HAM TO HAM - #4 January 1996

Conducted by:

Dave Miller, NZ9E

7462 Lawler Avenue

Niles, IL 60714-3108

Since we began this column three issues ago, a number of worthwhile ideas have been received and they continue to arrive daily. Many thanks to all who've contributed their time and effort to make this a success and I'm hopeful that the response seen so far will grow even more enthusiastic in the future...remember, this should be your column, with your ideas and suggestions. Here's a basic rundown of what I'm looking for in the way of reader input.

Everyone has different experiences in amateur radio, with different equipment and varying setups; each of us has encountered any number of instances where we've made improvements, both major and minor, to what the equipment manufacturers "think" that we want. Some of the modifications are primarily operational in their nature, others make the equipment more technically sound. I'd like to hear about your innovations, with the possibility of passing them on to all of the others reading this column. That's what Ham To Ham means, one ham's helpful hits, ideas or suggestions to another who might be in a similar situation or have similar needs. Hams have always enjoyed helping one another, that's part of the fun of the hobby, and this coulmn is a way of extending that helpfulness to many, many others all at once.

Not everyone wants to write a complete, long, drawn-out article on an innovative idea that they may have come up with, so here's your chance to jot down the basic details, in casual form, and I'll put the finishing touches on it, if need be, for the column. Don't worry about your writing style, just try to be as complete as you possibly can, but even if you're not sure, send it to me and I'll let you know if I feel that more information is needed. I'm primarily interested in down-to-earth, practical ideas that others can duplicate with relative ease. Here's an example of a simple idea, something that's easily duplicatable by most hams, yet many may have been reluctant to apply it to their own situation.

Pilot Lamps and Today's Radios

Have you noticed how difficult it's become to change a burned-out pilot lamp in most of today's modern transceivers? In the "good old days" - when radios and pilot lamps were both a lot bigger - changing a dead lamp was a pretty straight-forward, easily accomplished job. Lamps were usually mounted in sockets, and normally just a twist of the wrist popped the lamp right out, ready to install a new one. Not so anymore! Most lamps are now on tiny wires, soldered in-place and often buried deep within the radio's front panel wiring. It's become such a problem that many hams don't bother changing the lamps when they do burn out, or they wait until something else much bigger in the set needs attention, changing the pilot lamp then as something of an aside to the larger problem.

What follows may not make the task of lamp relacement any easier, but it might just double or triple the time between pilot lamp failures, and it's not usually too difficult to accomplish.

Here's the first tip: when a pilot lamp does burn out, most hams probably think in terms of going to the radio's manufacturer for a "direct replacement", but often that's the most expensive and time-consuming route to take. Here's an alternate approach that's ordinarily much, much easier. Radio Shack stores stock a number of small, low voltage lamps, most of which will either fit directly, or can be easily adapted to fit into most modern transceivers with a little "creative thinking". Their #272-1141 lamp, for instance, is a 12 volt, 25 milliamp mini-lamp that I've been able to use in a number of cases requiring a small replacement lamp with insulated wire leads. You can simply cut the wire leads from the old lamp, carefully splicing the news lamp's leads in their place and insulating the connection with either heat-shrinkable tubing (which I prefer) or neatly applied electrical tape. There are times when you'll have to be very careful in transferring the little colored plastic bootie from the old lamp to the new one, but I've always been able to somehow do it in the end.

Radio Shack also stocks what they call a 12 volt micro-lamp, their #272-1092, which may well be just the ticket as a replacement for the tiny bulb often found beside or in back of LCD displays these days. Hobby stores can also be a source of what are sometimes called "grain-of-wheat" lamps, which are very similar, but be sure that the voltage is correct in terms of what your transceiver supplies as pilot lamp voltage - usually 12 volts DC. By the way, using a replacement lamp rated at a higher voltage is fine, just as long as it will provide enough brightness once it's in place. In fact, a higher voltage lamp will last quite a bit longer than one that's rated right at the nominal supply voltage. That leads us into the rest of the story, how you extend the life of those tough-to-get-at little pilot lamps?

If you lower the supply voltage to any incandescent lamp, you'll increase the lamp's life dramatically. Tests have shown that the life expectancy of an incandescent lamp zooms upward as the voltage across it goes down, and of course, vice versa. Lowering the voltage across a 12 volt lamp by just a couple of volts, for instance, will extend the lamp's expected life by two or three times! Just a series resistor in the lamp's supply lead will do it for you, but there are some other considerations. You'll have to determine how much light loss is acceptable in your particular operating configuration, because there will be loss of brightness if the voltage to the lamp is reduced. Once you determined how much illumination you can afford to lose, then you'll need to know the value of the resistor needed to drop the voltage just enough to accomplish that objective. Finally, the power to be dissipated by the resistor will determine what wattage resistor is needed.

Lowering the voltage across an incandescent lamp will definitely reduce its light output and it will also shift its color somewhat toward the red region. The more the voltage is lowered, the more red-shift occurs. I have a couple of flood lamps illuminating the front of my house each night that are wired in series, ie., the lamps run at 1/2 voltage across each lamp. They're very much on the "warm side" - red shifted - but have been going for years and years each night without burning out, becuase voltage-rating-wise, they're loafing along!

