MIDGET IN A LUNCHBOX - TALES OF A RE-BORN SIXER
By Bill Meara, N2CQR
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This is the story of a receiver that came together through a process very similar to spontaneous combustion. In the corner of my basement hamshack, I'd been slowly but surely filling up a cardboard box labeled "good junk." (I'm sure many of you have similar boxes.) In the aftermath of each hamfest, a few new (old) pieces would be added to the pile. Variable capacitors, tubes, sockets, transformers - you know, the good stuff. Close-by, a collection of ham radio magazines from the '50s and '60s grew at a similar rate.
The accumulation seemed very innocent. Little did I know I was approaching critical mass.
The spark came from the April 1966 issue of QST. Page 49. In his "Beginner and Novice" article Lew McCoy presented a little three tube superhet receiver for 80 and 40 that promised to be "a real performer." This was the receiver companion to the "Mighty Midget" transmitter project that had been presented a few months earlier. (The transmitter was presented as a contest: The first novice to finish building it and make contact with stations in ten ARRL sections won!) I liked the looks of the circuit, but at first I was uneasy about the metal work that would be required. I have some deep, primordial ham radio memories of hacking away (with inadequate tools) at the chassis of a homebrew power supply for an HW-32A. The experience made me a believer in PC boards. But this receiver circuit looked very cool. The thought of tuning in CW, SSB and AM signals through all of 80 and 40 on a homebrew rig was very appealing. I could even do some shortwave listening on 40! But still, there was the metalwork problem...
Suddenly, while pondering a photograph of Lew's receiver, it occurred to me that the size and layout was remarkably similar to that of an old Heathkit Benton Harbor lunchbox (Sixer) that I'd picked up a while back. The wheels started turning. A quick check of the junkbox revealed that I had most of the "hard-to-find" parts on hand. A look the Sixer Chassis showed that it would save me almost all of the hated metal work - I could make use of holes that were cut in Benton Harbor during the 1960s! Soon I knew for sure that that Sixer would never again vibrate the ether of the "Magic Band." I justified my decision by noting that the rig had been moded almost beyond recognition (I swear) and would not therefore be of historical value. I started gutting it with gusto!
The old handbooks recommend covering a new chassis in wrapping paper so that the placement of every socket and screw can be carefully planned. ( I often wondered if there were real hams who were patient enough to actually follow this good advice. ) On this project I just took the major parts and moved them around on the chassis until I had a satisfactory layout. I tried to keep the tubes away from the local oscillator coil and cap. There was one big hole where the old electrolytic capacitor had been. I used some spare PC board material to patch over this hole, using it as the mount for the FT-243 crystal holders for the IF filter. Underneath, I just threw in a large number of terminal strips wherever I thought they'd be useful (those with lots of grounded terminals are the best!).
My junk box provided most of the components. The conveniently-timed Manassas, Va. hamfest helped me fill some of the gaps in my parts list. The slug-tuned coil for the BFO was a particularly happy discovery at that 'fest. I used the variable caps out of an old junker Swan 240 transceiver. The power and audio transformers came from the Sixer. I had to make a trip to the local pharmacy to pick up the empty pill bottles for the coils. I figured they would be suspicious about my request for what could (I suppose) be considered drug paraphernalia, so I brought with me a copy of the April 1966 QST and showed the pharmacist the picture of the Mighty Midget chassis, with its very cool-looking pill bottle coil forms. I immediately "scored" four free pill bottles! It was a lot of fun to wind the coils and I think they look very fine on the chassis. They add a real HB touch to the project.
The really difficult components were the two 455 kc crystals for the lattice filter. I discovered that these rocks have become quite rare! A major manufacturer of crystals (who will remain anonymous) gleefully offered to make me the rocks - for a mere $75.00 dollars each. Ouch! But then the amazing fraternity of cyber-space solder melters came to the rescue: James, W5LWU, donated three surplus FT-241A crystals. Unfortunately I was unable to get them to function in the filter. I get the impression that these rocks do not age very well. I'm also told that even under the best of circumstances homebrew lattice filters were hit and miss affairs, with more misses than hits.
Anxious to get the radio going, I substituted a 455 kc IF transformer for the crystal filter. This was very easy, because the IF can fit perfectly into the two adjacent FT-243 crystal holders on the chassis. A quick rearrangement of a few leads and I was in business. This setup left the receiver a bit broad, but I kind of like it that way. Phone signals (particularly AM sigs) sound very nice. While I couldn't get the complete "single signal" effect with this system, with careful placement of the BFO frequency I did notice a very significant attenuation of the "audio image." I may eventually experiment with lattice filters, but for now the IF cans are doing just fine. I received many suggestions about making the IF stage regenerative or using a Q multiplier - I haven't tried this yet.
