A Contact with my Old Azorean DSB Transceiver

I've been thinking about balanced modulators, and I wanted to see how some of my early circuits performed.  So I pulled this OLD Double Sideband rig off the shelf and fired it up. The balanced modulator -- and everything else! - worked fine,  and I soon made contact on 17 meter SSB with Gene, AB9GK. 

This was the first DSB transceiver that I ever built.  I made this out in the Azores, probably in 2000 or 2001.  Years later I had replaced the RF power amplifier with a "JBOT" (Just a Bunch of Transistors) designed by Farhan.  

Over on my YouTube channel a comment came in from my friend Jack:  

@dhakajack

"Looking inside and seeing the o-scope probe in place while the radio was on the air reminded me of neurosurgery where the patient is awake and talking while the surgeon probes different brain regions soliciting feedback. Sure, ham radio isn't neurosurgery, but it's not too far at times. Also, you already have rocket science covered."

Here is an article about my build of that first rig:

https://www.gadgeteer.us/17METER.HTM

I think the article captures well the trials and tribulations faced by new homebrewers, perhaps with the twist that comes from being out in the middle of the Atlantic ocean.  

Mike WU2D is having similar fun with his homebrew 10 meter DSB transceiver: 

https://www.youtube.com/watch?v=uZt1LTGyoHc

I was struck by how similar Mike's early QSO experiences were with mine.  We both put our DSB transmitters on the air before they made their way into real cabinets or boxes. 

Here's mine from 2001 in the Azores: 

SolderSmoke Podcast #168 Software Inefficiencies! DSB Blues! Schematic Errors! QRO Confessions!

SolderSmoke Podcast #168 is available. 
22 November 2014 

http://soldersmoke.com/soldersmoke168.mp3
The benefits of software inefficiency.
Don't throw a wet blanket on computer baby steps.
DDS-ing Pete's old boatanchors and Bill's BITX. 
Bill's DSB amplifier woes: a JBOT unfairly scorned.
Getting ready for solar-powered beach DSB.  
Michigan Mighty Mite Crystal Offer -- FREE ROCKS!
Tribal knowledge: Beware of mistakes in published schematics!
QRO update: Working Japan on 17 meters. 
Happy Thanksgiving! 

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N2CQR WINS ARRL SWEEPSTAKES! AGAIN! (video)

We did it AGAIN!   Winners in the Homebrew Double-Sideband QRP Northern Virginia Category!  Sweet!  I can't wait to pick up the trophy!   

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Swamping or Q-Killing

Sounds like some new TV series on the Discover Channel, doesn't it? Actually it refers to efforts to stabilize oscillators by reducing the Q of components that may be helping to turn the amplifier into an oscillator.   A 300 ohm resistor across the primary of the output transformer on my most recent version of the JBOT amplifier recently solved my instability problem.  As part of an effort to understand how and why I came across a CQ magazine article by Doug DeMaw.  The relevant passage appears above.  

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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! 

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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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JBOT Thoughts

Posted today to the BITX-20 Group:

Good to see so many JBOTs under construction. I recently built two of them. Farhan has asked me to share with the group my experiences using -43 core toroids. Ironically, TV cores are a bit harder to find here, so I had to go with the fancier toroids!

In my first amp (used in a 17 meter DSB rig) I used the smaller FT-37-43 cores in all three transformers. T1 and T2 were wound and placed in the same way that Farhan did with the TV cores. For T3 I used superglue to stack 4 of the cores 2x2, then wrapped them with a bit of electrical tape. I wound T3 so the input wires were on one side and the output on the other. (On this amp, I had started out using some large binocular cores from the junk box, but I had a tough time getting the amp stable with these cores, possibly because using them resulted in longer lead lengths. So I went back to the smaller FT-37-43 cores).

In the second amp (used in a 17 meter SINGLE Sideband rig) I used FT-50-43 cores for T1 and T2. The bigger gores were a little easier to work with. I glued them vertically to the copper clad board. For T3, I again went with 4 FT-37-43 cores. (I tried using 4 FT-50-43 cores for T-3, but I found that output was low, so I went back to the smaller cores for T3). Note that on the second amp, I put some insulation between the heatsinks and the copper clad board (gorilla tape) -- I was worried about possible short to ground if the anodized layer on the heatsinks got breached.

In building my amps, I used the photo on Farhan's Phonestack page as my guide.

As is always the case with amps that I build, I found that a certain amount of "taming of the beast" had to take place before I got the devices stable. This is certainly the result of the kind of homebrew devices into which they are being inserted! Following advice from Doug DeMaw, I made liberal use of ferrite beads. I put one on the lead carrying 12 volts to the amp board. I put another on the lead from the 12 volt line to the final's RF choke. And I put one on the line carrying the .6 volt bias to the secondary center tap of T2.

