April 19, 2010

Long live the hamfest

This weekend I went to a hamfest, and spent a few dollars--the ticket price and gas to get there cost more than what I spent once I got there, but I am very pleased with what I found.  For the radio hobbyist looking to build up the junkbox and locate parts for building stuff, most hamfests are still potential goldmines.

A case in point:  while digging through a box of junk on one of the swapmeet tables, I found a box that contained a one-pound spool of #30 Belden Polythermaleze magnet wire--the kind with heat-strippable enamel coating--for $5.00.  Just for grins I looked it up on the Mouser Web site and found their price: $83.73.  I checked the price for similar magnet wire from one of the parts and kits places, and they were selling 15 feet of similar stuff for $2.00.  My one-pound spool contains about 3140 feet of wire!

I also found a NOS (new old stock) air-variable capacitor for fifty cents that would probably cost me around $15 if I tried to buy it new from a supplier.

For a quarter I bought a 10-turn turns-counting dial that would have cost $8-10 new. 

For one thin dime I got another nice hank of magnet wire--maybe 50 feet or so.  For $5.00 I got two hanks of multi-strand copper antenna wire and a length of ladder line--enough for a nice end-fed Zepp antenna. 

I found a box of random parts that was marked $1 (for the whole box)--it contained a bunch of integrated circuits, including several ULN2003A Darlington transistor arrays, some quad op-amps, some MC1556G op-amps, some CMOS digital chips, several LM317 voltage regulator chips, a 75-to-300-ohm TV balun, and a bunch of other little odds and ends, including about a dozen short banana plug test leads--a lot the rest of it is useless to me, but the stuff I can use would have cost me a lot more than a dollar. 

I bought one UHF bulkhead connector from a commercial vendor--a whopping $3.85--but everything else I bought was from the swapmeet tables.  Plus I had a great time connecting with some hams that I knew from the local club, and it was a lot of fun to see a bunch of old radios and test gear and antennas and other junk.

If you like to build stuff, and if you are lucky enough to have a hamfest happening nearby, don't think twice about it--take the time to go and see what you can find.  You may not be able to find everything you need for that next project, but chances are you'll be able to save some money and collect parts for a future project, and you might even learn something in the process!

73 de aa0ms

April 10, 2010

I love building receivers

There is something very, very cool about building receivers.  You start with a handful of parts, and in a few hours (or a few evenings), you flick on the power switch and (if all has gone well) you hear signals.  Of course, quite often I have to spend a few more hours troubleshooting things before I can get my receivers to work, but that can provide very helpful experience.

I've been wanting to build another regenerative receiver, just for grins and giggles, and hearing Bill, N2CQR, speak on the Soldersmoke podcast about how he suspects regens are possessed and possibly evil made me want to try another one of these guys again.  I have built a couple of different regen rigs--one was a regenerative W1AW code-practice receiver built around a 3.579 MHz color-burst crystal, and the other was a Ten-Tec multi-band regenerative shortwave receiver kit.

I had found an old article by Charles Kitchin, N1TEV, titled "A Simple Regen Radio for Beginners," from the September 2000 issue of QST, that didn't require any tough-to-find parts (except for the air-variable capacitor), and after about 15 minutes of going through my parts bins and drawers, I had gathered up just about everything I needed.

I started by building an enclosure--well, it's really not an enclosure, because the back, top and sides are open to the air.  I suppose it's more of a chassis, of sorts--really just a base and a front panel, made of double-side copper-clad PC board.  I used a 150 pF variable capacitor for the main tuning control, and I wound the coil on a plastic pill bottle.  The article called for 22-gauge solid insulated wire for the coil, but I didn't have any around, so I used some enameled wire I harvested a few years ago from a TV degaussing coil.

I drilled holes in the front panel and mounted the pots and the tuning capacitor, then began breadboarding the circuit with the schematic at my side.  My aim was to finish the receiver in one evening, so I worked quickly (more quickly than I should have), and my layout admittedly leaves a lot to be desired.  And, of course, it didn't work the first time I turned it on.

N1TEV kindly included a few voltages at various test points in his schematic, so I checked those, and found that proper voltage was not getting to the transistor, though it was not apparent why.  I suspected a bad diode, so I replaced it, but to no avail.  I gave it up for the evening.

So the next evening I decided I was going to make this radio work one way or another, and I started poking around and found that the voltages that were incorrect the day before were magically correct.  (How does that happen?  Probably a bad solder connection or a short somewhere that was remedied by my poking around.) I turned the power on and started tuning through the entire range, to see if it would be possible to hear it oscillating on my store-bought shortwave receiver on the bench.  The good news is that I did hear it--the bad news is that there was still nothing in the headphones, so I turned my attention to the audio part of the circuit.

I had used an old LM386 amplifier chip that I had salvaged from another project--it occurred to me that it might actually be dead, so I replaced it with a new one, and when I powered back up I began hearing things in the headphones.  I connected three or four feet of hook-up wire for an antenna and just draped it across the bench, and was able to hear dozens of signals.  WWV came in loud and clear, along with a number of shortwave broadcast stations in a variety of languages.  Sweet, sweet success!

Tuning a regenerative receiver is definitely an acquired skill and it takes some getting used to, but eventually you get the hang of two-handed tuning (one hand on the main tune control, the other on the regen control).  My radio is really sensitive to hand capacitance, meaning that once I tune a station in, I have to keep my hands on the controls, because moving them away changes the tuning.  If I had gone to the extra trouble of using insulated shaft couplers, I might have avoided this problem.  (As you can tell from the photograph, I could also have built this thing with a lot more care given to lead length and layout, and that might have helped a bit.)