I'd originally planned on making a completely new front panel, but I came up with an alternative that allowed me to make use of the old Sixer panel: Home Depot sells some very light, thin aluminum sheeting material (used for storm or screen doors). Using tin shears, I cut out a "false front" for the Heath front panel. I got mechanical stability from the sturdy Lunchbox panel while covering up the scars of mods from days-gone-by. I secured the main tuning cap to both the chassis and the front panel - this provided a very noticeable improvement in stability. I found that I really didn't need a reduction drive for the main tuning cap. I put DX-60 knob with pointer on it. An old CD cut in half and affixed to the front panel serves as the dial.
The receiver went together very smoothly. Debugging was unusually easy. I was amazed to find absolutely no unwanted oscillations to be stamped out - this was particularly surprising given the small size of the chassis, the use of pill bottle coils, and the fact that I didn't pay a lot of attention to shielding. I had to experiment a bit with the coils for the local oscillator to get it vibrating on the proper frequencies. I also had to put a little trimmer cap on the tuned circuit in RF amplifier's plate to get it to track well with the grid circuit. Putting the RX inside the cabinet seriously detuned the BFO, so I had to come up with some way of adjusting the chassis- mounted slug-tuned inductor. I ended up putting another hole in the already well-perforated cabinet. With a slightly widened plastic nut-starter I could easily reach down and adjust the BFO. Coming up with little solutions like that is one of the little joys of homebrewing.
I added a few mods to Lew's original design. With AM operation in mind a "BFO OFF" switch was obviously a necessity. I also put a fuse in the power supply circuit. I mute the receiver by lifting off ground the IF amplifier's cathode and the ground connection on the RF gain control.
I thought about adding an additional stage of AF amplification but things were getting a bit cramped so decided to leave well enough alone. When I want to use a loudspeaker I simply plug one of those little computer speakers with an internal AF amp into the headphone jack. I realize many tube purists will find this distasteful, but let me point out that I briefly considered an even more unpleasant option: placing a little LM386 AF amplifier chip in there among the 6U8s, variable caps and pillbox coils. Somehow it just didn't seem right - the external AF amp option seemed to the be the lesser of two evils.
Lew McCoy was right when he promised that this receiver would be a real performer. The two tuned circuits in the RF amplifier seem to take care of all the image problems and very effectively keep the intense Northern Virginia AM broadcast energy out. Sensitivity is very good - I often have to back off on the RF gain and I can easily copy the Australians on 40 SSB in the morning. After a warm-up period the receiver becomes very stable.
Even though we're dealing with the beloved technology of yesteryear, let me throw in a good word for something that we didn't have back in 1966. My enjoyment of this project was greatly enhanced by a very active Internet discussion that accompanied the building process. We used the rec.radio.amateur.homebrew USENET group and the GLOWBUGS and HOMEBREW mailing lists. It was great fun to share progress reports with a large group of fellow solder melters. As a result of this Internet chatter, a number of other Mighty Midget receivers are now under construction: Sandy, W5TVW, has experimented with 6T9s in place of the 6U8s. Rod, N5HV, is building one on the chassis of a dead Heath Twoer. (Will there be no end to this carnage?) Giovanni, IT9XXS, is planning an Italian version. Jose, EB5GAV, (a well known Boatanchor fan) is gathering 6U8s in Valencia, Spain. Cedrick, N9YXA, hopes to use the MM project to take his mind off the snow this winter. Collin, N4UTA and Chris, KX0Y are both planning MM projects. Michael, AB5L, found a Frank Jones article that suggested a way of powering this receiver without a transformer: rectified and filtered AC line voltage to the plate with the AC voltage dropped to filament levels via a lightbulb in series. (You get a desk lamp in the bargain!) Finally, Eddy, VE3CUI, was delighted to learn that he is not alone in his enthusiasm for this simple receiver. Stimulated by the 1969 ARRL Handbook, Eddy built a version of the MM three years ago. His is a very "souped up" version that operates on 160 and 80 with additional stages of IF and AF amplification.
This was my first tube-type construction project and I really had a lot of fun with it. When you build solid state gear on PC boards, it's all very one dimensional, very flat. But this tube project was like building in 3-D! As you add components above and below the chassis, you really get the sensation that you are building something substantial. And of course there are other aesthetic rewards: the warm glow of the firebottles and that wonderful smell that comes from oil and rosin heated by filaments.
For those of you who are (like me) metal-shop challenged, I strongly recommend the recycling of old junk-box chassis. This project has caused me to take a new look at some of the old junkers sitting around the shack. There's another Lunchbox that may end up as a companion transmitter for this receiver. And then there's that DX-60 carcass that could serve as the foundation for the 160 meter rig I've been thinking about... Hmmm let's see...
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