Two Guys, a Hotel Room and a Radio Fire

Can you build a HF SSB radio transciever in one weekend, while on the road, at parts from a swap meet? I can, but apparently not without setting something on fire.

Of course the swap meet I’m referring to is Hamvention, and Hamvention 2016 is coming up fast. In a previous trip to Hamvention, Scott Pastor (KC8KBK) and I challenged ourselves to restore tube radio gear in a dodgy Dayton-area hotel room where we repaired a WW2 era BC-224 and a Halicrafters receiver, scrounging parts from the Hamfest.

Our 2014 adventures were so much fun that it drove us to create our own hacking challenge in 2015 to cobble together a <$100 HF SSB transceiver (made in the USA for extra budget pressure), an ad-hoc antenna system, put this on the air, and make an out-of-state contact before the end of Hamvention using only parts and gear found at Hamvention. There’s no time to study manuals, antennas, EM theory, or vacuum tube circuitry.  All you have are your whits, some basic tools, and all the Waffle House you can eat.  But you have one thing on your side, the world’s largest collection of surplus electronics and radio junk in one place at one time.  Can it be done?

Options for HF transceivers are very poor at the sub-$100 range especially those made in the US which are often collectible and valued much higher if there is a chance of functionality (e.g. Heathkit, Drake, and others).  We would likely end up with some very early SSB gear or possibly a late 70’s or early 80’s solid state rig.  The pickings are slim.

Choosing the ‘Donor’ Radio

A crusty but serviceable Ten Tech Omni A for a very reasonable price.
A crusty but serviceable Ten Tech Omni A for a very reasonable price.

Early Friday morning we found a Ten Tech Omni A with a jammed-up PTO (permeability tuned oscillator) for $80. This radio did not have the optional frequency counter, instead it had a broken slide-rule tuning indicator.  I liked its modularity, you can clearly see the signal flow by looking under the top cover.


Early on Friday morning Scott located an old vacuum tube powered Hallicrafters SR-160 project radio for $60. Unfortunately 53 years of age resulted in a few dried electrolytics and drifting resistor values. It did have a few things going for it, it came with a power supply, microphone and I brought my stock of high voltage caps so we could re-cap it. Re-capping and a possible re-alignment would be time consuming.

Friday evening the Omni-A powered up and seemed eager to run. After several quick-fix attempts with the PTO we tried a Google search and realized that the mechanical jam was a very common problem. The best way to fix it was to completely disassemble the mechanism, soak it with isopropyl alcohol, then lubricate with lithium grease.  There was no time to completely disassemble and clean the mechanism (it’s akin to a fine pocket watch) and the only lubricant we could find was a small can of WD40 from the gas station next door. After spraying the PTO mechanism and working the threaded brass pieces with a pair of channel-locks it came loose.  This radio was able to tune in numerous SSB stations with a 20′ length of wire hanging out of its antenna jack.

Next we tried to re-string the tuning indicator, this didn’t work out too well.  So we ended up with a working transceiver but no idea what frequency it was on. We did consider boot-strapping the Halicrafters VFO into the Omni-A, both are 5 MHz VFOs, but this was abandoned in favor of a faster solution.

To determine what frequency we were on we used an old Hammarlund HQ-170 receiver that had been procured separately and not part of the competition (when I see a nice boat anchor it’s hard to resist the buy).  This radio worked without need for repair thanks to Hammarlund’s use of ceramic disk capacitors.  We used the HQ-170 to zero-beat the bottom range of our frequency privileges then decided to only reply to CQ’s or jump into another ongoing QSO.  Frequency awareness problem solved.

Antenna: Outdoor Wasn’t an Option

For almost zero cost we bought a couple of spools of cold wire, some random lengths of coax, and a few WW2 surplus variable capacitors knowing we could easily wind inductors with the scrap wire. Our raw material stash was complete when someone gave me an old VSWR meter for free. It seemed like the dipole antenna or random wire option was the best bet. Unfortunately the window of the hotel room would not open. With nowhere to string the random wire antenna it was back to the drawing board.

First attempt at a Mag Loop transceiving antenna using RG58 coax and loop-coupling.
First attempt at a Mag Loop transceiving antenna using RG58 coax and loop-coupling.

