cause a short, and be nearly invisible - so use a bright light and optical magnification if possible.
You can use a DVM to check for shorts between nearby components, or shorts to ground, shorts to the supply voltage.
Using these techniques, I have a 100% success rate so far in finding faults in the receiver chain and getting them
working.