I put a bit of shielding (copper-clad board) between the low pass filter and T3.

These steps allowed these amps to work nicely with my contraptions.

As long as we are talking about JBOTS, I have a question for the group:

On my second amp, I noticed that the output was a bit lower than it should have been -- only about 1.5 watts. I did a lot of troubleshooting, then I finally checked the bias currents in the first two stages. I found that BOTH were running at about 50 ma.

I looked at Farhan's JBOT notes and found that the second stage is supposed to be running at 100 ma. I found that the base bias voltage on the second stage was 1.13 volts, while the first stage had 1.9 volts on the base. That didn't seem right.

I then turned to the bias.exe program that comes with EMRFD. I plugged in the values from the JBOT schematic and, sure enough, the predicted collector bias current was 50 ma, not the 100ma the design was looking for.

Using that program, I made some changes to the base resistor in the second stage. I found that with 150 ohms instead of 100 ohms, the collector bias current goes up to the desired 100 ohms. This also seemed to bring power output up to the desired range of 3-4 watts (output should be a bit lower at 18 MHz, right?)

More info (and pictures) on my JBOT adventures can be found here:
http://soldersmoke.blogspot.com/search/label/JBOT

GOOD LUCK WITH YOUR JBOTS!

73 Bill N2CQR
http://soldersmoke.blogspot.com

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My .02 KW Linear Amplifier

Once again, die-hard QRP guys should probably look away at this point...

My Cycle 23/Cycle 24 refurbishing project continues. The JBOT amplifier in the 17 meter SSB rig is now working nicely, but unfortunately just as I got ready to make some contacts, the Coronal Mass Ejection hit us and conditions on 17 deteriorated badly. Looking for something to do, I noticed that in some pictures of my old Azores station, sitting alongside the SSB transmitter there was a small cabinet with a QRO/QRP switch on the front panel. Ah yes! My Ramsey Kit linear amplifier! This is a MOSFET-based device. Mine was designed for 20, but I changed the low pass filter and put it on 17. Later, in Italy, it went on 20 meters and for some reason I went back to the 20 meter low pass filter.

Yesterday I blew the dust off this thing, pulled out the 20 meter LP filter and replaced it with a filter that will let my 17 meter RF reach the antenna. Today I fired it up. Wow! 20 Watts! Look out 17 meters!

I really like the QRO/QRP switch -- it gives me the opportunity to lean back and say (just like the big guns) "Wait a second Old Man, let me turn on my LINEAR!" I even have a little fan in the box that you can hear when I go to the QRO position. Unfortunately it doesn't make the lights flicker. But we can dream...

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Putting 17 Meter SSB Station On the Air

Magnificent, don't you think? The sun is rising over Northern Virginia, and 17 meters is starting to come alive. Yesterday I finished the tweaking and peaking of the JBOT amplifier for the transmitter. It is nice and stable now. (And yes, Steve, it has a low-pass filter!) On top of the transmitter cabinet is the receiver. It is a Barebones Superhet designed by Doug DeMaw and built on a FAR circuit board by Dale Parfitt, W4OP. I changed it to 17 meters and broadened the filter response for use on SSB. Both the transmitter and the receiver use variable crystal oscillators, with two crystals in each (switch-able from the front panels). The frequency coverage of of the transmitter and the receiver match up fairly well (good thing!). Wish me luck! Today I will venture forth amidst the coronal mass ejections and try to make a few SSB contacts on good-ol' 17.

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Toroidal Travails II

Steve Smith sent me a good article on broadband transformers. Reading through it, it occurred to me that perhaps using a heavier gauge wire in that PA output transformer might help. So I rebuilt the FT-50-43 transformer and put it in the final. No joy -- output was still down around 1 watt. Then I tried adding two turns to the secondary (on his web page Farhan advises experimenting with the turns ratio in an effort to improve output). Again, no joy. So I went back to the FT-37-43 transformer with 12 turns on the secondary. This yields the best results so far: about 2.5 watts. Still a bit low, but for some reason, the smaller cores seem to do better. Toroidal transformer tweaking to continue... Stay tuned.

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Toroidal Transformers: Does Size Matter?

I continue to tweak and peak the JBOT amplifier in the Azores 17 SSB transmitter. On this version I used some FT-50-43 toroidal cores instead of the smaller FT-37-43 cores recommended by Farhan. This morning I was experimenting with the output transformer. I seem to get noticeably more output with a transformer made with four FT-37-43 cores than I do with one made with four larger FT-50-43 cores.