How do you make a decent indoor antenna that would work within a concrete structure? The magnetic loop seemed like a good option. A mag loop works on the magnetic field component of the electromagnetic field. Mag loops are used in almost all AM broadcast radios and often used by radio amateurs in Western Europe where space is at a premium. Unfortunately they’re not too efficient.

Efficiency depends on the ohmic losses in the mag loop conductors at your operational wavelength.  Furthermore, the directivity (antenna gain is the product of directivity and efficiency) of a mag loop is usually poor due to the small physical aperture of the loop. For best performance we want to make the loop as big as we could reasonably fit in the hotel room and also use the thickest-available cable or wire for the loop itself.

Locating stuff at Hamvention is not like shopping the Digi Key catalog.  It is a hunting and gathering exercise.  In my experience it is difficult to seek out one specific item, the mode of shopping at hamventions is more akin to ‘you will know what you want when you see it,’ rather than searching through your favorite parts catalog.

block diagram and schematic
Schematic of mag loop coupling and tuning network.

After failing to get a 4′ loop of RG58 working on Friday night I was on the lookout Saturday for a replacement. For $10 a length of ¾” hardline or heliax coax cable (a very large industrial coax), larger gauge hook-up wire, and some terminal lugs were procured. Now we had everything needed to make a serious mag loop.

Rzather than an inductive coupling we decided to use capacitive coupling.  One variable cap for resonating the loop and a second in series for coupling.  We used short lengths of #14 wire and terminal lugs back-filled with solder to wire it up.

After peaking both caps for maximum receiver noise the loop tuned up well at low transmit power, easily achieving less than 1.5:1 SWR.  I did notice that the loop de-tuned when I touched the feed coax.  Because of this I rolled a shunt balun with 10 turns of RG58 directly in front of the coupling capacitor. Hand capacitance problem solved, SWR was well behaved, the loop was now being fed with a balanced line.

Time to get on the air.  We turned up the power on the Omni-A and keyed the radio. Unfortunately it did not seem like we were transmitting 100 watts. The lights were not flickering in the hotel room or even on the radio itself. Nothing in this radio/antenna system seemed like it was trying to transmit.  The output power meter on the radio read flat, one or two watts.

We did confirm good SSB transmit audio with the Hammurland, so something was getting out but not much. After goofing around for a while with the OmniA is seemed like it was a transmitter problem within the radio. As a last ditch effort the Omni-A’s manual was consulted (isn’t that alwasy the way?).

Automatic Level Control (ALC) is kind of like an automatic gain control (AGC) for a receiver except that it is used for transmit.  With this you can achieve a higher average power than transmitting a simple linear voice SSB signal. ALC is something I have not incorporated into my own radios that I’ve built.  Stupidly enough and not being at all familiar with the Omni-A we had left the ALC control at 0. Who has time to read the manual anyway?

We turned it up to about 3/4 clockwise, whatever that meant.  The transmitter kicked on strong.  Lights in the hotel lights dimmed to the SSB carrier, power meter on the radio read a solid 75+ watts. The Omni A worked after being neglected for decades, what a tough radio!

Electrical Fire

Time to get on the air.  Scott was performing some ‘Check 1 2 3 4 tests’ at full power when I observed a very bright arc near the tuning network.  To be sure I asked Scott to try again, I observed something near the mag loop’s coupling capacitor that appeared to be breathing fire in proportion to his voice.

The tuning network was made up of two very large and old variable caps, these were large WW2 surplus variable caps, one of which used ceramic insulators and the other was made of slate. These should be able to handle the RF current and voltages. These caps and the associated wiring were laying on the carpeted floor at the base of the loop which was propped up against the hotel bed.

Omni A back on the air!

I had used very short lengths of #14 wire to tie the network together.  The insulation break-down voltage was rated for 600V.  This is standard stuff you could get at the local hardware store. I asked Scott to key the radio again, while I observed the tuning network and there it was! A solid arc. Wow it was a sight. Do it again! Another arc. On the third try something lit on fire. Smoke filled the room.

It was easily put out but the smoke was nasty. We cleared it out of the room and luckily did not set off the smoke alarms. Upon inspecting the tuning network wiring it seems that the insulation had broken down and burned on the cheap #14 wiring. To fix this, I re-adjusted the wiring to put more space between the wires and placed a hotel ice cube tray under the network for added insulation. We were back on the air without further trouble.


