A Re-balanced Modulator for my Re-cycled Rig

My effort to bring my Azorean 17 meter DSB rig back to life continues. This is proving to be harder than I thought. I'm still taming my version the JBOT amp. It still seems to break into oscillation -- I think the oscillation freq is around 1 MHz. But I am making progress.

I had to go back and work on the balance modulator circuit. I really like the simple two diode singly balanced mixer circuit. But my original Azorean board had, like the oscillator board, been through a few too many rounds of modification and repair. I decided to start over. See above. I even came up with my own little innovation on this circuit. Doug DeMaw's original design called for a 100 ohm pot at the junction of the two diodes, with the signal coming off the tap. You adjust the pot to balance the circuit and null out the carrier. But I didn't have a 100 ohm pot. Lots of 1K controls were available, so I put in two 50 ohm resistors in place of the pot, then put the 1k pot between the diodes, with the tap to ground. This balances things out nicely. See below.

I hope to get a podcast out next week, but I've been suffering from a nasty headcold that would have introduced all kinds of weird audio effects.

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Sunspots! QRP DSB contacts on 17 meters

You can see here why I had so much fun in the summer of 2001 with my Azorean Homebrew QRP DSB rig. And why I am now re-CYCLING (get it?) and refurbishing the 17 meter gear that I built last cycle.

I've had the DSB rig on the air yesterday and have worked seven stations (2 watts peak DSB to a dipole).

Back to the Future! Someone on QRP-L said the SFI was recently at 180! Go Sol!

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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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My New Shack Heating System: Heath, Halli, Hammarlund and Drake

Temperatures dropped over the weekend and I had to fire up my new heating system for the SolderSmoke shack. See above. Heating by Heath, Halli, Hammarlund and Drake (sounds like a law firm doesn't it?). I'm happy to report that those old filaments take the chill off quite nicely while adding a nice aroma to the room, along with some very pleasing lighting effects (I especially like the green glow from the DX-100 tuning dial).

The Azores-17 DSB JBOT project is (I think) complete. And I did include a low pass filter. In keeping with the finest of ham radio traditions, now that it is cold I will go out and work on an antenna.

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Azorean Oscillator Re-build

With my JBOT amp ready for installation, it was time to go back and check out the rest of the circuitry on my old Azorean DSB/Direct Conversion 17 meter "Magic Carpet" transceiver. I was surprised to find that the oscillator, well, wouldn't oscillate. The MPF102 that I had in the main oscillator circuit was blown. I suspect voltage spikes from the T/R relay. I installed the required diode across the relay coil. (I seem to blow up a lot of MPF102s. Is it just me, or are these devices really fragile? They are junction FETs, not MOSFETS, so I thought of them as being more robust. But I seem to go through a lot of them.)

After messing around with the oscillator and buffer circuits, the nice clean Manhattan isolation pads that had been there at the start were all messed up, with big piles of solder with the ends of multiple axial leads stuck in there. I decided to start afresh. Out came the little PCB box that housed that housed the oscillator, buffer and amp stages. Off came the walls of the box. (When I built this thing the first time, I didn't realize that I would need an amp to get to the 7 dbm needed to drive the diode ring. I ran out of room on the main board and ended up building the needed amplifier on the back wall of the box!)

So I got to build this little circuit again, ten years and three countries after the original build. It was fun. I like building oscillators. There is that satisfying sense of closure and completion when, at the end of the effort, you turn on your receiver and hear the tone from your creation.

There was also a fun little bit of troubleshooting. After rebuilding the oscillator circuit I noticed that applying power to the "on the wall" amplifier caused the oscillator to shut down. At the buffer, I was seeing RF in and RF out, but the whole thing would shut down when I powered up the next stage. Obviously there wasn't a lot of BUFFERING going on! Sure enough, the MPF102 was bad. I replaced it from my dwindling supply, and all was right with the universe.

Now the amp goes in. But first I will build the low pass filter. I promise.

