Smith-Kettlewell Soldering Series Part 5

Table of Contents

Soldering I
Soldering II
Soldering III: Tinning Stranded Wire
 Soldering IV: Popular RF Connectors
Soldering V: RCA and Motorola Plugs
Soldering VI: Resistance Soldering
Soldering VII
JA3TBW Solder Guide
Soldering Kinks
Vinther Fingertip Soldering Iron

SOLDERING Part V: RCA and Motorola Plugs

By the late 1930's, there were consistent needs in consumer products for low-cost "shielded" (coaxial) connectors for use at audio and low radio frequencies. Two such connectors were developed to address these needs--the "RCA phono plug" (now the standard for interconnecting hi-fi components), and the "Motorola plug" (now the standard for automotive receiving antennas). (Note that in traditional terminology, a distinction is drawn between" "shielded" and "coaxial" cables and plugs in discussions of audio and r.f. signal lines, respectively. Since single-conductor shielded cables such as those used on these plugs are coaxial, the distinction will not be made in this article.)

These two connectors are very similar in concept. Both have a tubular center-conductor pin into which the "hot lead" is soldered. Both have a four-segment shield shell, to which the "cold connection" is made, which snugly fits a matching shell on the socket. The shell of the RCA plug is a bell-shaped affair which surrounds the shell of its socket; the shell of the Motorola plug is a sleeve-like structure surrounding the cable which fits into the shell of its matching socket. Having four segments and being made of springy metal (usually a hard brass plated with tin), these plugs can easily be adjusted to tightly fit their respective sockets, thus eliminating the need for close-tolerance machining in their fabrication ("stamping" and "extruding" are the only fabrication processes necessary).

Noting the principles of soldering as stated in Part I. of this series (summarized at the beginning of Part II., Winter, 1981), rare is the soldering job which requires as many wholesale violations of the rules as must be done in installing these connectors. The procedures for installing the plugs along with the rules to be broken are listed below:

Efficient heat transfer only occurs when there is a continuity of alloys between the iron and the work, i.e., the iron should be in a solution of solder along with all the work pieces. This cannot happen with the iron on the outside of the pin and the soldering going on inside the pin. Furthermore, in order for all pieces of the work to heat up together, they must be in firm physical contact. Since the wire is usually much smaller than the inner diameter of the pin, firm contact is unlikely.

As with the center conductor pin, the iron is used to heat up the outside of the plug while soldering takes place inside the shell. A solution of solder cannot be established between the iron and the work, since spillage of solder onto the outside of the shell would prevent its fitting into the socket.

Where possible, all components of the work piece should be in contact with the iron so as to heat up simultaneously. Where this is not possible, the iron should at least be in contact with the item of largest heat capacity, the largest piece of metal. Unfortunately, in the case of the RCA plug, the iron will be resting on the thin braid wires and not on the metal shell of the plug. By the time the shell is brought up to soldering temperature, the braid wires will have been heated for a considerable length of time, and they often conduct so much of this heat to the cable that the center insulator becomes severely damaged.

The points discussed in this section describe how we can cheat the laws of physics with "tricks of the trade".

Soldering Tubular Pins

Because these pins fit into spring contacts in their socket, it is imperative that no spillage be allowed to change their size and shape, which would damage any socket receiving them. One thing in our favor is that these pins are often plated on the outside with a metal less soluble in solder than the base metal (they are often plated with cadmium). Therefore, solder is less likely to stick to the outside than it is to the inside. When going into solution, solder loves to "wick" along, around, and into highly-soluble materials via "capillary action". Therefore, we can expect solder to guide itself in the direction of soluble surfaces; it will gladly flow into and around the inside surface of the pin as soon as the necessary soldering temperature has been reached, whereas it will just tend to "bead" on the outside of the pin. (If solder is melted directly under the iron, it will alloy with some of the plating.)

The diameter of the lead is very often much smaller than the inside diameter of the pin; being very loose in the pin, it may at times not be in direct contact with the inside of the pin. As the pin fills with solder, it becomes a sort of "secondary soldering iron"; it is intended that heat be conducted from the pin to the lead via the molten solder. However, two roadblocks can foil this plan. First, you may not be able to spill enough solder into the pin to make a good conductive puddle, or by the time you do so, some of this solder may have melted through the cable insulation to cause a short behind the plug. Second, the flux may be used up before the materials inside the pin have been deoxidized to any significant depth. By generously pre-tinning the lead and by inserting a piece of thin solder into the pin along with the lead, these problems are minimized.

