Ham To Ham #11 - August 1996

73's Ham To Ham column

c/o Dave Miller, NZ9E

7462 Lawler Avenue

Niles, IL 60714-3108

In Ham To Ham, back in the June 1996 issue of 73, I mentioned a practical use for those small laser pointer pens - using one to help locate a soldering pad on the foil side of a circuit board by shining its beam over the part to be unsoldered on the component side. Asking for other interesting uses, I received this from Harry, SM0VPO in Upplands, Sweden: "Those little laser pointers are about the easiest way that I know of to catch a fly! That's right, just shine it on the little bugger's head and it can see nothing else but the laser; now you simply pick it up! If you've ever wanted to catch a live fly - without damaging it - for study under a microscope or for macro-photography, this is the best method I've found...I even have the pictures to prove it!"

Now there's a novel use...from the land where zippers were invented too!

Well grounded!

Here's an easy "quick modification" to keep in mind the next time you have your ham transceiver - or other accessory piece of gear - apart on your workbench. Check to make sure that the area on the "inside" of the metal cabinet covers is capable of making good ground contact when the cabinet covers are fastened back into place. It may seem obvious at first, but I've noticed that most ham manufacturers are guilty of "over-spraying" the inside of the cabinets during the cabinet spray-painting process. This over-spray can vary anywhere from very light to quite heavy...depending upon luck I suppose. At any rate, sanding, scraping or otherwise cleaning the areas on the cabinet's inside - at least around the screw-holes - can sometimes increase the RF shielding properties of the enclosure dramatically.

Take a look at Figure 1 and see what I mean. It's a drawing of the inside of the enclosure cover for the Alinco DT-1200T 2 meter ham transceiver...simply as an example. The areas shown as diagonal lines should be scraped clean - down to shiny metal - of any paint over-spray - in order to assure good RF ground contact when the outside screws are tightened-up. I like using a small, hand-held "motor-tool" (such as the one made by Dremel), with a conically-shaped burr-style grinding bit in it for the job...it makes it fast and accurate.

Remember, what might be an reasonably adequate ground at DC and audio frequencies, may not be effective enough at RF frequencies...especially at the VHF's and beyond...and it doesn't take all that much to make sure that it is.

DE Dave, NZ9E

An unwelcomed battery load!

Erich Kern of Murrieta, California wrote to say:

"It was interesting to me to read your piece on conductive adhesives in the November 1995 Ham To Ham column. I too have had occasion to encounter the same effect, but with masking tape instead.

About 20 years ago, I learned quickly that some masking tapes can become semi-conductive after a time, creating unexpected leakage paths. In my case, I was using the masking tape to cover the exposed terminals of spare 9 volt batteries, only to find that the tape eventually became partially conductive - measuring about 20K ohms with the probes spaced 1/2" apart across the tape. Needless to say, the "protective tape" was also discharging my spare 9 volt batteries! It's difficult to say whether the tape adhesive was the culprit (after it dried out to some degree) or if the paper-based masking tape merely picked up moisture from the atmosphere and then became a semiconductor. Whatever the reason, others should be aware that the problem can and does happen under the right (?) conditions."

Moderator's note: In the May 1996 HTH column, I made the suggestion of using Johnson & Johnson 1/2" wide waterproof First Aid tape on conductive pathways, instead of other tapes that might become hydroscopic or otherwise semiconductive with time. I've been very happy with this product in my own equipment so far. Are there any other tapes that readers have used with long-term success, ones that don't become goopy after a while? I also ran into the conductive adhesive pitfall again...this time in a computer color monitor that I purchased at a local hamfest. The manufacturer had used an adhesive on some of the components that he didn't want to move during shipping, etc., but it began to become semiconductive when it hardened with time. It became noticeable in the horizontal deflection section of the monitor, causing the width of the screen's raster to change from time to time...very unpredictably. Once I removed the problem adhesive (which had become very hard, brittle and probably hydroscopic), the problem was corrected...Murphy strikes again! 100% silicone bathtub sealant seems to be best for holding down heavy parts on PC boards...it will last virtually forever and doesn't pick up moisture; just take care to let it cure for the time recommended by the manufacturer before applying power to the circuit and don't use it directly on the copper foil itself because of the acidic chemicals that it contains.

Ridin' the rails

I ran across a product that model railroaders often use, but which may also have practical application within ham radio...it's called "Rail-Zip" (registered) and is made by Pacer Technology, 9420 Santa Anita Avenue, Rancho Cucamonga, CA 91730-6117 (1-800-538-3091). It's a multi-purpose liquid product that cleans, lubricates, conditions and protects metal surfaces from mineral scale and rust.

