Re: Thanks to the trouble shooting guide.


Thanks Dave, I am about to start building!


Colin – G8FRA/M5FRA


From: <> On Behalf Of David R. Hassall WA5DJJ
Sent: 16 June 2018 22:18
To: QRPLabs <>
Subject: [QRPLabs] Thanks to the trouble shooting guide.


Dear Members,


I was putting together my 12th QCX transceiver as a receiver for my WA5DJJ’s QRSS ALL BAND GRABBER which is going to be used for the 2200M band.   Having built 11 other ones with no difficulties, I was a bit miffed when this one didn’t  do the I/Q balance or either of  the phase lo and phase hi adjustments.   The first thought was Solder connections which I went over each one very carefully but could not find anything amiss.   So, from reading this message board a lot of this kind of indications were attributed to the winding and installation of T1.   So, I carefully inspected T1 trying to make sure that I wound and installed it correctly and all of that looked good.   So, it was time to dig out the Hans’s troubleshooting  guide and see what it could tell me. 


Though I do the items in the General Checks before starting as a matter of looking over the board as I am building it, I did go back again and recheck the solder connections, the installation of the IC’s and other parts, I didn’t see anything wrong or that would cause the adjustments not to work.  So, went to SIGNAL TRACING WITH AN OSCILLOSCOPE.  The Digital section was OK, No problems there so on to the RECEIVE signal path.  The BPF peak was also OK and capacitor C1 peaked in the proper location.  I am now down to I-Q and Phase Balance fail to show a minimum.   Signal generation of the SI5351 was exactly as the screenshot as well as the channel 1 and 2 signals at T1. The phase measurements of channel 1 and 2 were also 180 degrees out.  (so that eliminates T1 as being the problem) I also checked the CLK0 and CLK1 test points which showed the 90degree phase shift of those two outputs from the SI5351 synthesizer.


Quadrature Sampling Detector IC4 measurement took a bit of juggling the oscilloscope controls but I finally ended up with a close representation of the screen shot for the signals.  They were a little different that the screenshot but they were there and that was considered a good indication and I went on the IC5.


AUDIO PRE-AMP IC5.  I was able to get the signals in this screen shot rather quickly and they were real close in amplitude and frequency.


IC6 and IC7 PHASE SHIFT CIRCUITS.  Moving my O’scope leads from pins 1 and 7 from IC5 to IC6 there was NO Output at all.  ????

I checked input pins 5 and 6 and there was a signal but nothing on output pin 7.   The A section of IC6 was completely void of signal.

So I found the spot where the signal disappeared but not the cause and started searching for the cause.  (REMEMBER HANS’s WORDS.  Look for the simplest cause of the problem)  NO IC6 was not bad.  NO all the resistors and capacitors were their proper values. (while checking values and looking over the top of the board I found the culprit) The wire lead from R22 was shorting out against the metal body of the rotary shaft encoder SW1.  Standing R22 up straight cleared the short and restored proper operation.


I am an experienced kit builder. I have built 11 other QCX receivers for my grabber.  But I failed to see the resistor touching the back side of the metal encasement of the shaft encoder.  From now on I will add a step in my notes to go to the top of the board and stand all the resistors up straight.  That will force me to check each one individually to make sure something is not touching something it shouldn’t.

Also notice that it was a simple thing that was causing the problem.


I Hope this email will help some other exasperated builder, Who when he turns on his QCX for the first time and finds it doesn’t work like it was described to take heart and start looking around carefully for something simple that can be causing the problem.   Soldering is one of them but parts positioning is another.   If I had been more careful in looking over both the top and bottom of my board, I could have saved a few hours of tedious O’scope troubleshooting.   It can happen to any of us, experienced or not.


Take care and have fun.  

73 Dave Hassall WA5DJJ  Las Cruces, New Mexico





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