But Why Did Swamping Resistors Cure JBOT Oscillations?

OK,  so as I mentioned in SolderSmoke 148 (apprently with screeching in the background -- sorry) I managed to tame a bad JBOT oscillation problem by placing 680 ohm resistors across the primaries of the broadband transfomers in the JBOT.  Now I'm wondering how/why this worked, and what we could do to make this kludge unnecessary.  

Am I correct in suspecting that at the problematic frequency of 250 kHz, the one or more of the tranformers are self resonant, and that the resistors just bring down the Q of the -- in effect -- tuned circuit?  This lowering of Q makes feedback and oscillation more difficult, right?  

Here is the background.   Read from the bottom up.   

There is an old saying in Spanish:  "No hay mal que por bien no venga."   More or less this is the same idea as: "Every dark cloud has a silver lining."  Well, the dark cloud was my techno-agony with the parasitic oscillations (see my plea for h elp from yesterday).  The silver lining was the e-mail from Edgardo, LU1AR, in Buenos Aires that helped me get rid of them.    Edgardo advised putting a resistor across the primaries of the JBOT amplifier stages.  This is an old "lower the Q" trick, the idea being that lowering the Q might help prevent the amp from self-oscillating.     I used 680 ohm resistors.   First I put one across the primary of Q1.  No joy. Then Q2.  No luck.  Then I put one right across the primary of that big output transformer.  That did it!  The parasitics disappeared.   And I still get a nice 4 watts of output.  Thanks Edgardo.  I hope to make a contact with this rig today. 

The real silver lining in this story comes, however, in the form of Edgardo's blog site.  Wow, what an inspiring example of Argentinian Knack.  Radios, telescopes, auto-giros.  This guy is also into homebrew DSB rigs.  Wonderful stuff.  Check it out.  (Google will translate it for you, if necessary, but even if you don't read Spanish, the pictures tell most of the story.) 

http://www.lu1ar.blogspot.com.ar/

I put this URL blog up on the SolderSmoke blog.  

Thanks to all who sent advice and encouragement.  Merry Christmas! 

--- On Sun, 12/23/12, lu1ar  wrote:

From: lu1ar
Subject: [BITX20] Re: JBOT taking off at 250 kHz
To: BITX20@yahoogroups.com
Date: Sunday, December 23, 2012, 9:56 PM

First. You must secure the RF decoupling of +B line. Use a bunch of small capacitors in paralell 1n and 100 nF with 22 uF electrolitic.
Second: Use 1Kohm resistor in paralell with primary of the transformers. Begin with the driver, then the 1st amplifier and then the output transformer.
Let us know results.
Regards.
Edgardo Maffia LU1AR
Buenos Aires - Argentina

--- In BITX20@yahoogroups.com, "iam74@..." wrote:
>
> 
> 
> --- In BITX20@yahoogroups.com, Bill Meara wrote:
> >
> > I'm building my FOURTH JBOT and this one is giving me more trouble than all the others combined.   I seek the help of the group. 
> > Here are the details of my tale of woe:
> > 20 meter rig.  Double Sideband.   The JBOT is fed by a simple two-diode, trifilar toroid singly balanced modulator.  NO DIPLEXER TO SPEAK OF.  At the output of the balanced modulator there is a 1000 uH choke to ground and a .001 uF cap to the input of the JBOT.  AF amp is an LM-386.  VXO is a very simple MPF-102 one stage ceramic resonator VXO with no buffer stage.  7 element low pass filter (three coils, four caps). 
> > All the transformers are wound on FT-37-43 cores.  T3 is four such cores stacked 2X2.
> >  
> > The arrangement works beautifully into a 50 ohm dummy load.  But as soon as I connect it to an antenna (a dipole out in the yard, fed with 50 ohm coax) the output signal gets ugly.  
> > Looking at it on the scope, instead of the nice figure eight pattern (similar to the two tone test pattern of SSB) I get ugly fuzzy outcroppings from the peaks.  Looking at the signal more closely I can see that in addition to the 14.2 Mhz signal, there is another LF oscillation at around 250 kHz. 
> > I've been struggling with this.  I can't get rid of the LF oscillations. The leads are all short and the inputs are far from the outputs.   I've beefed up shielding, and decoupling.  I've sacrificed a chicken to Papa Legba. Nothing seems to help.  
> > I THINK the feedback/oscillation is taking place in the JBOT itself -- not through the
> > audio amp or the balanced modulator or the VXO.
> > I watch the RF and the AF inputs to the balanced modulator to see if there is any difference between the stable situation (with the 50 ohm dummy load) and the unstable situation (with the antenna),  I don't see any differences. 
> > I even put an antenna tuner between the final and the antenna anb made sure that the antenna looks like 50 ohms non reactive.  This seems to help a bit, but the ugly instability is still there. 
> > Help me! 
> > 73  Bill N2CQR    
> >
> 

