Note: If you would like a copy of any of the circuit diagrams or figures referred to in
this column, simply send a SASE to Ham To Ham Column, c/o Dave Miller NZ9E,
7462 Lawler Avenue, Niles, IL 60714-3108 and indicate the month and circuit or
figure name on your request. No requests will be honored without a self-addressed and
adaquately stamped envelope (SASE).
Ham To Ham #24 - September 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
With this column (#24), Ham To Ham concludes its second year in the pages of 73 Amateur
Radio Today Magazine. Thanks to all of those who've been supportive over the last two years
by sending in their many tips, ideas, suggestions and short cuts for the rest of us to benefit from.
With an average of four or five tips per month, that's well over a hundred ideas that we've been
able to relay so far ... not bad! I always welcome more of your input, however, so don't feel that
your own ideas aren't worthwhile submitting ... they are! The basics are pretty simple, what do
you feel would be of interest to your fellows hams and electronic hobbyists? That's what I'm
hoping to see from you. Anything having to do with ham radio and electronics is appropriate (not
all readers are licensed hams...yet!). Since ham radio encompasses just about the entirety of
electronics, it's conceivable that just about any tip in this field of general electronics will likely
have some application in our ham or SWL shacks. So fire up your word processor and send me
some of your favorite shortcuts. You can send them by Uncle Sam's slow-mail, cyber-mail or via the Ham To Ham home page feedback button on the Web at: http://www.rrsta.com/hth. Take some time to browse the neat page that Mark Bohnhoff WB9UOM has come up with for us. Now,
onward to this month's offerings:
Nailing it down!
From Frank Brumbaugh W4LJD "If you haven't tried it, Liquid Nails (reg. trade mark) is a good
general purpose adhesive for holding down trimpots, small relays, electrolytic capacitors and any
number of small parts on a printed circuit board or metal chassis. As hams and electronics
experimenters, we often find ourselves adding parts to an otherwise completed item of
equipment, and as such, don't usually have the luxury of solder pads and pre-planned holes right on the PC board itself for mounting these additional necessities. Super glue is one answer, but it's expensive and a standard tube holds very little. Liquid Nails (manufactured by Macco
Adhesives, The Glidden Company, Cleveland, OH 44115) on the other hand, is less costly, and
a 4 ounce tube will last quite a long time. It's sold to the construction trade as a drywall and
paneling adhesive in larger caulking-gun size tubes, but the 4 oz. consumer-sized squeeze tube
is a great size for occasional use by the home hobbyist. It will work with ceramics, metals
(including aluminum), rubber, wood, plastic, etc. It has a very thick consistency (unlike super
glue), so it won't run and get into areas where it might be unwelcomed! It's very sticky stuff and
will begin to set-up quickly upon exposure to the air, so you'll need to have the parts that you
want to "nail" down ready, before opening up the tube. It takes a day or so for full curing, but the
parts will hold pretty well in place considerably sooner than that. Cleanup can be done with
mineral spirits, and of course, as with any petroleum distillate product, adequate ventilation is a
must. By the way, it comes with a 50-year guarantee ... think the equipment you're working on
will last that long?"
Moderators note: Frank's right, Liquid Nails is a nice product to have around the ham work
bench. At Frank's suggestion, I tested it for RF transparency in a microwave oven and it passed.
When completely set-up (cured), it doesn't seem to absorb any appreciable microwave energy,
so it should be reasonably safe to use around RF circuitry. In case you're not familiar with the
"microwave oven test", putting a sample of insulating material into your home microwave oven,
along with a small cup of water, is a good way to test for RF absorption. If the sample doesn't
get warm when the oven is run for a minute of so, then it's pretty safe to assume that it's
transparent to radio frequencies (which all insulating materials used in the RF area of ham gear
should be). The only other caveat that you have to watch for is moisture absorption (the
adhesive becoming hydroscopic). Some adhesives will absorb moisture from the air, with time
and age, and can become semi-conductive. Obviously this can cause all manner of strange
problems when the adhesive is laid across two or more copper printed circuit board traces. Even
a hundred K ohms or so of resistance can upset high impedance circuits quite a bit, and lower
resistance can even do some irreparable damage. Keep an eye on that with any new product
that you might try.
Two ears are better than one
Here's a far-out tip from Thomas Hart AD1B: "Last year I discovered the fun of hamming through
the RS-12 satellite and have been busily chasing DX using it ever since then. My usual mode of
operation has involved running my Kenwood TS-430S in the dual-VFO mode, and transmitting
on 15 meters while receiving on 10 meters. This is what's called mode K operation. It's
necessary to estimate the Doppler shift off-set (by the hit-or-miss method) because the 10 meter
receive frequency on the transceiver is silenced during transmit, but it's a reasonably
uncomplicated way to utilize RS-12 and I've been able to confirm 38 states and 16 countries in
just 7 months working with the satellite in my spare time. A true ham never being satisfied, I
recently added the luxury of being able to listen to my return signal on 2 meters with the addition
of another receiver (see last month's Ham To Ham column), and this necessitated yet another
innovation as described below.
