Every once in a while, most of us come across a task that requires that we learn a new skill or buy a new piece of equipment or both. Such was the case when I decided that I had to learn how to install SMA connectors in order to modify a new speaker mic that I purchased at the Orlando Hamcation.

           My first attempts were very poor. I got too much solder on the center pin which made it impossible to insert said pin into the housing. Eventually I got the job done properly but it was very obvious that a different approach was needed. Several years ago I wrote a evaluation of several battery powered soldering tools (QST June 2007) One of the tools tested was a battery powered resistance soldering tool called "Cold Solder", I think. I could be wrong. It has been over 10 years and I am getting on. In any case that prompted an Internet search on the subject of resistance soldering. The resulting hits were astonishing. Proper commercially available resistance soldering tools cost anywhere from $400 to $1200. WOW! The saving grace was that there were a number of articles on building a DIY version for MUCH less cost and achieving the same result. The two articles that I referred to most were both from the model railroad hobby where the construction of very detailed model steam locomotives requires much soldering and also requires that almost all of the solder be out of sight. No blobs or swoopy fillets here. Some of the builders have thus embraced resistance soldering. The two authors that I referred to most were Paul James from Blenheim, Ontario, Canada and Vance Bass from Albuquerque, New Mexico. Both are from the model railroad community. I combined Paul's power unit design with the tweezers that Vance designed. 

            A bit of background may be useful here. The concept of resistance soldering is fairly simple. One places two electrodes in contact with the work piece you wish to apply solder to. Current flows between the two electrodes and because the resistance of the work piece is higher than elsewhere in the circuit the area of the work piece between the electrodes heats up and the solder flows. Depending on the size of the work piece this can happen quite quickly so most units use a foot pedal or switch to turn the electric current on and off. This leaves both hands free to manage the application of the electrodes. One advantage is that nothing outside the space between the electrodes heats up at all. Sensitive components stay cool and fine wires do not suddenly dissolve. 

             As usual, my first order of business was to download both articles from the Internet and then begin to search my copious parts containers for materials that could be used. I did have to do a bit of shopping after the search. Purchased were a transformer similar in size and output voltage to the one Paul James used. I got a very good deal from my local electronics shop as the transformer was used and the proprietor  was not aware he even had it so I got it for less than $6. A computer style power cord was found at the same shop for about $2.  An enclosure was fashioned from a 4" x 4" x 4" NEMA box from Home Depot. The bamboo tweezers were obtained from Bed Bath and Beyond for less than $2.  Almost everything else came from my overly stocked parts stock. The only   item I had to order through the Internet was a pair of brass  fittings that  are called 90335-KNGI ACE Connector, 2/BG, 8-18awg - They were obtained from Galco Industrial Electric

           Once I had most of the parts in hand, construction commenced. I had already done some testing to determine what voltage the transformer put out of the secondary taps and based on my research decided that the two taps that resulted in approximately 8 volts would be used. I had also determined which two of the three primary wire inputs would be connected to the mains power. I was very careful when testing these aspects because the 110 volt power was out in the open and not insulated from a careless hand. I got through that part of testing without difficulty but extreme caution is advised. 

           I planned out the holes that needed to be drilled for power in, power out, the dimmer switch and the LED power indicator. Once they were drilled, assembly began. First the transformer was placed at the bottom of the NEMA box where it was a neat fit. I thought about adding some wood shims to wedge it in place but I found that the weight of the transformer was enough to keep it in place. The box is unlikely to move off my workbench very often, but "your mileage may vary" as they say.

              Next the dimmer switch was wired in between the transformer primaries and the mains power cord. The ground lead from the dimmer was connected to the ground line of the power cord as well. About three to four feet of 12 gauge zip cord was installed through a pre-drilled hole in the front of the unit and secured to the secondary wires from the transformer by first crimping with bare metal but splices and then flooding the connections with solder to keep resistance to a minimum. Remember the resistance at the work must be the highest resistance in the circuit or it is some other connection that will get red hot and fail. 