With the above principles in mind, the following three methods can be used to solder a wire into a tubular pin:

  1. A tinned lead is prepared whose length is just short of reaching the end of the pin. If the lead diameter permits, slip a piece of thin solder into the pin alongside this lead and cut it off at the end of the pin. Clamp the connector in a vice so that the pin points straight upward. Using fairly thick solder (perhaps .05 in. diameter), vertically orient and position a straight piece of this solder directly on top of the hole in the pin, positioning your hand so that you can feed about 5/8 in. of it straight down into the pin. Being sure your iron is wiped free of residual solder (you don't want it to deposit solder onto the side of the pin), find the side of the pin with the iron and hold it there until the solder melts. Heat transfer will be best if your iron has a screwdriver or chisel tip; turn the iron until you feel its flat side rests squarely against the side of the pin (getting your feedback through the handle of the iron). Reciting perhaps 17 stanzas of your favorite verse, wait for the solder to melt--then feed it straight down into the pin. If anything goes wrong--if you cut the solder in half with the iron while looking for the pin, or if you lose the pin while feeding the solder--let the project cool, clear away any droplets from around the work and try again.
  2. Prepare a long section of well-tinned lead whose length is sufficient to protrude about 3/4 in. beyond the end of the pin. This time, it is essential that a piece of thin solder be inserted into the pin alongside the lead, primarily to provide fresh flux for cleaning the pin's inner surface. You needn't cut the solder off as it emerges from the end of the pin; you can use this same solder in making the eventual connection. Clamp the connector in a vice with the pin straight upward. Hold the solder off to one side in preparation for feeding it to the protruding wire, not to the pin with the iron, find the point at which the wire emerges from the pin; rest the iron so that it contacts both the wire and the end of the pin. The solder in your hand will quickly be disconnected from the pin, at which point you should fish for the wire above the iron. Feed perhaps an inch of solder to the wire, taking care to avoid the pin.

    (The theory is that most of the solder will wick down the highly solder-soluble wire into the pin. For this scheme to work, plenty of fresh flux must be available to make the pin's inner surface soluble, otherwise the solder will wick everywhere else. Of course, the wire must be small enough in diameter to leave room for solder to run past it on its way into the pin.)
  3. The connector pin can be pre-tinned--actually filled with solder--and the tinned conductor can then be run through the molten solder as the pin is reheated. Clamp the connector in a vice with the pin horizontal, or perhaps pointing slightly downward. Feed a straight piece of thick solder (0.05 in.) into the pin from the rear of the connector until it reaches the end of the pin; hold on to it behind the connector in preparation for feeding about an inch of it forward into the pin. Find the end of the pin with the iron and press the solder forward until it is melted by the iron, and proceed to fill the pin with solder. After withdrawing your piece of solder from the rear of the plug, slide the iron sideways off the pin and let the project cool.

Prepare a suitable length of well-tinned lead--its length is not critical. Fish for the rear entrance of the pin with this lead and prepare to drive it up to the hilt in the connector as you reheat the pin. Once again, find the end of the pin with the iron and wait for the solder to melt, which will be indicated by squeakiness and by the fact that the lead can be moved forward. Slide the lead in as far as it will go remove the iron and wait for the solder to solidify without jarring the cable. If the lead is longer than the pin, you will feel it run into the iron--in which case, follow it out with the iron, letting the iron be pushed away from the end of the pin.

This last method may be more difficult with Motorola plugs, because the shell is so long that the pin is difficult to find with the center lead. With the continuity tester connected between the hot lead at the other end of the cable and the connector pin, you can verify having hit the solidly-filled pin with the lead. The pin rarely stays completely filled when being tinned, since some of the solder is bound to run out the forward end. Therefore, you may feel a depression at the rear of the pin with the lead, and you may even achieve partial insertion before the pin is reheated.

Beads and icicles of stray solder can be removed with a clean iron, a knife, or a file. If the pin's plating did not take the solder, spillage can easily be picked off with a fingernail. The important thing is that the pin be round and of an appropriate diameter for its matching socket.