To listen to my two radios at the same time via headphones, I bought a Sony stereo headset at a
discount store for just $6 and completely reworked their wiring. I removed the original cable and
replaced it with two separate lengths of light-weight 2-conductor speaker wire from Radio Shack
(reg. trade mark). Each of the new 2-conductor wires then terminates in a connector (also from
Radio Shack) appropriate for the headphone jacks of the two radios that I'm using as downlink
receivers...so that I've ended up with 2 meters in my right ear, and 10 meters in my left ear! It's
interesting. At times, one frequency may be more audible than the other (due to propagation,
background noise, antenna pattern differences, etc.). At other times, reception in both ears is
quite good and I have excellent "stereo" perception using this arrangement. It was a little spooky
at first to hear my uplink (sidetone monitor) in my left ear and my satellite downlink CW in my
right ear, but now it seems perfectly normal and I'd miss it if I were to go back to the old way!
This is an inexpensive way to give this method a try."
Moderators note: If you'd rather not completely re-wire the headset as Tom did, you could
achieve similar results by "splitting" off the two phones of the stereo headset by using a standard 3-conductor stereo jack and coming from it with two lengths of 2-conductor speaker wire...to each of the receiver's headphone outputs. This works only if both receivers share a common ground point and if one side of their headphone outputs is not "above" ground (which is usually the case). See Figure 1 for a basic drawing depicting this idea.
The old hobby knife trick!
Another hot tip from Phil Salas AD5X: "Here's a quick tip to remember the next time that you
need to cut out any type of large hole in a plastic project box (such as those boxes sold by Radio
Shack (reg. trade mark) and others). I use a heated hobby knife (like the X-acto (reg. trade
mark) knife), heated with my normal bench soldering iron. Here's the way to do it...tin the upper
part of the knife blade (the part that's closest to the handle) with some solder, then hold your
soldering iron on this tinned spot as you carefully cut the plastic. Always carefully pre-draw the
figure that you want to cut and scribing it with a sharp metal scribe will also help to keep you on-
track for the final removal cuts. It's actually pretty easy to do after you've practiced with it once
or twice on a piece of scrap plastic. You can cut round meter holes, or rectangular LCD display
cut-out's using the heated knife method, once you've developed a 'feel' for what it's like."
Moderator's note: It's also best to use the type of hobby knife handle that's insulated with a
cushioned rubber covering, to prevent burning your fingers from those calories conducted up the
handle from the heated blade. If your X-acto (reg. trade mark) handle is of the bare metal variety
however, you can slip some large diameter cable insulation over the handle instead...the outer
insulation from RG-8/U should be able to be pressed into this type of service.
Improved linear amp input circuitry
In the May 1997 Ham To Ham column, Rich Measures, AG6K, of Somis, CA suggested a method
of more closely matching your transceiver's output drive level to your amplifier's maximum linear
input drive level (the maximum RF drive level that should be fed into your linear if you expect to
keep it linear). Rich explained how to determine the value of an RF input "pad" and then how to install this "pad" in series between the amplifier's input coupling capacitor and the amplifier tube's cathode. The following is a quote from Rich's dialog from that column:
"If you look at the specs for today's popular amplifier-tube crop, you'll find that the bulk of the
new ceramic triodes requires anywhere from 22 watts to 75 watts input RF for full output,
anything beyond that results in saturation drive (distorted audio). Yet how many popular
transceivers are rated in the 22 to 75 watt range? "
Here Rich continues with another note on the subject and some additional worthwhile information
on improving the SWR for your linear amplifier's input tuned circuit: "When I first installed a
series/parallel resistance combination in-line with the RF drive feed in a commercially available
amateur HF amplifier using a single 3-500Z tube in a grounded-grid configuration - see the May
1997 Ham To Ham column - I experienced a better SWR match on some bands and a slightly
degraded SWR on others. This variation was added in the direction that the original input
matching circuits were off to begin with. In other word's, if the input matching circuit's impedance
was too low to start with, the additional 20 ohms of resistance in the amplifier tube's cathode
actually improved the SWR match, but if the original input impedance was too high, the SWR
degraded. The answer of course is to achieve a better match on the amplifier's input via the amp's input tuning network, and this brings up another interesting point that a lot of hams tend to miss: the actual impedance transformation of a pi-network tuned-circuit can not be optimally altered without changing at least two of the three pi-network components, yet most commercial amplifiers allow you to change only one. Figure 2 shows how the input matching circuits of most commercially made amplifiers are configured. A slug-tuned coil adjustment (if that's what your amplifier's tuned input circuit uses as the variable element) will rarely deliver the lowest possible SWR when tuned just by itself! Although it will indeed move the resonant frequency of the network, which will change the indicated SWR, the final SWR may not be the lowest level possible because only one element in the pi-network has been varied. Anyone who has experimented much with their linear's tuned input circuit has probably made this discovery for themselves.
