HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Antennae on March 09, 2014, 0135 UTC
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There are different kinds of crystals out there. I have an HC-49 crystal. It got hot 3 times for about a minute each time when it was working. (Like say-"ouch"-out-loud-hot with the fingerometer.) I'm pretty sure it broke. I have it in a Michigan Mighty Mite transmitter. Its metal casing is small compared to RC car crystals.
Does the small case mean its a wimpy crystal type? I wonder if I was misusing it. What kind of crystal should I be looking for to replace it?
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If the crystal is running hot, it means your drive level is, well, excessive. You could also have some DC present that shouldn't be there. You can order crystals in different packages, HC-49 and HC-6 are the most common.
Taking a look at the circuit, it appears this transmitter is a power oscillator coupled to an antenna. The crystal is either not suited for this purpose (microprocessor crystals usually aren't happy in these kind of circuits) or you may have the coil the wrong way around, driving the crystal too hard.
+-RH
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Yeah the HC49U/S micro xtals are the worst. They are VERY easily overdriven. As redhat said, in a MOPA circuit (like the Mighty Mike), there's too much current running through the xtal branch for a small rock. Also, the smaller the rock, the more susceptible they are to frequency change due to capacitance. In your situation the best xtals to use are physically large rocks like Butch (HC6) and Sluggo (FT243). HC18s are also usable, but will need a bit more care to keep from overdriving. I often use HC49s but only in chip oscillator circuits like those built around an HCU04 unbuffered inverter or a 7400 NAND gate. These circuits are easily found in a zillion places online and probably could be used to drive your transmitter. The smaller the rock, the smaller the power level it can tolerate.
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Oh man, I learned the hard way! It was $19 for that thing shipping included. Thanks, I'll look for a Butch or Sluggo chunk of crystal.
I realize I didn't mention that when it was working, it ran hot when the transmitter's capacitor wasn't tuned. Otherwise it wasn't hot. Another thing, after the failure, I found I shouldn't have connected the antenna's ground to the oscillator circuit ground. :P I wonder if that helped it die.
Regarding the coil winding, I made sure L1 and L2 were wound the same direction. I can't find anything else on how to wind L1 on this diagram. L1 has 21 turns total and its tapped at 7 turns. My question is if it matters where the 7 turns and 14 turns go. Like should the 7 turns section go toward the top of the diagram or the bottom? Refer to the attachment. I wrote notes in green to aid in my question.
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i put the tap at the bottom for a short lead through the circuit board
ft243 easy to grind up to freq you want and can be had fairly reasonable on ebay
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should the 7 turns section go toward the top of the diagram or the bottom?
Definitely towards the bottom. Another thing - the entire 21 turns resonates on freq and your 4 turn secondary should be wound over nearly the entire length for best results. You might already be building your Mighty Mike from Make RF's website but if not, check it out:
http://makerf.com/posts/mighty_simple_shortwave_transmitter
He has an excellent site for newbie transmitter builders - lots of good stuff! Once you get your unit working his Altoids can Low Pass Filter is a highly recommended add-on.
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One more thing Antennae - your xtal might still be good BUT your transistor is possibly blown. Energizing it without the antenna ground connected might have zorched the transistor, however mebbe not. It's hard to tell considering the low power level of the Mighty Mike. I mention it only to alert you to the possibility. Ain't transmitter building FUNNNNN? Now you know why its a "Black Art"...
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Its a fun experiment for the 40meter ham band. I can't wait to make some contacts with it.
-Yup, its from makerf.com. He's a great resource!
-Whew, thats how my coil ended up with the 7 on the bottom. And thanks for specifying to spread the secondary over the whole thing. That was another question. I did that because I was able to see it like that on makerf's pictures.
-It even worked on the breadboard with a coil made from enamel wire from an old RC motor. It was all lumpy so I got some new wire for my purdy version.
