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Soldersmoke DCR challange

This page was last updated : 20-Mar-25 12:39:33

As loyal follower of the Soldersmoke podcast, I was aware of the SolderSmoke DirectConversionReciver Challange (check out the dedicated discord server channel) and end February 2015 I decided that I was going to take it on. But as so many things, I wanted to go my own way with it and give it PH2LB twist. So I decided I wanted to use own designed 3D printed part like the "A ACTION glue stick holder for a PTO.". That PTO is based on the GlueStick PTO design from OM Nick M0NTV.

After a few hours thinking about what I would be needing, I sat down and used Tinkercad to draw various parts like :

• a speaker holder
• a set of glue stick holders
• a 9V battery holder
• a variable resistor stand
• a BNC chassis part stand
• a stand for the on/off switch and a DC jack.   

And using my 3D printer I materialized the parts to real life objects.  

TODO ADD TEXT

Part 1 : the PTO. 

And after a evening playing around with component setups, part 1 "The joy of oscillation" was ready. 

Because I didn't have the correct Silver Mica capacitors yet, the frequency isn't for the 40 meter band.

My local supplier didn't have a wide range of Silver Mica capacitors, so i had to look for another supplier wich I found in the Dutch web shop from Tonefactory. They are specialized in parts for building or restoring guitar tube amplifiers. And they sell a large collection of Silver Mica Capacitors in various values.  So I ordered a set of various values and Tuesday they already got  delivered. 

Update 2025-03-04 afternoon 

Because my PTO uses the glue sticks, the capacitors would have different value vs the original 660pF. I used 12 turns on the my glue stick and used a combination of 100pF / 220pF parallel. I hotglues a piece of rond 15mm and 36mm length copper tube as  Permeability material in the insert.  

With these values I get frequency range from  6.852 to 7.580 MHz. Which perfectly covers the 40m hamradio band and the 41m broadcast band.

 

Hmm it's clipping, what did I do wrong??? 

Update 2025-03-04 evening

This distortion was getting under my skin. So I sat down looked at the schematic, verified the components and could figure out what I have done wrong. 

So after measuring all various parts again, replacing the FET 2 times (did I have a bad stock?), I started to replace R6 with different values, and at some point I was getting improvements.

The best I could get out of it was replacing it with 47 ohm and removing the C21.

This did give a nice stable signal, although some distortion in the negative part of the signal, it was much better then I had and even the harmonics are much better this time. 

For testing I hooked up a 40dB attenuator (nice 50 ohm RF load) to my TinySA to measure the spectral harmonics. 

 

But was still curious where the distortion came from, so I used my scope to look into the points mentioned below to get a idea if the distortion was already in the signal or due to the buffer part.

• CH1 is the LO output (loaded with a 47 ohm resistor).
• CH2a is the original oscillator signal 
• CH2b is the oscillator signal after decoupling C4 

Screenshot below is whowing CH1 vs CH2a, so the buffer inverts like expected but also doesn't have a gain but some attenuation (half the signal). 

Screenshot 2 is showing CH1 and CH2b, same as CH2a so my conclusion is that the distortion is all ready in the oscillator part. So I can shift my investigation to that.

 But enough for this evening. 

Update 2025-03-06 evening

I dropped a message with  the image above on the Soldersmoke discord server and with a few hours from Tony VA3ZOT  telling me that it looked fine and with the mixer attached it would change again. Andy KB7ZUT confirmed that he had the same kind of harmonics and that the signal was absolute good enough to put the mixer to work. 

Part 2 : the mixer

Now knowing that the PTO was within specs, I constructed the diode ring mixer and wanted to test it with a dual signal generator (aka controlled environment) to see it worked. 

So I hooked up my Rigol DG1022, configured CH1 as the RF source with a 1.2 KHz modulated AM signal and CH2 as the LO oscilator.

Using a 50 Ohm resistor to have a little load on the ring mixer, I injected the modulated RF signal at RF-IN (L7) and LO signal on LO-IN.

I hooked up my Rigol DS1054Z to the RF signal (by a T-adapter) to CH1, the LO signal to CH2 and the AUDIO to CH3. 

Because the RF and LO where on the same frequency, the AUDIO was instantly "decoded". 

