QRP Labs QCX Troubleshooting Manual Download Page 17 | Manualshive

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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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Summary of Contents for QCX

Page 1: ...er 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...

Page 2: ...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...

Page 3: ...t 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...

Page 4: ...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...

Page 5: ...1A 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 u...

Page 6: ...o 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 no...

Page 7: ...e 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 d...

Page 8: ...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 sh...

Page 9: ...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 seei...

Page 10: ...nd 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...

Page 11: ...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 configu...

Page 12: ...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...

Page 13: ...els 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...

Page 14: ...ocal 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 Quadra...

Page 15: ...elected 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 solde...

Page 16: ...annot 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 freq...

Page 17: ...of 25 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...

Page 18: ...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...

Page 19: ...z 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 amplit...

Page 20: ...ching 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...

Page 21: ...put pins 1 and 7 Inspection revealed no obvious flaws and I decided there must be a problem with IC6 and I decided to replace it It was VERY difficult to remove because it is sandwiched right between...

Page 22: ...Si5351A Clk2 output to be gated through to the PA transistors The 74ACT00 is also supposed to produce 5V peak peak at its output so that the three BS170 MOSFETS Q1 Q2 and Q3 are saturated in the ON O...

Page 23: ...ace 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 conti...

Page 24: ...y between the pads of L4 Be sure to touch the DVM probes on the pads NOT on the wires themselves If there is no continuity apply the soldering iron and some more solder and hold it there for maybe 10...

Page 25: ...ltage 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...

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