December 2012 – SolderSmoke Daily News

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

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!

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!
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LadyAda Named Entrepreneur of 2012

Watch this video and you will see that Limor Fried has The Knack!
She and her company have brought homebrew electronics back to lower Manhattan.
Some kind listener sent me a gift certificate for Ada Fruit. We will put it to good use, probably
with Raspberry Pi and Arduino gear.
Three cheers for Lady Ada!