Getting back to our pilot lamp discussion, you can easily determine what resistor to use in series with each lamp by using Ohm's Law:

Voltage drop desired divided by the lamp's rated current equals the correct resistor needed

and

Voltage drop desired times the lamp's rated current equal the resistor's wattage

By way of example, let's take the Radio Shack #272-1141 lamps mentioned earlier, which have a current rating of 25 milliamps - or .025 amp - and assume that we would like to drop the 12 volt lamp supply by 2.5 volts - down to 9.5 volts. Plugging in these values we get:

2.5 volts divided by .025 amp equals 100 ohms

and

2.5 volts times .025 amp equals .06 watt

Now we know that we'll need a 100 ohm resistor in series with each lamp, and that a 1/2 watt dissipation rating on each resistor will give us a good margin of safety.

Another way to lower the voltage to all of the lamps in the set at one time, would be to use a 3-terminal adjustable voltage regulator supplying something less than 12 volts to the lamp supply bus, but in most cases, that's something of an overkill. You would also have to be sure that nothing else is tapped off of the lamp supply bus that might be adversely effected by a slight voltage reduction. The individual resistor in series with each lamp is usually the safest approach unless you know the transceiver's circuitry well.

Remember, the more you can lower a lamp's voltage from its nominal rating, and still have enough light output from it, the longer that new pilot lamp will last once its in service.

Reducing the effects of scratches in plastic meter faces, etc.

Sometimes accidents happen, and a plastic meter face, dial cover or even a wristwatch face-plate can become marred with an ugly scratch or dig, often making it difficult to even read through the scratch-mark.

Next time that happens to you, try using a small amount of metal polish, such as Brasso R, to polish out the imperfection...it really works. Most of these products are composed of a very fine abrasive, that if used properly, will eliminate or minimize many scratches - even relatively deep ones - restoring the damaged piece to usable condition again.

Try folding a paper napkin, and with the tip of your finger backing it up, apply a small amount of polish to the damaged surface. Take your time, applying only the amount of pressure needed to remove enough of the surface plastic to polish-out the scratches in question. You might want to try this first on a couple of scrap plastic pieces first, just to get a "feel" for the proceedure. Rub over the scratch several times and then wipe the dried polish completely off. If the surface has "fogged" a bit, repeat the proceedure, but this time using progressively lighter pressure.

For very light scratches, even ordinary typing paper may be just abrasive enough to polish the surface sufficiently. Try using the paper just as is - without any additional external abrasive applied to it.

Most reasonably sized marks - and even some pretty big ones - can be removed or greatly reduced in this manner. It even works on scratched CD-ROM discs, but again, better to practice on a "throw-away" one first.

Submitted by:

Herb Foster, AD4UA

3020 Pennsylvania Street

Melbourne, FL 32904-9063

Moderators note: most cleansers and polishes will exhibit the abrasive effect that Herb refers to above, so you might want to try a number of them - keeping notes on which work best for you - on several samples of scrap plastic. You might also want to try a mild-abrasive automobile rubbing compound or an optical lense grinding compound as well. Jewelers rouge, a very fine abrasive in an oil based cake form, is also very useful in the same manner - you might check with a jewelers supply house or your own neighborhood jeweler to see about obtaining a small container for the next time you need it. Jewelers rouge is also excellent for cleaning tarnished connector contacts and intermittent battery connector springs, as well as your XYL's gold rings and bracelets! Brasso, mentioned above, is a registered trademark of Reckitt & Coleman of Wayne, NJ.

Simplicity plus flexibility in a two-stage QRP transmitter

All the way from Israel, Lavee, 4X1UF contributed this interesting little two-stage, 5-watt, 20-meter CW QRP transmitter circuit for experimental purposes.

In the circuit diagram of Figure 1, the first 2N3375 transistor is wired as a Pierce Oscillator, and is capacitively coupled to a second 2N3375 that functions as a buffer as well as an impedance converter. Lavee writes, "by drawing as much current as needed by the stage", the second 2N3375 "acts as an automatic antenna tuner. The lower the impedance, the higher the current" drawn from this stage. The need for an external antenna tuner is therefore minimized, except perhaps as tuned circuit for harmonic reduction, since no tuned circuits are employed within the two-stage transmitter itself. These days, some sort of harmonic reduction would be well advised.

Lavee built his unit into a very small copper box, bolting the 2N3375's directly to the box and bringing the RF out via an RCA type "phono" jack. Direct point-to-point wiring is used, with all grounds soldered right to the copper box itself with the shortest possible leads. With 24 volts keyed DC applied, he was able to realize about 5-watts of RF output on 20-meters. He further cautions that the box can get very warm after a time, so either additional heat-sinking might be required, or key-down time kept to an absolute minimum - lower Vcc input voltage would also reduce heat, along with the output power. A small 12VDC fan might be a worthwhile addition if the heat dissipation is still too great. Lavee commented that after final testing, he covered all of the internal components with a heat-dissipating epoxy for additional component mechanical stability as well as better heat transfer.

From 4X-land, an interesting circuit with lots of potential for QRP enthusiasts. Questions and comments should be directed to:

Lavee Israel, 4X1UF

c/o International Electronics Services

12/21 Got Levin St.

Haifa 32922

Israel

That's our Ham To Ham column for this month, please keep your ideas, tips, suggestions and short-cuts coming and we'll use as many of them as possible in each of the coming months. Please send them to the address in the masthead, not to 73 Magazine directly.

73, DE Dave, NZ9E

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.