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Move over Altoids Tins! Legos for Raspberry Pi Case

Too bad the Raspberry Pi board is a bit too big for our beloved Altoids tins.  It would have been a very fitting match:  British board in a British box.   A twelve year-old has solved the problem with Legos:  http://blog.makezine.com/2012/06/11/lego-raspberry-pi-enclosure/
Will this lead to raids on the toy boxes of the grandchildren of SolderSmoke fans? 
Billy's RasPi is running nicely (but no Legos for him).   

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Signal Generator Ideas

Hi, Bill

Howzabout a generator that goes from 1 Hz to 40 MHz with 1 Hz resolution, adjustable cursor so you can step any of the 8 digits, presettable offsets for if frequency, repeater offset, or r.i.t., directly synthesized output so almost no phase noise and 12 bit waveform accuracy? How about a price tag of less than $20 for all the major parts? Start by Googling dd_synth.asm for control software, and rather than buying the PCB recommended in the notes at the head of the program, search for AD9850 on Ebay for the synthesizer board and LCD 16x2 for the display. A 16f628a PIC is a couple bucks. Both the display pinout and the synthesizer pinout match standard perfboard. For a little more than $10 more get the AD9851 board and the upper frequency limit is 60 MHz.

A better way to transistorize the old tube-type generator is to use a dual-gate RF mosfet for the oscillator. You can trim the gain for waveform purity by adjusting the bias on gate 2. Once you find the optimum bias on each band, diode-switch the bias level with the bandswitch. Since gate 2 has an extremely high impedance you can use resistors in excess of a megohm, so it won't affect the Q of the tank coil.

Jim Daldry
Raleigh NC

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Opening Scene of Contact (CQ at the end)

Mike Herr's video on his use of an Arduino to call CQ SKN made it onto Hack-A-Day 
yesterday and provoked some discussion there of the meaning of CQ.  One fellow posted 
this opening scene from the movie Contact.   I thought you guys would like it. 

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Arduino Does Straight Key Night

Bill,
     Hope a good Christmas and excellent New Year. Here at WA6ARA we had a blast for Straight Key night . In the past I have used new keys, old keys and multiple keys. This year we celebrated SKN with a little twist. At Pacificon I purchased a hugh CW key, all made out of wood, about 3 times the normal size J-38. This isn't a QLF key but a piece of beautiful wood working, and fully functional. This was my main key for SKN.
But SKN can require the sending of CQ SKN for a considerable amount of time. Normally a programed keyer would do the job fine. But this is not in the true spirit of SKN, the use of the straight key. So, using a trusty Arduino board and a few lines of programming,  I solved the practical and ethical problem. The Arduino drives a model airplane servo. The servo is positioned such that it's arm presses down on a miniature J-38 key, thus sending the "CQ SKN". Also the miniature J-38, about half size, sort of balances out the larger size of the
hand key, and keeps the universe in check. I've enclosed a short youtube for your viewing enjoyment

http://youtu.be/Rn7Ky9Ib7ug

73
-- 
Mike Herr
WA6ARA
DM-15dp
Home of The QRP Ranch

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Like a Hamfest, but kind of EVIL

For a look at how the hackers do their version of the Dayton ham-vention, check out this video on DEFCON:   http://vimeo.com/56234900

Looks sort of like "Four Days in Hell" to me!  Finally, a group that makes hams seem normal and well-balanced! 