We made contact with a station in Florida, received a signal report of 5 by 7.   Good enough.

Hamvention is made all the more interesting with improvised radio sport. Previously antique radio receivers were brought to life, and now a parts radio was put back on the air and out-of-state contact made. What should be the next challenge?  Who else wants to join the fun? Let us know in the comments below.

gregory-charvat-bioGregory L. Charvat, Ph.D, likes to put old stuff back into service in short periods of time, is CTO of Humatics Corp., author of Small and Short-Range Radar Systems, co-founder of Hyperfine Research Inc. and Butterfly Network Inc. (both at 4catalyzer), editor of the book series Modern and Practical Approaches to Electrical Engineering, guest commentator on CNN, CBS, Sky News, and others. As visiting research scientist at MIT Media Lab he created the Time of Flight Microwave Camera.  As technical staff at MIT Lincoln Laboratory he created a through-wall radar imaging system that won best paper at the 2010 MSS Tri-Services Radar Symposium and is an MIT Office of the Provost 2011 research highlight. Greg has taught short radar courses at MIT, where his Build a Small Radar course was the top-ranked MIT professional education course in 2011 and has become widely adopted by other universities, laboratories, and private organizations.

45 thoughts on “Two Guys, a Hotel Room and a Radio Fire

  1. $80 for a broken TenTec is “reasonable”? I would not have given them $40 for it. The Ham radio hobby is getting to be a rich mans only hobby with broken outdated stuff going for $80. Or Dayton is turning into what most Hamfest are, old farts trying to get retail prices for their used stuff.

    One of the reasons I stopped going to them.

  2. This was an awesome read! Thanks for sharing this with us!

    Suggestion for next year: Build a complete, homebrew from the ground up, HF transmitter and receiver set from parts bought there. I’d really like to see that!

    1. A very good suggestion. I think the most feasible way to do it would be to make the radio in a modular sense where the rule can be that you can not buy an entire radio but you can buy blocks such as a PA, frequency mixer, crystal filter, amplified speaker, VFO, etc.

      1. Precisely!
        Rather than starting with a radio that had once been functional, you would have to build your radio out of either individual parts or modules, depending on availability.
        This would be very hard to do at my local hamfest, but probably a bit more feasible at Dayton due to the sheer volume of different vendors.
        You might have to do a lot of walking, though…

        1. Yeah, you could probably find a Collins mechanical filter here, a power amplifier there. I think you could at least build a SSB receiver. Transmitter could be difficult because trouble shooting transmitter oscillation in a SSB transceiver is tricky without a spectrum analyzer or a scope. There’s alot of IF gain and if you do not control it properly on transmit things can go crazy in the radio. Usually i like to switch off excess receiver IF amplifiers on transmit and just keep a small TX IF gain stage alive.

    2. I thought that was the point here.

      First paragraph starts with “Can you build a HF SSB radio transciever in one weekend, while on the road, at parts from a swap meet?”

      And the third paragraph starts with “Our 2014 adventures were so much fun that it drove us to create our own hacking challenge in 2015 to cobble together a <$100 HF SSB transceiver "

      They didn't build or cobble a transceiver. They assembled a station, which isn't the same thing as building a transceiver.

      The minute the piece turned to "resurrecting a transceiver", I checked, assuming this had been written by a third party, but it wasn't, so I have no idea where the hyperbole came from.


      1. Good grief… A short bio of an article’s author is a fairly common practice in technical publications, including amateur radio publications. I really can’t see anyone being intimidated by a doctorate title anymore than they would be by a call sign that indicates the author holds an Extra class amateur radio license.

        1. Hackaday isn’t a technical publication, it’s a place to hang out and share cool hacks. People do get intimidated easily, I see it all the time when they show up for the first day at a ham training class. A frequent question is “Do I have to be a math expert or have high school physics to be a ham?” Like I said, do the community a favor and keep the professional titles at work. Thanks.

      2. Worked the opposite way for me – Here’s an MIT PhD ham, who doesn’t know what the ALC control is for, and manages to set a fire with a measly 100 W transmitter, after seeing it arcing multiple times. I think it’s kind of humble to finish that off with an “about me” paragraph, like saying, “yeah, I should have known better”.

    1. In general I think they’re helpful to get some sense of who I’m reading besides what the article says. In this case it’s perfectly in keeping with the folks I’ve met at Hamvention.