But hey--it was a fun project, and it worked, and there was great satisfaction in being able to troubleshoot it successfully.  I think my next receiver project will be some sort of direct-conversion rig, and after that, I'll turn my attention to a superhet design--I have a whole bunch of surplus crystals that I might be able to use.

I hope you're melting solder in some useful (and fun) way!

73 de aa0ms

April 6, 2010

Scrounging for parts

I used to do this all the time--any time any electronic thing I owned finally broke down and ceased to function, I attacked it mercilessly, soldering iron and pliers in hand, and stripped it of every potentially useful component.  I did it partly because I am cheap and I like getting parts pretty much for free, and partly for the sheer joy of taking things apart to see what they are made of.

So today I went to my local thrift store to scrounge about for old electronics to cannibalize for parts.  I spent $4 on a small portable TV.  I was actually looking for a TV--an older TV--I didn't want one that was made of surface-mount parts.  I was looking for a TV because in some of the radio circuits I've seen on the 'net, the designers include transformers built with TV balun cores, and I figured I would start tearing up old TVs and harvesting their cores (and other parts).

I walked out with a small, old portable B/W TV--there were other, full-size TVs there, but they were more expensive, and they appeared to be newer, and for my purposes, I just felt an older unit was what I wanted.  And it only set me back about four bucks.

I know different people use different methods to scrounge parts from circuit boards--I've heard of the blow-torch approach, where you basically heat up the back (soldered) side of the board with a propane torch (after bending all of the leads straight up), then allow the parts to fall into a bin.  There's a similar technique that involves a heat gun, rather than a blowtorch, but it's basically the same thing.  I opted for neither of those approaches, but rather decided to unsolder the parts I wanted one at a time--remember, I was not as interested in efficiency as in the joy of the activity itself.

But here are some things I've learned when scavenging for parts in this way:
  1. Sometimes certain parts are glued down.  Electrolytic capacitors are often secured to the circuit board with a blob of some kind of adhesive, and it can stick very, very tightly.  I've peeled some of the plastic sleeve from more than one electrolytic cap because of that stuff.  It helps to try to unstick the part before you try to unsolder the part.
  2. Some parts are practically impossible to remove (using my method) without destroying them--those little IF transformers have not only multiple pins, but usually the shielding cans themselves are soldered to the board--the blowtorch method would probably be a more reasonable approach.  I've pulled many a pin from a transformer while trying to remove it.
  3. You've got to be careful with semiconductors, because too much heat can destroy them, and desoldering can require a lot more time than soldering.
  4. Don't get your hopes up about all the electrolytic caps in whatever you're taking apart.  If it's old, there's a good chance that at least some of the electrolytic caps are bad.  That usually doesn't deter me or keep me from trying to use them anyway, but it's just something to keep in mind.
  5. I don't often salvage resistors, unless they are installed in such a way that there is sufficient lead length so they can be re-used (which isn't often the case).
  6. Board-mounted pots are really hard to uninstall using my method.  I usually don't even bother if they are soldered directly to the circuit board.
  7. If you take a TV apart, be careful.  The CRT can hold a pretty significant charge.  Years ago I took a TV apart and I carefully discharged the tube; but while I carried it to the dumpster, it  developed enough of a charge from touching my clothing that I still got a nasty shock.
  8. If there are big electrolytic caps in the thing you're taking apart, make sure you discharge them before you play with them.
    So after a couple of hours of messing around (and even using my old solder-sucker, which never really has worked all that well), here's what I harvested from this little old TV:

    1 - 4.7-ohm power resistor (10 watts)
    5 - plastic knobs for pots
    1 - pot with a pulley on the shaft (for connection to dial-cord)
    Various little rollers that go with the pot above (for use with the dial cord)
    1 - pot with a switch
    3 - power transistors
    5 - small signal transistors
    1 - varactor diode (I think that's what it is)
    5 - miscellaneous fixed inductors
    1 - ferrite balun core (whoopee!)
    6 - rectifier diodes
    3 - signal or switching diodes (appear to be germaniums, but I haven't tested them yet)
    3 - thermistors (?)
    about 20 capacitors (mylars, ceramics)
    about 15 electrolytic capacitors
    1 -fuse
    1 - chunk of thick aluminum, used as a heat sink
    a few resistors
    a bunch of hook-up wire, including some coax and two-conductor wire
    a little pile of screws of various types
    3 of those little plastic-coated metal tabs with a mounting screw hole; they are used as wire tie points--you gather the wires together and bend the tabs around them (I like these little guys)

    In my book, that's a lot of parts for $4.00.  Just the transistors, capacitors or inductors alone could have cost that much.  As spring settles in, the local garage sales should net me a bunch of old TVs, radios, broken VCRs and who knows what other kinds of electronic gear to either fix up or cannibalize.  If you hit a garage sale toward the end of the day, people might very well give you their old electronics, just to be rid of them.

    Have fun!

    73 de aa0ms

    April 1, 2010

    Empirical evidence for black holes discovered

    Yes, it's true--there is, in fact, a black hole under my workbench.  Seems like every time I work on a project, the smallest parts are somehow sucked off of my bench and disappear forever under there.  I'm guessing that somewhere in the space-time continuum there are enough parts floating around to build several QRP transceivers.

    That's my story and I'm sticking to it.  Happy April Fool's Day, everyone!

    73 de aa0ms