Ham To Ham #27 - December 1997
73's Ham To Ham column
c/o Dave Miller, NZ9E
7462 Lawler Avenue
Niles, IL 60714-3108
USA
E-mail: dmiller14@juno.com
A new game in town
I recently received a big, thick catalog (designated September '97) in the mail from Tech America (reg. trade name), a new one on me. It seems that Tech America is a subsidiary of the Tandy Corporation, known primarily for its Radio Shack (reg. trade name) retail outlet stores. Tech America, however, is a significant variation on the Radio Shack theme ... it's strictly electronic components and test equipment, aimed at the serious electronic experimenters and professionals. You'll find 546 pages of electronic components, test equipment, and kits in the Tech America catalog, but not just the familiar Radio Shack parts sold (sometimes reluctantly?) in their retail outlets. Manufacturers like Clarostat, Bussmann, Bud, Belden, Molex, PacTec, Edsyn, Fluke, Leader, Wavetek, and others are also represented ... under their own brand names. The familiar Radio Shack parts are there too, but the advantage of the Tech America concept is that you needn't run from store to store to collect all of the parts that you'll need for a particular project. Instead, Tech America will have them delivered to your door for a very reasonable shipping charge, with fast turn-around, and via toll-free phone or FAX numbers. For me, that sure beats burning up the pavement (and my time) in the family buggy.
Here's a quick, though admittedly incomplete, rundown. The catalog contains 67 pages of books for the electronics professional and hobbyest, including several pages aimed directly at the ham radio buff. There are also 65 pages of pinouts and functional descriptions for most of the popular IC's that you're likely to encounter today, including logic, comparators, op-amps, audio amplifiers, timers & oscillators, interface devices, A/D & D/A converters, analog switches, display drivers, etc. These pages alone are worth the price of the catalog, which, by the way, is free.
The Tech America catalog also sports 15 pages of electronic kits, from audio amplifiers to a PC-based digital storage scope. There are a number of kits aimed directly at the ham radio community (a radio direction finder, a communications receiver for either 10M, 2M or 220MHz., a CW practice oscillator, a DTMF decoder, etc.). These are in addition to the pages and pages of test equipment and component parts stocked by Tech America. You don't have to run out to a Radio Shack store to order from Tech America. You can contact them at 1-800-877-0072 by voice, or 1-800-813-0087 via FAX. They accept credit cards, checks, money orders or have commercial accounts if you happen to be in that category. Tech America is located at: P.O. Box 1981, Fort Worth, TX 76101-1981 and currently has one walk-in store in Denver (1550 S. Colorado Boulevard), with more planned in the future.
In short, the Tech America catalog is surely worthwhile having in your catalog library, along with those of the traditional 73 advertisers of course!
DE NZ9E
"Beeper" Short Detective
This month's Ham To Ham column will deviate somewhat from the norm. Ordinarily I try to blend
a balance of ideas, so that there exists a greater chance of there being something for everyone. This month's idea is so universal, however, that I'll be devoting the entire column to it ... so hang on.
Anyone who's done any amount of electronic troubleshooting will have run into a few instances
where it's nearly impossible to find the defect, except by disconnecting just about everything
connected across the circuit in question. That can happen when there is a dead short
somewhere. No matter where you might measure, using a normal continuity tester (usually an
ohmmeter), the short is registered. If a visual inspection proves ineffective, the only thing left to
do is to begin disconnecting parts (or circuit branches, ugh) until the short disappears. It's generally a long and frustrating exercise. Not any more!
Thanks to Jim Wood of Inovonics, Inc. of Santa Cruz, CA, you can now track down a short in
"short" order! Jim Wood's circuit originally appeared as a piece entitled "Beeper" Finds Circuit Shorts, in Electronic Design Magazine for April 3, 1995, and was thereafter voted "Best of Issue". It appeared again in the October 24, 1996 issue of Electronic Design because of its popularity. It is reappearing here with permission from both Electronic Design Magazine and from the author, Jim Wood.