What's the answer? One of the fixed capacitors in the pi-network should also be varied...and
that's not as tough as it sounds. It's usually very easy to temporarily parallel one of the tuned
circuit capacitors with a compression-mica trimmer variable capacitor. You can then alternately
adjust the slug-tuned coil and the trimmer until the input SWR has been minimized (it should now
end up being close to 1:1 at some point in any selected band). If, by chance, there's already too
much capacitance in the network, then of course one of the fixed capacitors will have to be
disconnected temporarily from the pi-network and a trimmer of approximately the correct value
used in place of it. Once an optimum value has indeed been reached, the variable trimmer
can be carefully removed, measured and a new fixed capacitor (or parallel combination of fixed
capacitors) installed in its place.
Now as a final touch, reduce the value of the pi-network's variable inductance, just slightly, by
backing out the slug a little bit from its optimal point. This technique will raise the "Q" of the circuit somewhat; I've found that most solid-state transceivers prefer a somewhat higher "Q" for best performance. Eimac (the well-known amplifier tube manufacturer) recommends using a "Q" of 2 for grounded-grid amplifier tuned circuits, though in fact, many amplifier manufacturers design for a "Q" of 1. This too results in less than optimum SWR from the tuned input circuit.
The above procedure will have to be gone through for each of the tuned input circuits in the amplifier, usually 5 or 6 bands. Take your time when you take on this project, and make notes at each step along the way to answer any questions that may come up at a later time."
Moderator's note: Rich brings up a very valid point, you wouldn't expect to be able to a achieve a
nearly perfect match on the output of your linear if only one of the pi-network components was variable, so why would we expect any more from the input matching circuit? Making at least two of the components in the network variable makes good sense. A variety of small variable trimmer capacitors, in the general range of 2pF to 180pF, can be obtained from some of 73's advertisers, including: Mouser Electronics (Tel: 1-800-346-6873, see pg. 196 of cat. #588), All Electronics (Tel: 1-800-826-5432), Fair Radio Sales (Tel: 1-419-227-6573) and others.
Murphy's Corollary: Whenever a specialized defective part has been positively located, it will be the only one that is not mentioned anywhere on the manufacturer's "Complete Parts List".
By the way, if you need data on a specialized part, try looking on the Internet under the part
manufacturer's web page. Many parts manufacturers now have data sheets available for
downloading via the Internet. This makes it much quicker, in most cases, to obtain needed
information than writing or calling the manufacturer directly. Use one of the many search
engines (such as Yahoo, Web Crawler, Alta Vista (all copywritten names)) or others to find what
your looking for if you don't have an exact address. And while you're browsing the Internet, be
sure to check out the Ham To Ham column's home page on the world wide web at:
http://www.rrsta.com/hth.
As always, many thanks to this month's contributors, Including:
Frank Brumbaugh W4LJD
P.O. Box 30
c/o Defendini
Salinas, PR 00751
Thomas M. Hart AD1B
54 Hermaine Avenue
Dedham, MA 02026
Phil Salas AD5X
1517 Creekside Drive
Richardson, TX 75081
Richard L. Measures, AG6K
6455 La Cumbre Road
Somis, CA 93066
Note: The ideas and suggestions contributed to this column by its readers have not necessarily
been tested by the column's moderator nor by the staff of 73 Magazine, and thus no guarantee of
operational success is implied. Always use your own best judgment before modifying any
electronic item from the original equipment manufacturer's specifications. No responsibility is
implied by the moderator or 73 Magazine for any equipment damage or malfunction resulting
from information supplied in this column.
Please send all correspondence relating to this column to 73 Magazine's Ham To Ham column,
c/o Dave Miller, NZ9E, 7462 Lawler Avenue, Niles, IL 60714-3108, USA. All contributions used
in this column will be reimbursed by a contributor's fee of $10, which
includes its exclusive use by 73 Magazine. We will attempt to respond to
all legitimate contributor's ideas in a timely manner, but be sure to send
all specific questions on any particular tip to the originator of the idea,
not to this column's moderator nor to 73 Magazine.