I've used it on a small Z-gauge layout that I have, and it seems to work. Model railroad track has the tendency to become a poor electrical conductor due to the "micro-sparking" (my term) that goes on between the engine's pick-up wheels and the track everytime it's run. Rail-Zip is supposed to penetrate the corrosion that's the cause behind this tendency, and restore proper conductivity. It's also claimed to help prevent new corrosion, which will minimize future sparking for 8 to 12 weeks. Sound like it might be useful for connectors and switch contacts in our ham gear?

I wrote to Pacer, out of curiosity, and this is a para-phrased summary of their reply:

"Rail-Zip penetrates to produce a clean metal surface, leaving an organic barrier that prevents further scale from forming. It's formulated for use on all metalic surfaces, pH stabilized, water soluable and safe around plastics. An oxygen scavenger works as a corrosion inhibitor to prevent cavitation, erosion and pitting on metalic surfaces. It's completely organic, biodegradable, environmentally safe and non-toxic to handle. Though not tested on connectors and switches used in amateur radio equipment, we feel that it will restore the electrical conductivity of them, and provide a corrosion barrier as well."

It might be worth keeping in mind, especially for outdoor antenna connections where corrosion can often be a big factor in intermittents; Rail-Zip is available at hobby stores and via mail-order from catalogs that cater to the model rail hobby.

DE Dave, NZ9E

The right packet path

From William Thim, N1QVQ, of Broad Brook, Connecticut:

"Having been involved in VHF packet for a couple of years now, I've become conscious of a lot of hams who might be newer to that mode, questioning on 2-meter packet and voice, what's available in the way of node hopping, BBS's, HF gateways, etc. in our area. I usually jump in and try to help - with tales of my own experiences - but if it's on a repeater, more often than not the station is mobile, and unable to write down the information that I might have to pass on.

In view of that limitation, I've taken to placing special information messages on my home PBBS, addressed to ALL, but only within my home district. A short description of the subject also helps, such as NODE HOPPING or perhaps HF GATEWAYS. I know that a significant number have benefited from these messages, and I've benefited myself with additional information coming back to me from others who know of paths that I wasn't aware of. It pays to share!"

Freebe sealant!

From Bob Boehm, N8EXF:

"Here's a readily available source of sealing compound for those outdoor coaxial cable connectors or weather-exposed antenna relay control boxes...and it's free for the asking. It's the windshield edge sealant used by the aftermarket automobile windshield replacement guys that you see in people's driveways, replacing damaged windshields right on location. I was able to acquire a nearly unlimited supply of this sealant simply by asking one of the repairmen for any 'tail-end' rolls that he had laying around...rolls that usually just end up in the dumpster.

Since locating this 'free' source, I've been using it exclusively for the past half-dozen years on all of my outdoor coax connections. While it isn't 'flat' in its acquired form, it's easily re-worked with just the fingers, to make an effective sealant for even the tightest of nooks and crannies. If you're worried about any adverse chemical reactions between the sealant and the metal on the connectors, simply wrap the connector first with one layer of electrical tape before applying the auto window sealant...it also makes for a 'cleaner' removal should you ever want to open the sealed connection for some reason. You can't beat the price, and it gives you a chance to tell a prospective newcomer (the windshield repairman) about the wonders of the world of ham radio!"

Broaden your scope!

From Michael Fratus, of Richmond, Texas:

"Even an inexpensive oscilloscope can be made to act more like one of the very expensive ones with a little ingenuity! Many of the 'big dollar' scopes offer on-screen voltage and frequency measurement as part of their selling points, but you can have nearly the same features for a lot less money.

By using a couple of inexpensive BNC 'T' connectors on the scope's input, you can easily hook

up both your DVM and your external frequency counter, so that you'll have a voltage and

frequency reading - along with the scope presentation - wherever you place your scope probe

during troubleshooting. It's a good deal easier than trying to read DC voltages by the amount of bounce on the scope trace, and frequency by converting microseconds into megahertz! And it surely beats paying $8000 for a later-model scope!"

Parts hunting made easy

From Peter Albright, AA2AD, of Lakewood, New York;

"One of the greatest challenges in any servicing endeavour (ham radio equipment included) is in identifying parts and finding serviceable replacements for defective ones.

The manufacturer's technical service manual (of course) should be consulted first, if available, but what if you don't have access to one? My favorite semiconductor cross-reference is the EGC Master Replacement Guide, which is just chocked full of semiconductor data...in almost "painful" detail! It's less than $10 if purchased new, but it can sometimes be acquired free from service shops (when a new one comes out) or from some parts dealers, if you're a 'high enough' dollar customer. ECG is a division of North American Philips Corporation, and you can contact them at: Philips ECG, 1025 Westminster Drive, Williamsport, PA 17701. Incidentally, the ECG parts numbering system has also been adopted by several other 'generic' parts suppliers.