Spillage of solder around the base of the pin, the application of too much solder, or over-heating of the project can all cause short circuiting of the plug and/or cable. After each and every step, it is wise to check for shorts between the pin and the shell, and between the pin and the cable shield.

Installing Motorola Plugs

In principle, braid is folded back over the outer sheath and tinned before inserting the cable into the connector; this braid is to be soldered to the four staves of the shell at the rear of the connector. Critical dimensions are listed below:

  1. The tubular pin is about 1.1/16 in. in length; the center lead should be prepared accordingly.
  2. In order for the shell to reach the full insertion of 5/8 in. into the socket, its four springy staves should be free to expand against the sides of the socket for this distance, making it necessary to solder the braid behind this point. Allowing 1/8 in. for the insulator to which the tubular pin is mounted, there should be a spacing of 1/2 in. between the entrance of the pin and the point at which the braid is soldered. In other words, the center insulator should extend at least one half inch beyond the folded-back braid.
  3. This connector's shell is best suited for a braid whose outside diameter is about 5/16 in. RG59/U with its braid folded back over the outer sheath is about this size, while braid folded back on RG58/U is too small. Where necessary, masking tape or other non-contaminating material can be wrapped around the outer sheath of smaller cable so that the braid can be formed around something of appropriate size. 52

(Actually, it is a good idea to wrap a layer of this tape around the outer sheath of any cable, even RG59/U, to keep the intense heat of tinning and soldering from melting the plastic, thus contaminating the braid and making it unsolderable.)

Preparing and Tinning Braid

Of course in preparing the cable, a decision will have to be made as to which method of soldering the tubular pin you intend to use, since the appropriate length of exposed center conductor varies among the three alternative methods discussed earlier. Behind the exposed center lead, the following procedures should be carried out:

  1. Strip off about 5/8 in. of outer sheath--after the braid has been folded back, this will give you just over one half inch of center insulator to act as a spacer inside the plug.
  2. Before folding the braid back over the outer sheath, wrap paper or masking tape around the cable at the end of the sheath to protect it from the intense heat of tinning and soldering. If the cable diameter is smaller than about 1/4 in., wrap enough tape around it to bring it up to size.
  1. Fold the braid back over the sheath and smooth it out in preparation for tinning. You will probably have to unbraid about half of it to accomplish this; do so carefully with a Braille stylus or other instrument which is thin but not sharp. Sometimes, braidwires get rather sparse as they are expanded over the outside of the cable; in this case, twist the cable between your fingers and wrap them around the sheath near its forward end.
  2. Wrap a couple of turns of thin solder around the braid nearest the forward end, just over the end of the sheath. Gently wipe the iron over the circumference of the braid's forward end; tinning will not take long, since the expanded braid has very low heat capacity (you may wish to wrap the center insulator in a bit of masking tape to prevent it from being damaged by the iron when you miss the braid.) Let the project cool and survey the damage. An occasional lump of solder will not hurt anything, since these connectors do not need a "precision fit" between the braid and the shell. Any loose braidwires behind the tinning job can simply be folded up near the front of the braid and forgotten about.

Pre-Tinning the Motorola Plug

Tinning the inside of the shell is an essential part of the installation of the plug. There is no other efficient way of getting solder and fresh flux inside the shell and on to the shield connection.

Tinning the inside of the shell of a Motorola plug is good fun--it is one soldering job during which you cannot get lost. Clamp the tubular pin horizontally in a vice and turn the plug so that the stave to be tinned is at the bottom. Slip a piece of thin solder through a crack near the top of the shell in preparation for feeding about an inch of it down on to the bottom stave. Insert the tip of the iron into the shell and rest it on the bottom stave near the open end. Feed solder to this stave just ahead of the tip of the iron, then slowly withdraw the iron from the shell. Turn the plug so that this can be done to all four segments of the shell.

Soldering the Shield of the Motorola Plug

Slip the tinned plug over the prepared cable and solder the tubular pin; this will help keep the cable in position. (You can assure firm contact between the segments of the shell and the braid by bending the staves inward so as to tightly fit the cable. They can be bowed out again if the plug fits loosely in its socket.) Now clamp the tubular pin horizontally in a vice and reheat the staves one at a time. As the solder melts under the stave, you may feel it give a little under the iron--but more importantly, you will feel a rise in the temperature of the cable behind the plug. After doing this to all four segments of the shell, let the project cool. (You need not do all four segments at once; you can let the plug cool after each phase if you wish to.)

