I built 40m version QCX. It Works fine, I was surprised how this simple radio clear recieve.
But as radio has no AGC, I need use a volume knob a lot. I kept it most of the time almost on zero. The total gain is just too much for 40m (or 80m), I believe optimal for higher bands.

As a 'secure' solution I modified R37 from 1 K to 10K that decrease gain by 20dB. I kept R38 and C23 as it defines cutoff frequency (1326 Hz) for low pass filter.
It works fine, maybe sidetone is quiter. But I am still thinking how to lower gain on first stage IC5.
I cannot touch 100 ohm resistors R5-R9 (input impedance of amplifier) as they are tighten with quad. sampling detector. What about R7-C4, R10-C7 in feedback?
2k2+47n, gain -13dB
3k3+30n, gain -9.7dB
cutoff freq. remains the same

What do you think? Could it affect IP3 / sensitivity of whole in negative way?

73
Ondrej OM4DW

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I built 40m version QCX. It Works fine, I was surprised how this simple radio clear recieve.
But as radio has no AGC, I need use a volume knob a lot. I kept it most of the time almost on zero. The total gain is just too much for 40m (or 80m), I believe optimal for higher bands.

As a 'secure' solution I modified R37 from 1 K to 10K that decrease gain by 20dB. I kept R38 and C23 as it defines cutoff frequency (1326 Hz) for low pass filter.
It works fine, maybe sidetone is quiter. But I am still thinking how to lower gain on first stage IC5.
I cannot touch 100 ohm resistors R5-R9 (input impedance of amplifier) as they are tighten with quad. sampling detector. What about R7-C4, R10-C7 in feedback?
2k2+47n, gain -13dB
3k3+30n, gain -9.7dB
cutoff freq. remains the same

What do you think? Could it affect IP3 / sensitivity of whole in negative way?

73
Ondrej OM4DW

I reduced R38 (originally 120K) in my 80-meter QCX to 12K for a 20 dB reduction in audio gain.? It had way too much with the 120K.? About right now and I turned up the sidetone level a bit to compensate.? I'm think about ways to add AGC of some type to this really fine rig.? Seems like one could take advantage of the audio sampling that goes on with the audio1 line that is feeding one of the ADCs in the CPU.? I think that is the source for the S-Meter display data, so maybe use a JFET in the feedback where R38 is and shut down the audio gain that way.? More experiments to do!? :-)

72,

Jim, K8IQY

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I have a real simple AGC circuit that works perfectly with this kit. ?Im on vacation this week but will publish it when i return to my office next week. It uses a transistor and diode charge pump to control the gate of a fet for variable gain in the audio chain. ?By changing the charge capacitor or the parallel resistor you can make it variable anywhere in the fast to slow range.?

Bill,
I'd like to see your AGC circuit, if you wouldn't mind publishing it.? The QCX could sure use some AGC action.?

73/72,
Dean K5DH
20m QCX msn 612

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Ok im getting home late tonight from a road trip so will post tomorrow. I have two circuits, one audio driven that is plagiarized from the BIXT group and adapted for this application...easy to add because no butchering of the radio needed, and an RF chain driven circuit that is more complex and requires radio surgery.?

Completed the construction of my 40M QCX today. No problems, aligned nicely. As has been stated, too much overall RX gain for 40M. I added a 12K resistor across the original 120K at R38 (similar to what Jim K8IQY did, but I just added a 12K on hand across the original, on the back of the board. Gain is now reasonable, and plenty enough for 40M. Initial impressions are favorable, and I will bring it to the New England QRP Club meeting tomorrow for "show and tell".

Thanks Hans (and all involved for qrp-labs), for a very enjoyable build, and an outstanding value!
72/73,
Bruce, N1RX

--

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Bruce, your modification certainly tamed the audio gain, however I found the decode capability no longer worked satisfactorily.

73 Nigel M0NDE

All,

I found that when I changed R38 to 12K, I had to readjust the decode parameters a bit.? I have 5.1 Noise Blanker set to 20 and 5.3 Amplitude Average set to 50.? There may have been other parameters that I've missed that I also set, but my 80-Meter QCX decodes CW just fine.

72,73,

Jim, K8IQY

On 18 November 2017 at 01:14, Bruce Beford N1RX <n1rx@...> wrote:

Completed the construction of my 40M QCX today. No problems, aligned nicely. As has been stated, too much overall RX gain for 40M. I added a 12K resistor across the original 120K at R38 (similar to what Jim K8IQY did, but I just added a 12K on hand across the original, on the back of the board. Gain is now reasonable, and plenty enough for 40M. Initial impressions are favorable, and I will bring it to the New England QRP Club meeting tomorrow for "show and tell".

Thanks Hans (and all involved for qrp-labs), for a very enjoyable build, and an outstanding value!
72/73,
Bruce, N1RX

--

73 de Nigel Evans
M0NDE QTH Burnham on Sea, Somerset




GEO Shop

shop@...

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Nigel- Have you tried adjusting some of the decode parameters to see if that helps? (noise blanking, speed avg.) You could also experiment with the value of the paralleled resistor across R38 (increase it) to find a better balance between RX gain and decode performance.
GL,
Bruce N1RX

-- Have played with my 40 meter QCX for several weeks.? like others I was experiencing so much gain that the volume control was probably only being used in about 10% of its range.? I did not have a 12 k to replace R38 with but the 15k that I did have seems perfect for my taste. I was ab!e to remove R38 with quality solder wick and replace it on the top side? ?My resistor was a .5? watt?but it fit and I am happy with the results.? Now to tackle the AGC
?

Rick w9rea