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

A Nice Hallicrafters S38-E Knack Story

Bill - 

It was Christmas of 1958 (5th Grade)that my parents bought me the Hallicrafters S38-E that I still have.  I was recovering from multiple surgeries the previous summer to correct for the effects of Polio when I was 3.  I came down with Polio in the last year before the vaccine was released to the public.  

A bit of bio - because of my physical limitations as a child, I spent a lot of time indoors reading and listening to the radio.  Broadcast AM here at that time was still playing dramas like "The Lone Ranger" and "Big John and Little Sparky".  I really enjoyed them.  Then, on that fateful day I asked my mother why, since our radio plugged into the wall electric outlet and we could hear people talking, weren't other people able to hear us talking on their radio if we spoke into the loudspeaker.  She said, 'I don't know, but the library ( 2 blocks away) will have books about radio that you can read."  The Knack bit early and it bit hard.  

By the time I was in 5th grade I had discovered short wave radio.  I dearly wanted a short wave radio and I wanted a Hallicrafters.  Somewhere I had found a catalog and wow! nothing else would do.  What I didn't realize until I was in my teens was just how hard times were for my folks.  Most families then did not have medical and hospitalization insurance.  My dad was paying off the hospital and the doctors every month almost until I graduated from high school.

But, anyway, I did get the S38-E for Christmas.  At $69.95, it was the least expensive Hallicrafters available.  In reality, it is just an All-American - 5 with extended tuning range.  Tuning was as broad as a barn door, and above 15 MHz it is as deaf as a post.  But it was mine and I loved it.  The short wave bands were still hot in those days as we were just peaking through one of the most celebrated Solar Maxima of the last century.  Many the hours I listened to HCJB - Quito, and Radio Moscow, and Voice of America....   I also found people talking to each other - Wow!  Ham Radio.  And just look at me now.... And those strange beeping sounds - that was actually people talking? And what was that strange Donald-Duck sounding talk all about?

Yes - I still have that same S38-E.  It went to college with me, and has been with me for over half a Century, now.  Yes, it still works. (Needs a replacement IF transformer),  Yes, it will shock the bejeebers out of you if you plug it in wrong.   {I use two filament transformers back-to-back to prevent that.)  Yes, the Knack still has me, but that's another chapter.

72's 
Bruce  KK0S

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High Pitched Audio QRM in SS #148

Two listeners blessed with good high frequency hearing alerted me to the fact that the latest podcast had some annoying high pitched noise in the background.   Peter, VK2TPM was the first to report it -- he sent along the above spectral analysis of a gap between words.  Steve,  W1KF,  also heard it, and provided a clue that I think solved the mystery.  

Steve noted that the noise was not present during the sponsor ads that were included in this episode. 
That let me figure it out.   On Saturday when I was recording this,  I got through about half the show when the power went out (that happens a lot here).  The battery in the laptop I was using to record the podcast saved the day and no data was lost.  The power came back on within a few minutes, but the outage had turned off the main PC in the shack.  That PC (AND ITS FAN) remained off when I was recording the ads.   I did the same kind of noise removal processing that I always do, but this time the noise sample that I used was taken from the mic with the main PC (AND ITS FAN) turned off.  My guess is that the whine from the fan (which I can't hear due to tinnitus) didn't make it into the noise sample, and thus didn't get removed from the podcast audio.    I went back and did a couple of rounds of noise removal and low pass filtering.  I think (I HOPE) I got rid of the offending noise. 
I have uploaded a new and hopefully improved version of the podcasts.  Audio reports will be appreciated.   

One of my New Years resolutions is to improve the technical and substantive quality of the podcast. 

Thanks Peter, Thanks Steve. 

Happy New Year to all! 
  