                A simple bridge rectifier and resistor circuit was built to supply a red LED with appropriate current and voltage and it was installed across the hot and neutral from the power cord. It is intended and indeed the control method has been built to control the mains power going to the unit via a pedal operated switch which will be on the floor. Thus, when the foot switch is pressed, the LED will light up. When the foot switch is not depressed there is no mains power anywhere in the system. Safety First.

                  At the other end of the 12 gauge zip cord, a bamboo toaster tongs or tweezers were attached to the wires as is shown in the photo. Installed on the ends of the wires were the ACE connectors. The other end of each connector holds the soldering tips which I made from 3/32" copper clad steel welding rod. It was relatively simple to chuck a 2" piece of the welding rod into my drill press and then use a metal file to create a tapered tip on the rod. After that was done, I bent each tip in my vise to an appropriate angle for use. I secured the 12 gauge wires to each leg of the bamboo tongs using a combination of heat shrink tubing at the pivot end and wire ties on each leg. The ACE connectors are insulated with heat shrink as well and I also added a cover of ABS plastic to each leg as well. These were also secured with heat shrink tubing. The voltage across the tips may only be less than 9 volts but the current available is quite high, certainly enough to give the victim a nasty burn if not worse. Again, Safety First.

                 Using the tool is actually quite easy. Using the assembly of an SMA connector as an example, I tin the center conductor of the RG-316 coax using a standard soldering iron of about 40 watts. I then insert the center conductor into the pin of the connector and touch the pin with the resistance soldering tool. This melts the solder and connects the pin to the center conductor. It also leaves no solder on the outside of the pin meaning it can be readily inserted into the SMA housing. The crimp tube is then brought up over the back end of the housing to capture the shield between the outside of the housing and the crimp tube. A standard crimp is then applied to the crimp tube locking the shield to the housing. Job done. It is advisable to add a short length of heat shrink over the crimped tube for aesthetic purposes. 

                     If one is going to use this tool for regular soldering of components, a certain amount of practice is advised. It is necessary to have both of the tips in contact with the work and to apply solder only to the area between the tips. It is also vital to not apply power until both tips are in contact with the work. Otherwise a fairly large spark or arc can emerge and your work will have a major divot in it.The heavier the material, the longer it will take to heat up. Having said that, I not that Paul James noted that his design was able to heat up fairly heavy gauge materials easily. I have not attempted anything heavy yet. 

This is the top of the power module. The dimmer switch allows adjustment from 0 to about 8.4 volts AC. The dimmer switch also had an on/off function which is not used at present since the unit already has an ON/OFF switch. The dimmer had to be replaced since the first one was somewhat elderly and went belly up very quickly. The new one required some re-positioning of the voltage levels but appears to be working fine so far.

The foot switch that I used comes from MPJA.COM here in Florida and cost less than $6. It is rated at 10 amps. With rewiring it also makes an excellent foot PTT as well.

This is the front face of the power module. You can see the LED power indicator and the exit point of the 12 gauge zip cord. Ordinarily I would use a strain relief grommet here and at the back where the mains power cord exits, but the wall thickness of the NEMA box makes that impossible so I used wire ties on both sides of the wall to restrain the cords from shifting. I added an ON/OFF switch which cuts off all power to the rig for safety's sake. The hex wrench on top of the box is for replacing the metal tips of the hand piece when required and is now stored on the hand piece holder pictured below.

The hand piece is now complete. I installed the wires shown in the picture of the bamboo tweezers into the brass fittings and then placed the supplied heat-shrink tubing around them leaving the front set screw area uncovered so I can replace the copper clad steel tips when needed. I also placed two strips of 1/8" ABS plastic on top of each side of the tongs to minimize the possibility of electric shock or burns. . A brief test of the rig shows that it may need to be turned down from 8.4 volts to around 5. It certainly heats up the test work very quickly to soldering temperatures. At the 8.4 volt setting it is almost too fast, creating a control factor. 5 volts may be a better setting, testing will continue. The copper clad steel electrodes that I made from 3/32" welding rod are holding up well but I am going to continue testing with smoother conical electrodes to see if there is an improvement in control and positioning. Other styles of electrode will also be tested.