I noticed something similar on my previous JBOT: performance improved when I switched from some relatively large binocular cores and went to the recommended FT-37-43.

So, what do you guys think? Could there be lower losses using the smaller cores? Any other reason why the smaller transformers seem to be doing better?

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JBOT Installed in Azores 17 Meter SSB Rig

Even though one of their Coronal Mass Ejections is due to hit us soon, the radio gods have been quite kind to me this morning. I installed the JBOT amplifier board in the 17 meter SSB transmitter that I had built out in the Azores during the last solar cycle. The board went in without any trouble. And I was a very surprised when it DIDN'T break into oscillation and instability! Holy Cow! This one was stable from the start! Even when connected to my antenna! Eureka!

The rig still needs some peaking and tweaking. I'm only getting about 2-3 watts out and I should be getting 4-5. I did a quick and dirty "by ear" alignment --- I just listened to my own signal with my trusty Drake 2-B and moved the carrier oscillator freq around a bit until the SSB audio sounded good (you never have to do that with DSB!).

For those of you not familiar with this rig, here is some background:
-- Built on the chassis of an old Heath DX-40
-- Crystal filter at 5.174 MHz. Filter rocks and carrier oscillator rocks from an old Swan 240 I picked up in the Dominican Republic from Pericles Perdomo HI8P (SK).
-- Based on a design published in SPRAT by Frank Lee, G3YCC (SK).
--Heterodyne oscillator is a G3RJV Universal VXO circuit running at around 23.3 MHz.
-- That orange cord to the big meter that you are no doubt wondering about is just a little circuit that monitors total current drawn by the rig. It bounces up and down as I talk. I put it in there mostly because I wanted to make use of a beautiful old Simpson meter that I picked up in 1973 at the Crystal Radio Club (W2DMC) in Valley Cottage, New York.

Going around, clockwise from below the meter: G3RJV VXO, carrier oscillator and two diode balanced modulator board, crystal filter (with NE602 mixer and post-filter bandpass filter to the left), JBOT PA. Audio amp (using op amp) below the chassis. T/R relay in the center (antenna changeover relay below the chassis).

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Another JBOT Amplifier

Over the weekend I built another JBOT 5 watt linear amplifier (design by Farhan). I used a nice piece of copper-clad board that Dave, W8NF, sent me (thanks again Dave).

This time I chickened out regarding the possible conductivity of the anodized heat sinks. I didn't have any trouble with this on my first JBOT, but I worried that if the anodized layer gets flaked away, a heat sink might short one of those collectors to ground. To be on the safe side, I put small squares of Gorilla Tape on under the heat sinks. (PLEASE don't tell me that Gorilla Tape is conductive!)

For T1 and T2 I used FT50-43 toroids instead of the TV baluns used by Farhan. He had recommended FT37-43's as an alternative to the TV baluns, but I went with the slightly larger toroids. For T3 I rolled my own binocular core using four FT37-43 toroids stacked 2X2.

The amplifier has passed the smoke test. Next I have to put in the low pass filter (Steve Smith: Please note that I have left space on the board for the filter.) Then this version will face its real test when it goes into the 17 meter Azores SINGLE sideband rig.

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Amplifier Woes -- Help me! Help me!

When I look in the mirror and I see a haunted, obsessed look in my eyes. My wife senses that there is something wrong in the ham shack. She is right. I have an amplifier that wants to be an oscillator. Help me exorcise these gremlins! Guys, this problem is holding up the production of the next SolderSmoke podcast.

My JBOT amp works fine into a dummy load, but when I connect it to an antenna, it gets unstable. Here are some more details of the symptoms:

I am running the JBOT with a 5 element (two toroids, 3 caps) low pass filter (designed by Doug DeMaw and approved by Steve Smith).

With the antenna connected, all is well UNTIL I raise the power out (by varying the input) beyond about 1 watt. Below one watt, the amp is working fine, and it stable. As soon as I hit the 1 watt point, the amplifier seems to break into oscillation. This does not happen into the dummy load.

The antenna is a simple dipole fed by coax. It shows a low SWR. Even when I put an antenna tuner between the amp and the antenna and bring the SWR down to negligible levels, the instability problem persists.

With the amp disconnected from all other circuitry other than the antenna and the power supply, if I just touch the input capacitor, it breaks into oscillation. This does not happen when the amp is working into the dummy load.

I've bolstered the power supply filtering and decoupling. No luck. I tried some de-Qing of the transformers. No luck.

Any suggestions?

