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Audio pop on CW

K4LXY <zehrhj2@...>
 

I think I solved the audio pop problem on CW based on the VE1BWV muting circuit (see uBitx.net)

Earlier I tried this muting circuit, which mutes the audio during transmit, but it also muted the sidetone.

The solution was to use his muting circuit but insert the sidetone on the top side of the volume control, after the muting circuit, instead of earlier in the audio circuit as it was originally.
 
The idea was suggested earlier by Philip as part of his AGC solution (though he incorrectly identified the resistor involved as R235 rather than R253).
 
Here is what I did
Removed R253.
To the end of R235 that was also connected to C252, I added in series a 1K resistor and .1 microfarad capacitor, then going to the yellow wire on the volume control (top).
 
There are two things about my setup that make it different than Philip’s but it should work as suggested above.
 
1) I have a Hi-Per-Mite SCAF after the volume control, and it may provide some amplification.
 
2) Earlier I had replaced R250 with a variable resistor so that I could control the sidetone volume, so I have the option of fine-tuning sidetone volume.
 
73 Howard K4LXY

Bill Carpenter
 

Good work!  I hope that a step by step procedure could be posted for those of us who are schematic/theory challenged.  I can follow step by step instructions ala Heathkit/Elecraft but I totally suck at looking at a schematic and producing a working mod from it.  I love my uBITX but as a CW op that audio pop is a real problem.

73, Bill NZ0T

K4LXY <zehrhj2@...>
 

Bill
The muting circuit, with photos, is on the uBitx.net site.  I could probably send you photos of my addition.  The schematic is only partially helpful because of the gaps.
If you are interested, take a look at the muting description on uBitx.net, then let me know what more would be helpful.
Howard

Bill Carpenter
 

Hi Howard,

Please send me photos of your work.  I have a pretty good idea of what to do but another perspective would help a lot!  My email is nz0tham@....

73, Bill NZ0T

K4LXY <zehrhj2@...>
 

I'll be in touch, Bill, but am posting the following for others who may be interested.

The muting mod is on the uBitx.net site. Here is some info about the mod to get the sidetone back.

You’ll see in the attached schematic that R253 is X'ed out (removed) and a 1K and .1 are in series to the yellow wire on the volume control.
 
If you are looking at the the Bitx board from the front, along the front edge just right of center is a row of SMDs.  The first one on the right is R253.  Remove that, then solder your cap/resistor series combination to the end where the resistor was connected that is toward the back of the board. (You can see from the trace that it is connected to C252).  I soldered the cap to that point, with the resistor connected to the cap, then a wire to the volume control, but you could solder the wire there and attach the cap/resistor series at the volume control.)
 
Farther to the right on the schematic you can see another modification.  That is the sidetone volume control I added, which may not be necessary.  I believe it is a 50K or 75K pot.  R250, which is removed for this, is in the same line of SMDs as the above.

Skip Davis
 

Thanks Howard for the drawing, I also for myself drew it out last night from your description. A question is there any reason why you didn’t try connecting the 1 k resistor to the wiper on the volume control and is the cap needed?

Thanks Skip NC9O

K4LXY <zehrhj2@...>
 

Skip
I don't know the answer to either question - I just implemented Phillip's idea that was in his AGC adaptation.
Here is a low-res photo of my connection at the board.
Howard