The connector should not rotate freely with respect to the cable; even with the center conductor soldered, there will be considerable freedom of motion if the braid has not adhered to the inside of the shell. Furthermore, you should not be able to lift individual staves away from the cable.

Reheating parts of the plug can be done to correct problems unearthed during testing. If reheating fails to attach segments of the shell to the cable, there is a secret weapon we have to assure eventual soldering of the shell. Lay a piece of thick solder in a crack between two staves and place the iron on top of it just over the braid beneath. This will get some action; you can have fun filing the plug back down to size and polishing it off with emery cloth after this drastic measure has been taken.

Installing RCA Phono Plugs

In principle, the braid is flared out so that the plug can be slipped onto the center conductor and soldered, then the braid is closed over the rear of the plug (about the top third of the "bell") and soldered there. Critical dimensions are listed. below:

  1. The length of the tubular pin varies considerably--from as short as 1/2 in. to a length of one inch. Older units came in two sizes, long and short. A nominal 5/8 in. will serve as a practical estimate.
  2. There should be at least 3/16 in. of insulation between the flared-out braid and the exposed center conductor. The presence of this insulation is absolutely necessary, since this portion of the center lead passes through a small opening in the rear of the bell-shaped shell behind the tubular pin.

Tinning the RCA Plug

A fair sized puddle should be created at the back of the shell; when the braid is pressed against the connector with the iron, it will sink into this puddle and soak up the molten solder like a gauze. I tin the plug with its tubular pin held horizontally in a vice. I touch the shell with the iron and the solder in two or three places around the circumference of the shell (do this toward the rear and try to avoid the segmented section). After the unit has cooled, I check for bare metal with a fingernail--those portions wet with solder will be very smooth and gummy with flux. I usually find a spot which needs redoing, whereupon I turn the plug to make this point accessible and go after it again. Lumps of solder are not very important at this stage, but if they offend you, wipe them off with the iron after you have stripped its tip free of excess solder on your cleaning sponge.

Soldering the Shell of the RCA Plug--Solder the tubular pin to the center lead, making sure that the plug has been pressed back firmly against the flared-out braid. (If the cable is large and stiff, you can hold it vertically in the vice while soldering the pin. If the cable is small or very flexible, hold the connector in the vice while making sure the cable is being pushed up snugly underneath.)

Turn the connector over and clamp its tubular pin in the vice with the segments of the shell resting on top of the vice jaws. Arrange some way of holding the cable straight up from the connector. Having the cable held vertically is very important, since letting it droop sideways will bring the center insulator of the cable firmly up against the rear entrance of the shell; as soon as the shell is heated, it will cut through the insulator and short out the cable. Finally, firmly press and smooth the braid down over the shell. You may even wish to tie the braid in place with a piece of thin wire--this wire can be smoothed down with a file afterwards.

With the iron in one hand and the solder in the other, solder the braid in a few places around the shell. You will not need to apply much solder, since what you are mostly doing is providing the braid wires with fresh flux. You will only need to do this in four or five places around the shell, and you can take as long between solderings as you wish. You may even wish to turn the connector in the vice in order to conveniently reach all sides. If your vice has metal jaws, you can use them as a landmark by which you can first find the connector shell and then hop up to the upper rim of its bell-shaped structure. In doing so, be careful not to jostle the braid wires out of position.

After the project has cooled, check to see that adhesion has been accomplished on all sides. Any individual loose braid wires can be "tacked" down with the iron, most likely without applying additional solder.

With wire cutters, trim ragged edges of braid off the plug and file the plug until it is round and smooth. Do not trim or file away any material at the back of the shell, just around the sides.