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JBOT Amplifier Installed in Azorean DSB Rig

I've been remiss in posting to the blog, but I have a good excuse: I've been melting solder. I have working on the installation of my new Farhan-designed JBOT amplifier in my old Azorean DSB transceiver. I'm really enjoying this project, and I now see it as the first in a series. My shack has a number of creations that were built during the peak years of the last solar cycle, but have since fallen into disuse. Many of them were partially cannibalized -- usually it was the RF amplifier that was taken out. The JBOT was just what I needed. I plan to refurbish all of these rigs, adding a bit of India to each one of them. This is very much in keeping with our "International Brotherhood of Electronic Wizards" ethos.

The installation went fairly well, but with all of the ups and downs that accompany this kind of project. The amp worked fine on the bench, fed with a signal generator and into a dummy load. But of course, life got more complicated when I installed it in the rig. Yes, it took off on me. This was no fault of the amplifier -- I just needed to add some additional shielding. It is working fine now. See above. I moved it out of center stage and put it off in the corner to avoid feedback problems. More discussion of this in the next podcast (maybe this weekend).

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Oh No! Short Circuit Danger with Anodized Heat Sinks?

Our friend Brent out in Minneapolis sent me some pre-Halloween HB frights yesterday. I'm still concerned. Am I living dangerously with my anodized but collector-connected heat sinks glued to the ground plane?
What say the HB gurus?

--- On Tue, 10/18/11, KD0GLS wrote:

From: KD0GLS Subject: Re: JBOT Amp
To: "Bill Meara"
Date: Tuesday, October 18, 2011, 9:38 PM

Bill,

Well, it is nearly Halloween, but it wasn't my intention to frighten you!

I think it's hard to argue with the fact that it works as-is. The heatsinks you bought look like common anodized aluminum to me. Out of curiosity I dug a couple of similar ones out of my parts bins and measured them. I found exactly what you did - wide open on the surface, and, not surprisingly, a dead short just underneath. I've always assumed that anodize finish was metallic and conductive, but now I know otherwise. I like to learn at least one new thing each day, so thanks for providing today's opportunity!

The clearances are no doubt small, but I think with the low voltages present in your circuit you'll probably be fine. The only potential problem might be if movement or vibration might someday break through that coating. I'd certainly recommend a fuse in the power supply line if you don't already have one.

Anyway, it looks great sitting there in the middle of your old rig. It's inspiring.

Brent

On Oct 18, 2011, at 19:42, Bill Meara wrote:

Brent: You scared me with your question but I think what I did is OK. Maybe. I looked more carefully at the clip on heat sinks I bought. They are covered with a non-conductive coating, but f I scratch them up a bit I can reach the conducting part. So i guess you just have to treat them gently if you intend (as I did) to super-glue them to the PC board. I notice that Farhan did the same thing (see his JBOT page) so I guess I'm OK. I put some heat sink compound in there too. And then there is the super glue. I have no short circuits from the collectors. I guess I could have used a Dremmel to isolate the copper just below the transistors. Or I could have put some mica between the heat sinks and the boards.

The sinks are deeper than the transistors, so there is no direct contact between the top of the transistor and the PC board.

What do you think?
--- On Tue, 10/18/11, KD0GLS wrote:

From: KD0GLS >
Subject: JBOT Amp
To: "Meara Bill" >
Date: Tuesday, October 18, 2011, 5:27 PM

Bill,

The amp looks mighty FB, and maybe I should try it as well. But I'm curious how you've insulated the collectors of all those transistors from the ground plane. From the picture, it looks like all of the metal heat sinks are sitting right on the ground plane copper.

.73,

Brent, KD0GLS, Minneapolis

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My JBOT 5 Watt Linear (Farhan's Design)

There it is -- my version of Farhan's JBOT 5 watt linear amplifier. JBOT stands for "Just a Bunch of Transistors." But I think Farhan is being modest. I really like the design, especially the effort to make this something that hams around the world can reproduce from readily available parts. For example, Farhan's original version used TV balun cores for the transformers. I didn't have any of those around, but I found three "mystery spec" binocular cores in my junk box. They seem to be working just fine. (I tested them a bit: Farhan had written that FT 37-43 cores would work. T1 has seven turns trifilar. Seven turns on my mystery cores yielded 13uH. On an FT 37-43 core 7 turns yields around 20 uH. Close enough -- these are, after all, broadband transformers.)

Note how closely my build follows Farhan's schematic (which you can see in the background). When building this circuit, I just kept Farhan's web page on my computer screen, and scrolled up and down from his schematic to the photo of his version.

This is the first linear amplifier that worked the first time I powered it up. It didn't release any smoke, or leave transistor burn tattoos on fingers, or try to be a 14MHz oscillator.

This version is going into my Azores-built 17 meter DSB transceiver. See how nice it fits:
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