Junk Drawer Mighty Mite Issues

Discussion in 'Homebrew and Kit Projects' started by KC0YEJ, Jan 13, 2018.

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  1. KC0YEJ

    KC0YEJ Ham Member QRZ Page

    I'm trying to get back into the swing of things, having been only lightly active on VHF and UHF FM since my only functioning HF radio kicked the bucket, and decided that I could probably homebrew a transmitter and receiver to get started. I've got some electronics training, am an IT guy, and have brewed up a few odds and ends circuits in the past, so how hard could this be?

    I managed to brew up a direct conversion receiver with VFO with the simplest circuits possible to get started, and have heard a few things on it (with lots of digital hash, so more troubleshooting required, but it'll be enough to get started). Figuring that a simple Michigan Mighty Mite would be good as a starting point for a transmitter and to test the receiver, I found the schematic online and started brewing. I'll have to buy proper crystals to get into the ham bands, but for starters, I have some 10MHz crystals and a 50 ohm dummy load that I can use to see if the thing works. Queue the first issue.

    I don't have any 2n2222 transistors in my junk drawer anymore. The most common transistor on the planet, and I can't find any! But I found a bunch of BC182LBs in there, and figured they'd be functional enough. I followed the directions to turn a coil for 40 meters and assembled the remainder of the circuit. (I've ordered some, just in case that is actually the issue)

    Anyway, on to what is actually bothering me, and the thing I need help troubleshooting. The output waveform is not a nice, clean sine wave. Or, even, clean at all. Picture attached of what my o-scope found. Also attached a picture of the breadboarded circuit, in case there's an obvious assembly issue. Any advice as to where I went awry, or if the BC182 is the issue I'm fighting? Thanks!

    Brian
     

    Attached Files:

  2. KG5QBI

    KG5QBI Subscriber QRZ Page

    That is cleaner than other MMM outputs I have seen. They really do require a LPF on the output. I'm presuming you have tuned the variable cap for cleanest output waveform?
     
  3. KC0YEJ

    KC0YEJ Ham Member QRZ Page

    Yes, that's pretty much the best waveform I've seen with the adjustment I have available . Adding a low pass filet is quite doable, but will it make a difference that I could see in a scope?
     
  4. N1OOQ

    N1OOQ Ham Member QRZ Page

    Yes. A decent filter can turn some really ugly waveforms into nice sine waves. You definitely do not want to transmit that as is.
    Is that from a class C final? That's more or less how it would look without a filter.
     
  5. N1OOQ

    N1OOQ Ham Member QRZ Page

    Ok, I just looked up the MMM and it seems to be just an oscillator. The coil and the tuning cap are apparently supposed to resonate at the xmit freq, and that's the "filter". You're at 10 MHz, right? It might not be tuning far enough in whatever direction to resonate... maybe try spreading or closing up the windings on the coil.
     
  6. KC0YEJ

    KC0YEJ Ham Member QRZ Page

    Side note, typing on a cell phone is a bear! Sorry for the spelling errors!
     
  7. KG5QBI

    KG5QBI Subscriber QRZ Page

    Thinking some more, for the frequency crystal you currently have, you may need more capacitance than the variable cap is providing. You can try adding some capacitance in parallel, and see if it tunes up better. But my understanding is that the MMM virtually always needs an external LPF to be legal.
     
  8. KC0YEJ

    KC0YEJ Ham Member QRZ Page

    All right, here's an update using the fine advice you have all provided!

    I removed a parallel capacitor to the variable cap, removed some windings from the inductor to put it close to the 30M band, and added a single stage RC low pass filter. There has been some improvement in the signal. It's still not Quite Right, but I'm wondering if maybe a second stage of RC filter might clean it up a little more. See attached picture for current o-scope readout!
     

    Attached Files:

    KG5QBI likes this.
  9. KG5QBI

    KG5QBI Subscriber QRZ Page

    More LPF will remove more of the non sinusoidal energy. Any shape that isn't a sine wave is always representable as the fundamental sine wave plus other higher frequency sine waves, and the filters will react appropriately.

    If I recall, the CQX qrp cw transceiver drives the PA as a square wave, then uses a LPF to remove everything that isn't the desired sine wave.
     
  10. N1OOQ

    N1OOQ Ham Member QRZ Page

    Yah, another stage of filter might get that looking pretty good. What kind of amplitude are you getting into your dummy load?
     

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