Shifting the LO signal a few tenth of Hz immediately showed a degrees of the signal and mixing products appeared.  So the mixer was working as expected.

Next step was to hook up the PTO and try to tune into the 7 MHz RF signal. 

And after a bit tuning around I got the hang of it and got a nice kind of stable AUDIO signal. 

So the first 2 stages are ready. Next step will be band pass filter.

But that one for another evening. 

 

Part 3 : Bandpass filter

That evening would become a Sunday afternoon which for which I needed to be at home because of  few larger 3D print jobs (I never 3D print when I'm not home). 

After winding the toroids, I building it up on a small piece of PCB to be able to play with the values, I found out that for me C18 and C19 would be best when I was using a 100pF fixed capacitor with a variable 7-35pF mica capacitor. 

That combination allowed me the create a nice  passband show below with a modest -2 dB attenuation. 

After re-building it on the final PCB, a minor re-adjustment of the variable C's was enough to get the pass band back as before.

I might be spreading / compressing  the core windings to see if I could get out a little more of it. Or may even replace C18 .. C20 with Mica capacitors like I used in the PTO.

But that will be for another time. 

Part 3b : Bandpass filter with silver mica.

I replaced the fix value band pass caps with silver mica one's and it got me 0.5 dB less attenuation. Not much, but every thing counts :-)

 

Part 4 : 3 Stage audio amplifier. 

Today (Monday 17 march) after I finished house keeping, it went to the shack to build the final part of this project. The 3 stage audio amplifier.  

Although the build was very straight forward I had to improvise with the loudspeaker transformer. I only had a 600 ohm / 2x50 ohm model, but it seems to work out.

I isolated the audio amplifier and fed a 10mV (Vpp) 1 KHz sinewave signal into the audio input.

The audio sounded like a 1 KHz sine wave, but there was some distortion and checked out all the values of all the components (maybe I swapped a resistor or something). But when those all turned out to be ok,I measured the base DC voltage of the transistors. 

Q5 and Q4  they where set to 2.1 V. Q4 was something different, it was just 1.7 V. Hmmm. Would expect it to be kind of the same. And evening after changing R14 to 470 Ohm and replacing L4 with a 1K resistor, it still would get near the 2.1V.

So I started to probe the test points as shown in the schematic above. 

Test point 1 : base of Q5. No distortion. 

Test point 2 : collector of Q5. No distortion and some 19x gain (vs testpoint 1). 

Test point 3 : base of Q3. No distortion and due to audio taper lower signal as expected.

Test point 4 : collector of Q5. There you have it, some clipping is happening.

Test point 5 : base of Q4. As expected also some clipping there.

Test point 6 : collector of Q5.  is the incorrect transformer making the signal going banana's?? 

Just for testing, I disconnected C8 from the base of Q4. 

Test point 4 : collector of Q5. The signal now is OK again. And some 10x gain (vs testpoint 3) hmmm would expect something higher because the configuration is almost the same. But maybe that has something to do with the load of Q3. 

And just for good measures, I probed the signal on the floating pin of C8. 

I should have put a resistor to ground on the floating C8 pin. 

What I did do was hook up a amplified speaker (with line-in) and hooked it up to the floating C8 pin. And the audio was distortion free and clear as hell. 
So the main suspect in this case is the Q4 configuration. I swapped out Q4 with a new 2N3904.  

Using my Peak Atlas DCA55 I tested the original Q4 :

HFE=186, Ic = 2.5mA,  Vbe = 0.78V.  Ib = 4.63mA 

Then I tested a 2N3904 from the new batch : 

HFE=326, Ic = 2.5mA,  Vbe = 0.76V.  Ib = 4.64mA 

I find the difference in HFE quite a lot and even after replacing the Q4, the audio was distorted.

So I still have to look into that, but that's something for another evening. 

To be continued . . . . .
  

The (almost) final layout

The proof is in the pudding . . . . 

But even with a distorted output signal we can do some test. It's doesn't sound as nice as with the amplified speaker (with line-in) and hooked it up to the floating C8 pin, but it's a start.

And it's finished :-)

After installing the new transformer and re-soldering every pad, the audio amplifier is working as expected. Didn't verify the signals with my scoop but the audio sounds good to my ears :-)

 

The SolderSmoke DirectConversionReciver Challange : accepted, build, tested and finished.

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