Jim's circuit utilizes a pair ordinary op amps in a clever circuit that will produce a 1,000 Hz tone
when the circuit's test probes are connected together (simulating a short circuit), but will quickly
be lowered all the way down to a low growl or ticking sound when even a very small resistance is
placed between the tips of the probes. This means that as one of the probes is moved along a
printed circuit board trace that's shorted (moving toward the probe on the other side of the short),
the pitch of the tone will change with even the very small resistance of the copper PC board
trace itself! In fact, if you hold one probe on the wire lead of a brand new, small electrolytic
capacitor (before it's mounted in the circuit), you can actually differentiate between a movement
of the second probe along that lead of as little as 1/4" or so ... just that minuscule amount of
resistance is detectable.
It's easy to see why you can walk right up to a dead short using this technique, or at least get
within a fraction of an inch of it. As you pass the short, the tone will begin changing again, telling
you that you've gone too far. Sound like the kind of test instrument you could use?
But there's more, using Jim's circuit in reverse will also allow you to evaluate how much resistance might be present in test leads, connector pins, ground lugs, etc. that should be close to a dead short. Anything that is supposed to show nearly zero resistance can be judged
(on a relative basis) as to whether it really is making good contact. You can then clean or adjust the contacts of whatever you're measuring to see what improvement is possible. In addition to the examples just mentioned, you might want to judge how much relative contact resistance exists in a selector switch, toggle switch, relay, mechanical contactor, or other component that's supposed to have no appreciable resistive component in it. Jim's circuit is the answer.
Here's perhaps the best part ... duplicating the "Beeper" short detective is inexpensive, because all of the component parts used are readily available at any good sized hamfest, or from the component parts dealers who advertise here in 73 Magazine each month. Furthermore, the entire circuit is at either an audio or a DC level, so parts placement and wiring technique is also non-critical, as long as some reasonable care is exercised (as one would apply to any circuit where a degree of gain could potentially cause circuit instability). But it's not at all difficult to build a stable, usable unit when the parts specified are utilized. So I'd suggest that you build it "as shown" (in Figure 1) first, and only then, feel free to modify it if you think that you can improve upon the design for your own particular needs.
Most of the problems associated with duplicating any circuit, seen in a magazine or book, can be traced to not thoroughly checking over the final circuit for errors, using parts or values different from what the author suggests, or making modifications to the circuit before building it "exactly" as shown in the text. It's an error we all make from time to time. One modification that you can make that shouldn't pose a problem, however, is substituting a single LM1458 IC for the two separate LM741's. The LM1458 is simply dual LM741's in a single 8-pin DIP package that will save you a bit of board space. The pin-out for this IC is shown in Figure 4 if you decide to take that route.
I've built Jim's "Beeper" short detective, first exactly as suggested, then modified a tiny bit, and it works like a champ!
Take a look at the original design that Jim suggested (as shown in Figure 1). The "Beeper" short detective is a classical free-running multivibrator, built around a pair of the commonly available 741 op-amps, but with a few important additions. Transistors Q1 and Q2 deliver a plus and minus 10 volt square wave to resistor R4 and are capable of supplying 100 mA when the probe tips are shorted together. Resistor R5 assures that the open circuit voltage never exceeds 0.1 volt when the probes are open circuited. The gain of the U2 is equal to the R4/R5 divider loss, making the two op-amp outputs identical. The tone drops to a low growl or ticking sound, effectively silencing the speaker when the probes are in the open (or high resistance) state.
When the probe tips are a perfect short however, U2's inputs are effectively shorted and the speaker will produce a 1-kHz tone. Anything between these two extremes, even a very tiny resistance, will produce a different sounding tone, telling you immediately if you're headed in the right (or wrong) direction toward the short.