      Since I completed construction of my resistance soldering system. I spent part of the day testing it out. At the setting of 4.8 volts (approximately) I was relatively quickly able to built two RG-316 cables, one with SMA male connectors on both ends and one with an SMA male connector on one end and an SMA female connector on the other. I had no problems with either and both were tested and found free of short circuits. I plan to place SO-239 to SMA female adapters on one of the male SMA connectors on each cable thus creating two "rig savers", one for HT's that have an SMA female connector on the radio like Yaesu and some Wouxons and the other for the Baofang style where the connector on the radio itself is a Male SMA. I have already ordered the SO-239 adapters and they should be here by mid-week. The new electrodes and the resistance soldering rig were also put to the test by soldering two pieces of 3/32" copper clad steel welding rod (actually the same stuff that the electrodes are made of). The job took an extra few seconds to heat the welding rod up but the solder flowed nicely and made a secure solder joint length wise between the two pieces. Tomorrow, I am going to look for some brass material at my local hardware store to test solder larger and heavier materials. I will also see if raising the voltage setting will speed up the soldering of the heavier materials. I must admit that I am quite pleased with how this project has worked out so far. Better than I expected and very usable for the tasks I have tested it on so far.

Here is a photo of the bamboo tweesers with the zip cord wires temporarily attached. 

Here is the transformer as it sits in the bottom of the NEMA box. The mains power cord exits at the top of the picture and the 12 gauge zip cord exits at the bottom (not clearly visible in this picture. The coiled up orange, red and white wires are not used at all. Out of the picture are the dimmer switch and the bridge rectifier circuit for the LED. The transformer was marked as having a 120 or 130 volt primary, I used  the 120 V leads. The secondary had a max of about 17 volts AC, using the center tap I got 8.4 volts which was suitable for my purpose. Not shown here is the ON/OFF switch that was added later.

Here is a very rough and ready diagram of the circuit. The LED power indicator .is fed through a bridge rectifier via a 50K resistor and lights very nicely. The Dimmer switch controls the output voltage between 0 and 8.4 volts AC The transformer takes 110 Volts AC and puts out 8.4 volts AC into the tweezer handle. THe whole unit is fed via a foot pedal on/off switch that is operated by the user so as to keep both hands free.

N1GY- The simple Approach to Ham Radio

and My Model Railroad Hobby


    Here is the underside of the dimmer switch mounted on the lid of the box. The black wires connect from the hot wire from the power cord and to one of the leads from the primary side of the transformer. The green wire is secured to the green ground wire of the power cord.

     The dimmer shown unfortunately died before I even got to use the rig. I replaced it with a similar on that did not need the defeating of the push on push off switch as it had a much simpler on/off switch at the end of its rotation. Naturally the indicated voltage positions had to be altered as well. You will note that the corners of the mounting plate were clipped at a 45 degree angle to allow the dimmer to fit snugly into the lid.


This picture shows the connectors that will join the wires to the operating tips on the tweezers. The tips (not shown) are made from 3/32" copper clad steel welding rod with the ends filed to a conical tip. They are visible in the picture below. I have also changed the electrodes that I use for the SMA pins to ones that have flat surfaces facing each other so that the pin can be held by the hand piece for soldering.

This holder, rough and ready as it is, was made from a powdered drink mix container and mounted on a small slab of plywood. Holes were drilled in the plywood not quite through to hold the hex wrench and the spare electrodes. One set of electrodes has conical tips on the operating end and the other set has flat angled ends to perhaps give a better contact area with the work.

A Home-brewed Resistance Soldering Rig