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

SolderSmoke Podcast #148

SolderSmoke Podcast #148 is available:
http://soldersmoke.com/soldersmoke148.mp3
December 30, 2012

SPONSORED BY usedradiomall.com

-- Hurricane Follow-up
-- Subliminal Mind Control to Encourage Homebrewing (beep-beep)
-- The Shortcomings of the All American Five Receiver
-- Solid-Stating a Heath SG-6 (with Farhan's circuit)
-- VK3YE's Beach DSB Rig
-- Tek 'scope connection to frequency counter
-- Mod to W7ZOI/W7PUA power meter
-- JBOTing my 20 meter DSB Rig -- A Tale of Woe
-- LU1AR "The Most Interesting Ham in the World"
-- Billy's Raspberry Pi
-- Latest QST, SPRAT, Hot Iron
-- Videos: Landfillharmonica,  Knack in Sierra Leone
-- Santa delivered a Soldering Station
-- MAILBAG

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

He Should Have Used a Low Pass Filter!

http://www.sun-sentinel.com/news/broward/hollywood/fl-pirate-radio-hollywood-20121229,0,5142922.story

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

A Christmas Present from Argentina

There is an old saying in Spanish:  "No hay mal que por bien no venga."   More or less this is the same idea as: "Every dark cloud has a silver lining."  Well, the dark cloud was my techno-agony with the parasitic oscillations (see below).  The silver lining was the e-mail from Edgardo, LU1AR, in Buenos Aires that helped me get rid of them.    Edgardo advised putting a resistor across the primaries of the JBOT amplifier stages.  This is an old "lower the Q" trick, the idea being that lowering the Q might help prevent the amp from self-oscillating.     I used 680 ohm resistors.   First I put one across the primary of Q1.  No joy. Then Q2.  No luck.  Then I put one right across the primary of that big output transformer.  That did it!  The parasitics disappeared.   And I still get a nice 4 watts of output.  Thanks Edgardo.  I hope to make a contact with this rig today. 

The real silver lining in this story comes, however, in the form of Edgardo's blog site.  Wow, what an inspiring example of Argentinian Knack.  Radios, telescopes, auto-giros.  This guy is also into homebrew DSB rigs.  Wonderful stuff.  Check it out.  (Google will translate it for you, if necessary, but even if you don't read Spanish, the pictures tell most of the story.) 
http://www.lu1ar.blogspot.com.ar/
Thanks to all who sent advice and encouragement.  Merry Christmas! 

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Woe is me! Why Podcast is Late: Parasitic Oscillations!

I'm building my FOURTH JBOT and this one is giving me more trouble than all the others combined.  
Here are the details of my tale of woe:
20 meter rig.  Double Sideband.   The JBOT is fed by a simple two-diode, trifilar toroid singly balanced modulator.  NO DIPLEXER TO SPEAK OF.  At the output of the balanced modulator there is a 1000 uH choke to ground and a .001 uF cap to the input of the JBOT.  AF amp is an LM-386.  VXO is a very simple MPF-102 one stage ceramic resonator VXO with no buffer stage.  7 element low pass filter (three coils, four caps). 
All the transformers are wound on FT-37-43 cores.  T3 is four such cores stacked 2X2.
 
The arrangement works beautifully into a 50 ohm dummy load.  But as soon as I connect it to an antenna (a dipole out in the yard, fed with 50 ohm coax) the output signal gets ugly.  
Looking at it on the scope, instead of the nice figure eight pattern (similar to the two tone test pattern of SSB) I get ugly fuzzy outcroppings from the peaks.  Looking at the signal more closely I can see that in addition to the 14.2 Mhz signal, there is another LF oscillation at around 250 kHz. 
I've been struggling with this.  I can't get rid of the LF oscillations. The leads are all short and the inputs are far from the outputs.   I've beefed up shielding, and decoupling.  I've sacrificed a chicken to Papa Legba. Nothing seems to help.  
I THINK the feedback/oscillation is taking place in the JBOT itself -- not through the
audio amp or the balanced modulator or the VXO.
I watch the RF and the AF inputs to the balanced modulator to see if there is any difference between the stable situation (with the 50 ohm dummy load) and the unstable situation (with the antenna),  I don't see any differences. 
I even put an antenna tuner between the final and the antenna anb made sure that the antenna looks like 50 ohms non reactive.  This seems to help a bit, but the ugly instability is still there. 
Help me! 
\

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20