There is one precaution that must be observed. Notice that the schematic of Figure 1 shows two
wires going to each probe ... it's important that you follow this recommendation. U2's differential
input must have its own separate path to the probe tips in order to eliminate test-lead-resistance
from the measurement. The miniature 'zip-cord' style of speaker-wire that's readily available
makes a good 2-conductor test lead fulfilling this requirement. Along this same line of reasoning,
the test probe tips themselves must make very low-resistance contact with the short-circuit being
traced; a pair of H.H. Smith #317 probe-tips are ideal ... their tips are needle-sharp for piercing a
wire's insulation or for digging below the flux or oxide barrier on a PB board's trace ... and they're
easily replaceable as well. You can experiment with different probe tip ideas if you like, just
make sure that you end up with ones having the very least contact-resistance possible.
The LM741's must also have a true plus and minus DC power source, with the common of
the two supplies connected to the circuit common (illustrated as the ground symbol in Figure 1).
Don't let that stop you, however, because the power source requirement isn't all that difficult to meet. If you have a dual polarity bench supply, you can simply use that. Two small 12-volt batteries could also be used. I've operated mine using just two standard 9-volt transistor radio batteries (shown in Figure 2), and except for a slight reduction in speaker volume, it works just fine. You might also want to build a simple dual polarity supply dedicated to the "Beeper" like the one shown in Figure 3. The circuitry draws less than 100 milliamperes from each supply (or battery), so the supply can be of a reasonably low current design. Even two low current wall-cube power supplies could be used, just make sure that the proper polarity is supplied to the LM741 IC's supply pins, and that the other two leads from the cubes are tied together and to the circuit common (the ground symbol).
Looking at the circuit shown in Figure 2, you'll see a couple of small modifications that I made to mine. I used ten easily obtained 10 ohm, 1/4 watt carbon film resistors in parallel for the
1 ohm resistor designated as R5 in the schematic. I also used a series/parallel configuration
(of 100 ohm, 1/2 watt resistors) for the 100 ohm, 2 watt resistor shown in the schematic as resistor R4. Placing all of the resistors right on the board, and wiring them per Figure 2, takes up very little additional room. I also changed R6 to 100 ohms and added a 500 ohm volume control (R9) to the speaker circuit. This affords some control over the ultimate volume level of the unit's output. I found that returning the speaker circuit directly to the common point increased the obtainable volume level a bit, especially if you're planning on using 9 volt batteries in place of the recommended plus and minus 12 volts as in Jim's original design. Again, however, it's probably best to build the circuit up originally by following the schematic of Figure 1 exactly, then you can begin to experiment from there. I used a socket for the 741 IC's for ease in changing the op amps if needed, and I employed a very small "flat" speaker (ala a recent hamfest) for the sound output device. I built my own short detective into a Radio Shack (reg. trade mark) #270-283A Experimenter's Box and perf circuit board combo. I like this combo for small projects since it incorporates both the project box and perforated circuit board (with foil pads) in one easy-to-use package. There may well be other areas in which some improvements can be made to
better fulfill certain user needs for the short detective, and I'd be interested in seeing any that our Ham to Ham column readers might suggest.
So that's it for this month. A very special thanks to the originator of the circuit featured:
Jim Wood
c/o Inovonics, Inc.
1305 Fair Avenue
Santa Cruz, CA 95060
The very best of Season's Greetings to everyone. Sue (KA9UCK) and I hope that your stocking is filled with all of the toys that a ham could want! Well, almost all, there's always next year! We'll return with the column more in its normal format next time, but between now and then, try putting together Jim Wood's "Beeper" and give it a try ... I think you'll like it.
Please send any ideas that you would like to see included in this column to 73 Magazine's Ham
To Ham column, c/o Dave Miller NZ9E, 7462 Lawler Avenue, Niles, IL 60714-3108, USA. We
will make every attempt to respond to all legitimate ideas in a timely manner, but please send
any specific questions, on any particular tip, to the originator of the idea, not to this column's
moderator nor to 73 Magazine.