Alternative Methods for Attaching the Basic Connectors

Now that you know the "right way" to attach these plugs, perhaps you'd be interested in how most of us do it. Although not as elegant, the braid is often attached by gathering it into a "pigtail" and soldering it to the side of the shell--soldering is only done in one spot. Furthermore, since they are gathered together, individual braid wires are less likely to sever when the cable is flexed. This trick can be used on Motorola connectors by bringing the "pigtail" out the rear of the plug and lapping it over the end of one of the staves. (It should be noted here that this is how such connectors are attached to unshielded 2-wire cables, such as speaker lines and other non-coaxial applications.) A detailed example using an RCA plug is given below:

Generously tin the "pigtail" and one spot on the rim of the plug's shell. Bend the "pigtail" so that it curves around the upper rim of the shell, covering about 1/4 of the circumference. Grab the end of the "pigtail" in locking forceps and hold it in position while bringing the iron into contact with both items. Watch for the squeakiness of solder-wet metals and for a rise in temperature of the cable. Remove the iron and don't move. If it didn't stick, wrap thin solder (0.03 in.) around the "pigtail" where it is to contact the plug and try again.

There are two accepted ways of getting a "pigtail" of braid:

  1. For cables whose center conductor does not bend easily, the braid can be unwoven--simply combed out with a non-pointed instrument such as a Braille stylus. Once the braid wires are separated, they can be gathered together and twisted into a bundle.
  2. With careful surgery, the center conductor can be extracted from the braid at the end of the outer sheath, thus giving you a braided pigtail. Remove a generous length of the cable's outer sheath. Surrounding the braid with three fingers, massage it backward until there is a little "balloon" of it right against the end of the outer sheath. Sharply double the cable over at this point (at the balloon of braid). With a Braille stylus, gradually work a hole in the braid at the outside of this bend. Gently shift all the braid wires over to the inside of the bend, exposing the doubled-over center conductor. Being careful to miss all the braid wires, slip your stylus through the loop of center conductor and ease it out of its knitted sleeve of braid. Afterward, look for whiskers indicating severed braid wires at the base of the center conductor; get hold of them and simply pull these wires out and discard them. Finally, massage and pull the braid out into a nice thin pigtail.

Alternative Varieties of These Connectors

A wide variety of these plugs are available, especially in the case of RCA phono units. Your Editor's favorite variations will be listed here. In addition, although perhaps not honestly classifiable as "alternative designs", certain critical variations in the shape of basic RCA plugs will be discussed.

Motorola Plug

H.H. Smith makes a unit (12WJ) which includes soldering tabs at the rear end of two of the staves. Although I am not absolutely sure of their intended use, it appears that they are to be soldered to the braid without its being folded back over the cable's outer sheath. In any case, the tabs can be adjusted to accommodate any size of cable, or they can be treated as solder lugs to which a pigtail is attached. These tabs can be cut off and the plug treated as described earlier.

Basic RCA Plug Differences

Although I have used plugs without this feature, very often the bell-shaped shell of the plugs you buy will have a "double cup", that is, behind the large-diameter segmented section of the shell is a slightly smaller-diameter section (also having straight sides), and the braid can be tied into position around this portion of the shell with a piece of wire. (Actually, this small-diameter section is used to hold the phenolic insulator of the tubular pin.) As far as ease of soldering is concerned, I would not reject plugs which are not so shaped.

There are mean/nasty self-destructive plugs having sharp edges at the rear entrance of the shell which will damage the cable after it has been attached. A notable example is the Switchcraft 3501M, whose shell includes a sharp-edged, flared-out portion which they use for mounting this plug on various adaptors and cable clamps. The sharp edges at the back of this plug will quickly sever the braid wires as the cable is flexed. It is important that you render any such connector toothless before approaching it with the cable. This can easily be done with a file.

RCA Plugs with Cable Clamps

The basic disadvantage of all the above units is that stresses on the cable are borne by its fragile wires and not by its protective outer sheath. Plugs are available with wrap-around cable clamps which grip the outside of the cable and keep it from severing the wires as it is flexed.

The Switchcraft 3501MC is a basic plug with an attached crimp-on clamp. This clamp has two stages, one for the outer sheath and one for the exposed braid (although you will have less trouble if you use them both for the outer sheath and solder a pigtail of shield to the shell of the plug). Because it is not completely shielded, this unit is poorly suited for RF.

The Switchcraft 3502 is a very fancy unit with a screw-on cover. Its design has two major disadvantages. Its ground-connection device has no hole through which a shield pigtail can be inserted for soldering; the pigtail must be "tacked" on to the back of the cable clamp support. The other is that its tubular pin extends a full 3/16 in. into the connector housing, making short circuits between this pin and the cable shield all too likely. A fine hack saw can be used to cut this pin without affecting its structural integrity; this will separate the connection points and will give you room to fully grip the outer sheath in its clamp.

