These instructions were a clinic handout at the 1992 NMRA National Convention given by Don Thomas. Obviously with electrical projects, please exercise appropriate caution. If you are unsure about something, ask a qualified individual to help you! We can assume no liability for your use of these instructions.

HOW TO BUILD AN ECONOMICAL RESISTANCE SOLDERING UNIT

Don Thomas, Train Specialty Co, 11540 Auburn Rd,, Chardon OH 44024-9310

INTRODUCTION

The April 1992 Model Railroader article titled "Introduction To Resistance Soldering" by Rick Hesel does an excellent Job of describing what resistance soldering is and how It can be used.  The PBL video "The Art of Soldering As Applied To Model Railroading" covers the subject even further.  This clinic will go a step further and show you how to build an economical resistance soldering unit In lieu of buying a comercial unit.  The savings gained can be significant along with the pride of building your own..

BACKGROUND

Back in 1978 a friend of mine asked me to repair a basket case 0 scale brass articulated he had picked up for a "song".  I had considerable experience with soldering, so I said I would give it a try.  I had also Just read the November 1978 Model Railroader article on "Resistance Soldering" by Tom Dressler.  I was impressed with the idea and proceeded to gather the necessary parts to build my own unit.  I was able to build a complete unit for less than $40.00. It can be done today (1992) for about $60.00.

Considering the cost of commer  cial units in the $200+ range, this can put a unit in the hands of modelers.  The articulated restoration was a complete success and I have continued to use resistance soldering techniques ever since.

COMPONENTS REQUIRED

POWER SUPPLY:  FILAMENT TRANSFORMER rated at LESS than 7 volts and 6 or more amps.  I have been using transformers rated at 6.3 volts at 6.8 amps with complete success.  The primary (110 volt) wire should be 16/18 gage stranded.  The secondary (6.3 volt) should be 12/14 gage stranded wire.

CABINET to fit transformer.  Radio Shack has a variety of aluminum utility cabinets which work fine.  Their number 270-253 usually will hold unit transformers of the size needed.

BINDING POSTS or JACKS

Used  to mount the secondary wire from the transformer to the cabinet for a convenient and quick way to attach HAND PIECE and GROUND wires.  Walthers Binding Posts #942-403 work perfectly.  However, they have been recently discontinued and may no longer be available.  R/S #274-662 is a good substitute.

Assemble as follows:

1. Mount transformer and jacks/binding posts in cabinet by drilling appropriate holes as needed.  A handle can be added to the cabinet if desired.
2. Drill hole in back of cabinet for primary wire to enter. Install a small rubber grommet so wire does not rub thru the metal.
3. Slip primary wire thru hole and solder to primary side of transformer. Use wire and plug from the soldering gun used in making the hand piece.
4. Solder wires from the secondary side of the transformer to the two binding posts or Jacks.  Polarity has no bearing here since the circuit is AC.
5. Install cabinet cover.

FOOT SWITCH

* Pedal type with cord set is ideal.  When you push dam, it turns on the power supply.  When you release it, the power supply is turned off.  Do NOT use a switch that requires a push/release to turn it on and another push/release to turn it off.  ConnTrol International, Inc.  Single Switch w/cordset catalog number 864-1440-00 is ideal.  Radio Shack #44-610 Recorder Foot Switch will work, but must be hard wired into the transformer primary. This can add a significant savings to the unit.

HAND PIECE

Radio Shack #64-2067 30 Watt Soldering Iron is the ideal base for the hand piece.  Remove the soldering tip by unscrewing it.  The tip is not used. Unscrew the handle from the metal barrel and the set screw from the end of the barrel.  Save the 3 screws from the handle and the set screw.  Pull apart and cut off the 110 volt wires.  The wire cord with plug can be used on the primary side of the transformer.  Taking the end the original tip was screwed Into, remove the entire heat element by sawing It off.  A razor saw works well here.  Discard the element.  You should now have a handle, the chromed metal barrel, four screws and the end of the barrel which once contained the tip.

Additional items needed:

• Carbon rod 1/8" in diameter by about 4" long.  The rod is called Arch Gouging Carbon and is available at most welding supply or hardware stores. It comes in 12" lengths and is easily cut into 4" pieces using a fine razor saw.  Sharpen the end(s) using a common pencil sharpener.  
• Stranded 14 gage wire about 4' long.  The more flexible, the better.  
• Plug or appropriate connector to attach to power supply.

Assemble as follows:

1.  Strip one end of the 4' wire about 1/2" and solder it to the barrel Just inside where it screws to the handle.  Use a heavy soldering gun or torch to assure a good solder Joint here.  
2. Slip unattached end of wire thru handle and re-install barrel to handle using the original 3 screws.  
3. Attach/solder appropriate connector to match power supply binding post/jack.
4. Using a 1/8" or #30 drill bit, enlarge the hole In the end which contained the original tip.  The resulting hole should Just allow the carbon rod to slide thru.
5. Reinstall the end to the top of the barrel using the original set screw.  Slip a carbon rod in and tighten the screw to the point where the carbon rod will not slid up or down.  I have found that the original set screw is often too short and that a 4-40X 1/4" screw is needed as a replacement.

GROUND WIRE

Use a 14 gage stranded wire about 4' long.  As with the hand piece, the more flexible the better.

Assemble as follows:

1. Attach/solder appropriate connector to watch power supply binding post/jack.
2. Attach/solder a small alligator clip to the other wire end.  The clip will be used to ground the item being soldered.

COMMENTS

1. Transformers of heavier amperage can be used, but are not required.I have found transformers with multiple secondary outputs and wired them with SPDT or rotary switches to take advantage of the additional heat ranges produced.  Variable transformers rated at 1 to 1 1/2 amps wired into the primary side of the transformer will provide infinite heat ranges.
2. I have found FAR more use for the hand piece over resistance soldering tweezers.  I have a quality comercial set, but rarely find use for it.  I suggest using the hand before considering tweezers.
3. The hand piece built from the above instructions has the advantage of getting into much tighter areas such as the boiler of an HO locomotive.

Commercial hand pieces have a large base to which the carbon rod attaches making their use more limited.  The hand piece WILL work perfectly with any commercial unit.