      Go ahead, condense your resume/CV into 100 words and see how it looks!

      1. Haha.
        Duct tape, WD-40, and a Multitool can fix almost anything!
        I know almost NOTHING about radio (I understand how FM works vs. AM, and how important correct antenna length is), but I learned from this article.

  3. Long back I had seen something like this with a plasma chamber power source. The insulation between the twin-line RF conductors broke down and there was a sustained arc between them.

  4. Hope to see you there, as I plan to be there on Saturday. Hope to meet up with Brian from Hackaday as well. Might buy a “Boat Anchor” to tinker with when I get home. :) 73 de KC8KVA

    1. I had to look up ‘boat anchor (+electronics)’.
      Now I feel stupid. I have a several boat anchors! I would love to recap my 1980 o-scope, but don’t have time.
      I also have a hard drive with optical tracking! Has anyone seen this before?
      (Not MiniDisc) The actuator arm has a blade of plastic with microscopic lines on it. Can’t throw it away. Small anchor-for a Kayak I would say haha.

  5. Great story, sounds like lots of fun. Oh and don’t worry about the fire, considering the things you see in a hotel room with a UV light I’m sure a little smoke wouldn’t b other anyone.

    1. Ah Hahaha! Gross.
      I would rather sleep on the floor of a hotel instead of the bed. Then again, the floor isn’t pristine either!
      I bet the fire made the room cleaner!
      F-ing Jackson Pollock paintings in every rentable space.
      Okay. Back on topic. Great article I’m serious! Someday the Ham operators might be some of the few people that have long-range communication abilities.
      Carrington Event
      You can cry ‘Tin-foil hat!’, but it could happen again.

    1. That’s interesting! I wrote a book recently that talked about WW1 Zeppelin construction. Hadn’t even considered the dangers of operating a wireless rig on board the beasts. No wonder it was one of the first things thrown overboard when limping home, engines failing, balloons holed by bullets and AA fire and leaking gas profusely… A future Hamvention challenge perhaps… dredge up a WW1 German wireless from the English Channel and make it work…

  6. Hello Gregory, I enjoyed your story of the radio that caught on fire. The majority of the terms were over my head, but I enjoyed it. I am the type of person that hates to throw things away. At some point you will find the part to repair it, thus avoiding having to buy a new item that most likely is made of crap and will work as long as 2 or 3 days after the warrantee is up. Anyway, about 20 yrs. or so ago I came across a bunch of radio tubes, or TV tubes. Having had an interest in such things at one point in my life, I thought that at some time there would be somebody that would be able to use them. When I was about 12 yrs. old I purchased an intercom system from Fisher Scientific. This was the early days of transistors. The intercom came with a pile of resistors, transistors, and other stuff that had to be soldered on to a chassis. There were very detailed instructions, and every day after school I would race home to work on my intercom, Always checking and rechecking before I soldered any connection. I was very meticulous. Anyway, one day I got home a little earlier than usual and went up to my room and there was my older brother soldering away on my intercom. I checked some of my previous work with the instructions they weren’t the way {I had done them a day or a few days before. Trying to straighten my brothers work was something I did not think I would be able to accomplish so I packed up everything and sent it back to Fisher Scientific with a letter requesting that they check the progress so far. About three weeks later I received a letter from one of their Tech people saying that in the twenty some odd years that he had been repairing customers mistakes He had never seen anything that even came close to this. He could not find one connection that was where it was supposed to go and that perhaps I should consider a different hobby. Anyway, if you think that you might find a use for these tubes email me and I will send them to you..

  7. Awesome story! Reminds me of when I first got my ticket, I was sharing an apartment with my elmer, and we were using a Yaesu FT-101ZD, into a tuner to a length of copper wire strung out the window, across the parking lot to another apartment building, then back to ours. I noticed that when I was transmitting and touched the case of the radio, I got a tingling sensation. I mentioned this to my elmer, and he tried it, said a few well-chosen words, and told me it was basically an RF burn on my finger. He hooked up a wire between the chassis and the ground screw of the electrical outlet, and that fixed that. After that, we pretty much worked a WAS on 40m with the old Triple H net. We were only on late at night, so none of the neighbors ever complained about TVI or any other interference. This taught me the value of a good ground…

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