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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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A Good Day in the Shack -- 2B Fixed, SPRAT Arrives

The temperature has dropped to around 10 C today. Fall is in the air. So it was appropriate for me to do some work on an old tube radio. (Several of our correspondents have reported similar seasonal urges to melt solder and to heat filaments.) This week my trusty Drake 2-B was giving me trouble on 17 meters. It was working on all other bands, but not on 17. Today I put it on the bench, pulled out the schematic and started troubleshooting. I quickly determined that the problem was, in fact, with the 22 MHz crystal. The 2-B has a 24.5 MHz rock for tuning the lower portion of the ten meter band. When I put that crystal in the "E" socket (where the 22 MHz crystal normally sits), the local oscillator worked just fine. Putting the 22 MHz rock back in the E socket resulted in no oscillation. And when I tried to the 22 MHz crystal in the 10 Meter socket normally used by the 24.5 MHz rock... nothing. What causes a perfectly good crystal to go bad like this?

Consulting the 2-B manual, I saw that I could also tune the 17 meter band by using (in socket D) a 14.21 MHz crystal from my junk box (it had been used in my 20 meter NE-602 DSB. transceiver). It works great. I'm listening to DK9KW calling CQ on 17 right now. Makes me want to fix up my homebrew 17 meter transmitter. (I need a final for it, and am thinking of using Farhan's JBOT circuit). I may even buy some telescoping fishing poles and rebuild my Azorean rotate-able dipole (I have the mast and the wooden center support for the fishing poles). This magnificent antenna is shown above, spreading its wings above Sao Miguel island in the Azores (our home from 2000--2003).

Icing on the cake: I went out to the mailbox, and, instead of the usual pile of bills, there was a SPRAT 148 and the G-QRP Club's Members Handbook.

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

555 Contest Finale

Jeri Ellsworth's 555 contest is in the home stretch, and I've got my acceptance speech all ready. "I'd like to thank the members of the Academy... and above all the people of the Azores, who were put at risk by the large kite that carried my 555 timer circuit and 35 mm camera over their unprotected heads..."

Here's my entry: http://www.gadgeteer.us/KAP.HTM

Forrest Mims is one of the judges. Here is what he had to say:

[The participants] displayed an absolutely remarkable range of engineering skills, creativity and dedication to meet the deadline. Also impressive was the use of video and photography to illustrate many of the entries.

This contest helps restore my confidence in analog designers, who have become a very small minority in electronics these days. Some of these projects do with great simplicity and efficiency what exclusively digital designers would have great difficulty emulating. Moreover, a number of these projects have potential commercial merit. Hopefully the developers will be able to pursue this.

The winners will be announced tonight at 9 pm Eastern Time:
http://www.555contest.com/news/

(Oh no! There must be some sort of mistake! I checked the list of finalists and my kite entry is not there!)

Back to the Future: Packets from the Space Station

Readers will have noticed that I have an affinity for beacons. All kinds of beacons: WSPR, QRSS, 10 meter CW... (Tony Fishpool says I have broadcaster tendencies.)

My favorite was a VHF Digital Satellite beacon. Out in the Azores, I had my old Kantronics KPC-3 Terminal Node Controller hooked up to a 286 computer and a Realistic HTX-202 HandiTalkie. Antenna was a ground-plane made from a coat-hanger. My rig would burp out packets all day long, and occasionally either PC SAT or the International Space Station would fly over and relay my signal to stations on the European continent or, sometimes, on the east coast of North America. I had APRS data in my packet, including a nice little island with a palm tree icon which would show up on the on-line APRS maps.

Well, after watching the Space Station (and the Shuttle discovery) fly over a few weeks ago, I got the urge to get back into the outer space packet game. Last weekend I dug out the old HT and KPC-3. I even found the cables. For the computer I put to work the OLD Toshiba Satellite Pro (appropriate, eh?) that Kevin, ZL3KE, had helped me revive.

Yesterday before going to work I noticed that there would be a nice pass of the Space Station at around 0750 local. I took a little mag mount antenna and left it on the ground in the backyard. I tuned the HT to 145.825 MHz and fired up the Windows 3.1 terminal program on the Toshiba.

When I returned 10 hours later... SUCCESS! Lots of space packets on the screen: KB1GVR, W1TMS, W1CGT, W1GSH, VE2TMW...

So far I'm only receiving, but soon I hope to be sending
PACKETS TO SPAAAAAACE!