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Saturday, January 10, 2015

"Gentlemen, We have Oscillation!" Jacob's California Mighty Mite


Bill and Pete,

I bring news of the California Mighty Mite as Bill dubbed it.

I repeated the steps as documented in version two with Pete's parts. I had one hiccup. I was winding L1 with Pete's red wire when, on turn 42, I ran out. Luckily a search of the workroom revealed the last two coils of radio shack magnet wire that I bought with the Gold wire. I checked the thickness of the Green-coated wire and found it to be one gauge smaller than Pete's wire. To be ready in time for the podcast, the green wire would have to do. It was fussier that the larger gauge wires, but with my trusty electrical tape, the L1 coil stayed on the coil-form quite neatly. Using a scrap of gold wire, I wound L2 making sure to tape it evenly spaced.

Next I remeasured each of the parts to be soldered and arranged them in according to the Schematic. With the Soldering iron hot from warming while I would the coil, I detached the 365pf varicap from the MMv2. I strapped on the heatsink to the 2N3058 transistor and soldered the 27Ohm resistor to the Emitter. I attached one pole of the Colorburst crystal to the solder tab on the varicap along with the stripped end of L1 coil. I soldered the Collector to the Tap-2 of the L1 coil, then the Base to the other pole of the crystal. I then attached the 10k resistor to the base and attached another red-magnet wire to the side of the varicap. Almost done I looped a 47nF ceramic capacitor to the  2 power posts of a DC connector, then wrapped the Tap-1, 10k, and the lead from the varicap into the pin (positive) solder tab of the DC connector. I took a second red scrap and stripped the ends, stuck it in the sleeve (negative) solder tab.

Two applications of solder later, the multimeter was back out for a final continuity check. After checking across every joint, I plugged in my power supply to the Mighty Mite. Reaching over to the shortwave I leaned in the power button and tapped in the Colorburst frequency, 3-5-7-9. Static filled the room as I made sure the radio was how I left it earlier that week. SSB mode, attenuator off, and volume up. My finger tingled as caressed the straight key, now wired to the resistor and negative line. I was scared something would melt or flame up when I keyed down. Well no way to find out, except to do.

 The key spring resisted, but I felt metal below as the key bottomed out. The radio let out a sound like rubber going down a slide and faded as I held down the key.

Gentlemen, we have Oscillation.

Revision 3 was a success. Version 1 being breadboard and Version 2 being Bill's crystal and what I had on hand.

I tried tapping out CQ, but was faced with the chirping fact that I haven't practiced in a year. I don't even know TEST let alone CQ CQ KK6JTL TEST. I turned the varicap to see if the sick squeal got any better with adjustment. It did and the pitch out of the radio changed after a bit. I must be changing frequencies! I spun around and fired up the computer. The RTL-SDR would show me what I was doing and who may respond, along with who I might be interfering with.

With the waterfall display zoomed in, showing about 5kHz around 3579kHz, I went back to the Mighty Mite. I saw one CW QSO going on in 3575kHz and two other spikes at 3576-3578kHz. Any guess at what those are? I knew since Sunday night I had been listening to JT65-HF. I reached over and held the key down. A big red spike went up at 3576.5kHz

I waited for quiet and keyed down. I turned the varicap to lower the capacitance. The spike moved to a higher frequency. When the varicap was half out, the squeal began to sound like a tone again. Problem was I was still at 3578kHz and still stepping on the JT65 signals. I called my wife in in excitement to show her and asked her to help me tune the circuit to the official frequency. I'm going to leave the varicap there for now. So I start it at 3577 and have to dial it back to 3579 to operate. I can almost go to 3580 with this little Mighty Mite.

Finally I was curious. I had forgotten to check the output of the transistor Pete sent me on my LC meter. So I got the datasheet from the Internet. The BC547A I had used in Versions 1 & 2 were rated for 325mW. The 2N3058 is rated for 5W. Pete, how much power does this put into an antenna when fed with 12V? Apparently not enough to wake my Radioshack SWR/Power meter. But that thing is hungry. It needs more than 2 Watts for it to function correctly.

Pictures attached. The transistor heatsink looks like it it touching the varicap in the pictures but I made sure there is plenty of gap. Too bad none of the pictures of that gap turned out. The wiggle in the waterfall picture is due to me turning the varicap to show range. Pete left a grounding wire on the case of the crystal. I chose not to remove it, but did not attach it to anything.














Congratulations Jacob!  You really hung in there, overcoming obstacles including the US Postal Service's Crystal-Crushing Steam Roller.  The rig looks great! 



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