-Zorched transistor eh? I hope not, I tried it with another one I had and it still wouldn't work. If I had an extra variable capacitor I could do a test on the crystal and make certain what the problem is.
I can't help it, take a look at my baby!
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Oh yeah, I had the antenna and antenna's ground connected with a 50ohm load...mostly. BUT I had the antenna ground connected to the ground of the Mighty Mite circuit when it shouldn't be.
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It was the crystal that was busted. I installed a big honkin' FT 243 crystal and it works again, yay.
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It was the crystal that was busted. I installed a big honkin' FT 243 crystal and it works again, yay.
Hooray for Sluggo! ;D
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I made a long 30 below blog post of my building experience with pictures. I had no intention of making it today but it just burst out of me. I hope it helps other newbies!
http://hfunderground.com/blog/?p=369
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An excellent article Antennae! Reading it brought back memories from 30+ years ago of my plumbing mysterious radio circuitry in similar fashion. Congratulations on your successful build and welcome to the fraternity. As your knowledge and experience grows, may your signal always look "siney" and your curiosity never fade... 73!
ff
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UPDATE:
I got a ham license (hooray!) and tried the transmitter on air and it doesn't go across the street! It doesn't move the dial on my antenna tuner either. I used 2 different antennas. Full update is on the bottom of the article:
http://hfunderground.com/blog/?p=369
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These guys have been around for years.
http://jancrystals.com/
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These guys have been around for years.
http://jancrystals.com/
Yes, along with International Crystal and a few others who are still chugging along. Unfortunately, nowadays the cost for a custom-cut crystal is quite out of reach for many. Also, the manufacturers stopped producing FT243 size rocks over 20 years ago. Many no longer mess with HC6s. HC18s and similar are generally the largest physical rocks one can custom order nowadays. Hamfests, Ebay sellers, and online vendors selling used rocks are the only places one can find the big fellas now. Small rocks are fine for low power oscillators but for MOPA-type transmitters (which are usually beginner-type projects), the little chunks will overheat and fail. I still jump on FT243s in my frequency ballpark when I occasionally come across them. There's always some like-minded folks in need... ;)
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Last I checked, ICM only wants about $25 shipped for a standard HC-6 crystal. Doesn't sound that out of reach to me.
+-RH
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Last I checked, ICM only wants about $25 shipped for a standard HC-6 crystal. Doesn't sound that out of reach to me.
+-RH
Nor to me either. Admittedly Redhat I haven't actually purchased a custom-cut rock since the 90s. A few years back one of my buds had a quote from Jan for an HC-6U of about $40 shipped with a lower per rock price for multiple xtals of the same freq and specs. Buying one wouldn't hurt too much but ordering for several freqs might smart a bit, especially for some of the newbs...
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Honestly, if your going to go the multi-frequency route, a better way would be to build a simple frequency synthesizer based around a 74HCT4046 for about the same money in parts, and can generate frequencies up to 17 MHz. That's the way I'd go :)
+-RH
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Yes, I'd say the PLL is a good way to go for anyone who wants to invest the effort. Its funny you mention the 4046 Redhat. I'm toying with one right now, along with a 74hc74 flipflop to use as an xtal doubler. Have you ever played with one? I'd appreciate your thoughts...
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The last time I built something with one was several years ago. I build a device that had a 3 MHz oscillator in it, and divided it down to 300 Hz. That was compared to a pulse generator on the gear train of a machine that at the proper speed would also provide 300 Hz. The error voltage produced by the phase comparator was filtered and fed to a VFD to keep the machine at the right speed. It took some design and breadboarding time, but its still working :)
+-RH
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I agree that some kind of vfo is the way to go, however, if you come across an old rig with the crystal plugs it's much easier sometimes to just buy one and pop it in. Used to work for a guy in the 90's who had a box full of the old style crystals, (brown bakelite square thingys) mostly ham bands but he also had some bizarre frequencies in there as well. Wish I had bought the lot off of him.