The SK semiconductor parts series (from the former RCA Corporation) is also often used or quoted in replacement data as something of a standard...very much like ECG. The SK Replacement Cross-Reference is now available from: Thompson Consumer Electronics, 2000 Clements Bridge Road, Deptford, NJ 09096-2088.

Radio Shack also publishes a semiconductor replacement guide. It's available for sale or in-store use at most Radio Shack retail outlets. It usually only details parts actually being currently sold by RS stores, but it also contains a reasonably complete general cross-reference section.

Because of their wide-spread-presence, I'll often frequent my area Radio Shack stores first, when looking for replacement parts. Their parts are generally name-brands, simply re-packaged in single or double-lot bubble packs, but you generally can't beat the convenience and speed of obtaining a replacement in a number of cases. If they don't have what I need, however, I'll turn to the mail-order houses. One of my personal favorites is MCM Electronics, 650 Congress Park Drive, Centerville, OH 45459-4072 (Tel: 1-800-543-4330). They offer a good selection of parts, power supplies, tools, test equipment, computer accessories, etc. in their free catalog.

Other suppliers that I've used include: American Design Components, 400 County Avenue, Secaucus, NJ 07094 (Tel:1-800-776-3700)...Black Box Corporation, P.O. Box 12800, Pittsburg, PA 152421 (Tel: 1-412-746-5500)...Fox International, 23600 Aurora Road, Bedford Heights, OH 44146 (Tel: 1-800-321-6993)...Jameco Electronics, 1355 Shoreway Road, Belmont, CA 94002-4100 (Tel: 1-800-831-4242)...Jensen Tools, 7815 S. 46th. Street, Phoenix, AZ 85044-5399 (Tel: 1-800-426-1194). Tucker Electronics, 1717 Reserve Street, Garland, TX 75042 (Tel: 1-800-527-4642 Nationwide, or 1-800-749-4642 within Texas) carries a good line of test equipment, ham gear and computers and accessories.

The above listing is by no means complete, but I've personally found these suppliers to be reliable and often having the correct parts that I've needed in my repair work."

Moderators note: In addition to the suppliers Pete mentioned, Digi-Key Corporation, 701 Brooks Avenue South, P.O. Box 677, Thief River Falls, MN 56701-0677 (Tel: 1-800-344-4539) has a thick (free) catalog of parts and semiconductor components for the asking. I've had very good experiences over the years ordering from them and I love the name of that town!

A pin-up beauty!

From Klaus Spies, WB9YBM of Niles, Illinois:

"Often, when designing a new home-brew item, or if repairing a factory made piece of equipment, I've done some head scratching trying to remember the standard resistor values available. But

here's a chart that makes it much easier, so I'd like to share it with my fellow builders. It applies to both resistor and capacitor 'standards'."

Standard R/C Value Chart - use multipliers as required

1.0 1.1 1.2 1.3 1.5 1.6 1.8 2.0 2.2 2.4 2.7

3.0 3.3 3.6 3.9 4.3 4.7 5.1 5.6 6.2 6.8 7.5

8.2 9.1 10.0

"All of these values are then followed by the appropriate multiplier, for example: 4.7 47 470 4.7K 47K 470K 4.7Meg.

When 10% resistors were the norm, every other value was skipped, beginning with 1.1, then 1.3, 1.6, 2.0, 2.4, etc.

Today, with 5% resistors as the norm, the values shown in the chart are commonly available from the larger suppliers.

For those who might be curious as to how these values were arrived at, this seems to be the mathmetical explanation:

(1.0 + 5%) = 1.05, and (1.1 - 5%) = 1.045. This implies that if one value of resistor is at the high end of its tolerance, it will overlap with the next higher standard value. This allows a person to pick almost any value required for a particular project, even if the tolerances are demanding, presuming that you have enough stock in the various values to choose from.

By the way, experience has shown me that today, a new, unstressed resistor is seldom at the extreme of its tolerance range. In fact, modern metal-film resistors can be almost dead-on many times value-wise."

Moderators note: I've pinned up Klaus' chart over my workbench, how about you?

Etcetera

If you're looking for a Heathkit part at some point in the future, you can try calling Heath's Parts order desk at 616-925-5899. Heath has sold some of its parts stock to Spectrum Electronics, so if Heath itself doesn't have it, then try calling Spectrum at 616-742-0613. This isn't an ad for either vendor, it's simply intended as potentially needed information for fellow hams.

DE Dave, NZ9E

Tape tips

From Steve Thacker, KB5VCA of Vilonia, Arkansas:

"In my professional career as a superintendent for a mechanical and electrical contractor, I've come upon a number of types of tape for use in outdoor electrical situations, some of which might be of help to my fellow hams.

