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

    2 comments:

    Eldon R. Brown SR - WA0UWH said...

    Good stuff, several months ago I did the same thing.

    I now have a junk box with many parts for some of my projects, and for my very young shop friends (kids) to toil-over and to make simulated projects with a little solder - someday these kids will make great engineers and they will never forget their first adventures into the world of electronics.

    73's Eldon - WA0UWH - http://WA0UWH.blogspot.com

    -

    Doug, AAØMS said...

    Eldon--Thanks for your comment (and for reading my blog). You're absolutely right--scrounged parts make great "practice" parts for those who are learning how to do electronic construction, and for those who are experimenting. If you fry a few transistors or hook up an electrolytic cap backwards, you've learned a valuable lesson without it hurting your wallet too much!

    73 de aa0ms

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