HINTS AND KINKS

This column is near and dear to your editor's heart, and I would like to see it become a regular feature. Its next appearance will occur after I have received three or four new items. To those of you whose ideas did not appear this time, I apologize--they have either gotten lost in my developing filing system or I did not understand the ingenuity of your suggestion. Please resubmit anything you feel should be included. Do not short-change your cleverness by sending me abbreviated explanations; be specific as to the applications of your inventions and/or techniques.

Single-Cell Marking System for Quantifying Braille Dials

With dozens of braille dials around the lab, it becomes evident that a system for numbering major markings would be handy. Standard braille numbers become easily confuseable as they are rotated in space, especially without the presence of a number sign.

Roy E. Lockwood, W8CIL, of Charlevoix, MI, points out that use of the full cell reduces the confusion caused by rotating the characters, and he suggests using the following one-cell symbols to represent the numbers 1 through 10: "L" "th" "w" "ble" "er" "y" "p" "r" "v" "q" "and".

Numbers in the European Computer Code are written without the number sign: the numerals 1 through 0 are generated by adding dot 3 to the basic dot combinations a through j as follows: "ch" "gh" "sh" "th" "wh" "ed" "er" "ou" "ow" "w".

Since they're your dials, go ahead and "roll your own" system. Thank you, Roy Lockwood.

Negotiating Live Circuits on Printed Boards

John Lizza of Carle Place, New York, recommends wrapping your hand in a plastic bag when feeling your way around live printed circuit boards. Occasionally, your effect on a circuit must be minimized while fishing around with test probes, and in some cases, a plastic bag could reduce the shock hazard.

Components are often shipped in antistatic bags which might be more appropriate for using this trick with CMOS circuits. Thank you, John Lizza.

Tips on Soldering

Mike Bhagwandas, JA3TBW, of Kobe, Japan, recommends the following good ideas:

For soldering in hard-to-reach places among nests of wiring and among components of close proximity, a piece of thin fiberglass tubing can be used as a guide. One end of the tubing is rested on the connection of interest, with the solder being fed through the tube. The tip of the iron can then be slid down the tubing to the connection without fear of cutting the solder in half on the way.

(Editor's Note--I have been unable to find a source for small quantities of tubing; manufacturers apparently make such tubing to order rather than stock it. I am negotiating the purchase of 100 feet to be divided up and passed out in a future issue.)

I tried this scheme using 1/8 inch glass tubing which I filched from a chemistry lab in our associated hospital. The solder will gall against sharp edges of the glass unless the ends of the tubing are "fired" in the flame of a Bunsen burner or smoothed down by judicious use of emery cloth.

Holding the tube vertically, I support it between my ring and little fingers while feeding the solder with my thumb and first finger. Initial melting of the solder is facilitated by cutting the bottom end of the tube off at an angle, thus permitting the tip of the iron to touch the solder.

I was amazed at how much control in feeding the solder that this system affords and by how well this idea works. Given small diameter tubing, this may be just the thing for soldering components with short leads into printed boards.

In regard to attaching component leads to the pins of IC sockets, why not pre-tin the materials and then solder them together simply by reheating them once they are in contact with each other? In this way, you don't have to worry about feeding the solder, and you will be less likely to spill solder onto an adjacent terminal of the socket.

Since the socket pins are extremely small as compared to the leads being attached to them there should be enough solder on the lead alone to make the connection (this trick will surely work if you apply a small amount of flux to the socket pin).

Once the lead has been laid over against the pin, heat the connection. The solder will melt almost immediately; this will be indicated by the lead settling further over to the pin, by rapid heat transfer along the lead to its component, and by the squeakiness of solder-wet metals.

(Editor's Note-- "activated" flux is a good thing to have around. It can also be used in preparing the inside of tubular pins; a pipe cleaner which has been dipped into the flux is inserted into the connector pin. Kester activated flux can be gotten by the pint from

Marshall Industries (Kester No. 1544),
788 Palomar Ave.,
Sunnyvale, CA
94086
Phone: (408) 732-1100.

The price of the flux is about $5.00, but be advised that Marshall Industries requires a $15.00 minimum. Thank you, Mike Bhagwandas.