Everyone has probably heard of, and perhaps used, Scotch No. 33 electrical tape - it has particularly superior stretching ability in cold weather, won't harm copper or aluminum and can be sealed even tighter with Scotch 1602 spray sealer. 3M also makes a nice 'splicing

tape' ; it's a two-sided tape - one side is soft (like a good rubber tape) and the other is much harder and more weather-resistant. It 'cures' over a short period of time into an effective outdoor splice, the harder outside layer taking the brunt of the elements. Other manufacturers may make similar products, the 3M numbers are simply to give readers a point of reference from which they can start. Most electrical supply houses, and now some of the larger home centers, carry these products for shopping ease."

Cheap power zener replacements

From Richard Measures, AG6K, of Somis, California:

"Most HF linear amplifiers have a relatively high-power zener diode in the cathode circuit of their output tube (or tubes), for biasing them into the linear portion of their operating curve, and for providing a stable, fixed cathode bias. This zener diode is prone to zapping out, particularly if a high-power VHF parasitic oscillation should occur, because of the large grid-current pulse that always accompanies a VHF parasitic arc-over. It's a costly and sometimes difficult-to-find component to replace. A better solution, lies in the use of ordinary silicon rectifier diodes, here's why:

1.) Zener diodes are not adjustable.

2.) Zeners are relatively sensitive to high-current pulses.

3.) And, as mentioned, higher power zeners are expensive and harder-to-find.

Fortunately, the substitution of ordinary silicon diodes, addresses all of these objections.

Since ordinary power silicon diodes have a stable forward voltage drop of about eight-tenths of a volt DC (0.8 volts) across each diode, a series string of these relatively inexpensive diodes can be cascaded...to make up a stable cathode-bias regulator, for virtually any voltage required. Additionally,this 0.8 volt step-range gives you some desirable leeway in customizing a replacement circuit for your own particular linear amp. Now by simply adding or subtracting diodes from the string, you can adjust the zero-signal anode-current for your particular amp and tube combination parameters...guided by what the tube manufacturer recommends as the ideal idling current for that particular tube-type. That's something you can't do using a fixed-voltage power-zener.

Here's a quick example:

Seven silicon diodes will provide a stable cathode bias of 5.6 volts (7 times .8 volts), 8 diodes would give 6.4 volts, 9 would yield 7.2 volts and 10 silicon diodes would provide an even 8 volts.

You can make up a small perf board layout for the diodes, say ten diodes in all, then with just a wire jumper, bridge out the ones that aren't needed, in order to arrive at the target anode-current 'idle' figure that you're seeking. If a new tube (or set of tubes) later on changes this figure significantly, you can re-adjust the idle current by simply moving the jumper.

And don't forget, we're utilizing the 'forward voltage drop' of the silicon diodes - as opposed to the 'reverse voltage drop' in the case of the zener we replaced - so that the banded cathode end of this new stack will go to the point where the anode end went before, that is, just exactly opposite of the way the zener was wired into the circuit. You can use virtually any PIV rating for the diodes, since the voltage at the cathode is very low. The current rating of each diode in the string, however, should safely exceed the maximum expected cathode current, up to 3 amps maximum.

Here's a final advantage, if you should happen to be the unfortunate victim of another high-energy parasitic pulse, it's a relatively simple matter to replace the diode or diodes that may have suffered the brunt of it...and again, with a whole lot less cash outlay!"

We end this month's column after another of Rich's worthwhile ideas...watch for many more in the months to come. If your idea isn't in here...why not? Everyone has run into interesting problems and found their own unique solutions to those problems. If you'd like to share yours with other readers, send them to me at the address in the masthead and I'll let you know promptly whether it will be used or not, and roughly when. The column is prepared a few months before the time that you read it, because of magazine layout and printing considerations, so don't expect to see it next month, but chances are good that it will be used in a near-future issue. So click on the word processor program icon in your computer, and write a few lines about your favorite tip, technique or shortcut...don't forget!

73, Dave, NZ9E

Our sincere thanks to this month's contributors:

Erich F. Kern

24774 Shoshonee Drive

Murrieta, CA 92562

William Thim, Jr., N1QVQ

50 Miller Road

Broad Brook, CT 06016-9676

Bob Boehm, N8EXF

6821 Le Conte Avenue

Cincinnati, OH 45230-2935

Michael Fratus

Dept. of Criminal Justice

Inmate #650394

Jester - 3

Richmond, TX 77469

Peter Albright, AA2AD

28 E. Summit Street

Lakewood, NY 14750

Klaus Spies, WB9YBM

8502 N. Oketo Avenue

Niles, IL 60714-2006

Steve Thacker, KB5VCA

268 South Church

Vilonia, AR 72173

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.