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There is much controversy among various modellers about the uses of a Resistance Soldering Unit (RSU) and many entrenched views of what it can, and cannot be used for. I firmly believe that if the methods you currently use work for you then you should continue to use them. However, one does need to be aware of technical innovations and changes or improvements to what was once accepted practice. Simply because some method has been in use in a particular way for 25 years is no reason to continue with it if a better method emerges. By better I mean one that either increases the quality of the model produced, or makes the job easier or faster, thereby releasing more time for playing trains.

Raymond has produced a Guild video on resistance soldering.

There are a number of myths floating about on resistance soldering, possibly even more than there are about soldering with an iron; all of them complete nonsense. A controversial statement, but one I am confident I can support with evidence. I learned the black art of soldering by reading what I could find on the subject, trying it out and comparing notes with other modellers. One aspect of soldering that I really dislike is cleaning off excess solder. It is a waste of time and solder, and potentially dangerous. More than once has the scraper (usually a broken and ground old needle files) embedded itself in my hand.

The following views are based entirely upon my own knowledge and experience and naturally reflect my own prejudices. However, if they help others to work faster with greater accuracy to gain greater enjoyment from their modelling, my efforts will be well rewarded.

A definition of soldering could be persuading two or more pieces of (frequently dissimilar) metal to stick together permanently at room temperature. Over time a number of ways have been devised to achieve this end. I have used for instance, gas torches, blow lamps, soldering irons of many and various types and an RSU. I have even heard of the gas hob being pressed into service, though I do not recommend this latter as is it likely to upset the domestic authorities.

Aside from the gas hob these methods require the effective use of a tool. Granted, neither an iron nor an RSU is necessarily a simple tool, but it is a tool nevertheless. To use a tool successfully one needs to know how to use it; what it can and cannot, do. A tool of itself can never be bad, only its usage (or possibly poor manufacture, but I am assuming that you only buy good tools). The only use I have for a soldering iron is electrical work. The tools I use exclusively for all other soldering work are an RSU, plus a gas torch for rare use with heavy metal sections.

Over the years many people have told me that an RSU cannot be used to perform this or that soldering task. I disagree totally; an RSU to my mind is simply a technological improvement upon the soldering iron, provided that one takes the trouble to learn how to use it, to discover of what it is capable.

The RSU is an improvement over the soldering iron for a number of reasons. There is no need for various shaped bits, no need for special cleaning of coated tips, the bits cannot get welded to the iron, no need for the tinning of parts to be sweated together, little or no excess solder to remove. The method uses less solder and electrical power is required only when actually soldering, therefore no iron sitting on the bench all day (or all night because one forgot to turn it off) burning the pennies and, blessedly, no more burned fingers.

The ideas for the base plate and magnets discussed later are based upon methods expounded by the late lamented Bernard Weller. I use a readily available RSU, 179ºC solder cream and wire lead solder and, at times, standard 12% phosphoric acid flux. Rare earth magnets are also effectively an extra pair of hands.

How does an RSU work? The box simply contains a transformer with several output windings that produce differing voltages. In mine the outputs are of 1, 1.5, 2, 3, and 4.5 volts. However, these voltages are generated at very high amperages, in the order of 40 amps for the higher settings. The power to the probe is controlled by an on/off foot switch. The negative terminal is attached to a steel base plate on which the items to be soldered sit and the positive to a carbon probe. Aluminium is completely useless as a negative terminal as it oxidizes rapidly, preventing good conduction, and will not of course allow the use of magnets to hold parts in place. In use the probe is placed upon the work piece resting on the steel plate and the foot pedal depressed. This completes an electrical circuit that produces a high temperature almost instantaneously, but much localised, around the point of the probe due to the inherent resistance at the point of contact. This is the reason why there must be no loose connections in the circuit; these would create local high resistance spots and reduce the output where it is needed.

Some units are provided with a negative lead having a large crocodile clip at one end. I never use clips of any kind to transmit electricity to a model because I have in the past burned some neat sets of teeth marks into models. When using the chassis jig I have an extra negative lead bolted to the jig and am seriously considering making another to bolt to the bench vice for soldering laminated rods together.

At its simplest one holds items to be joined on the plate with the probe and applies heat using the foot switch. The solder cream between the parts melts very rapidly and joins the parts. As soon as the solder has flashed silver and or has stopped smoking, one switched off the power, still holding the parts together. With good quality solder cream the joint will 'go off' in seconds and the joint will be strong (I strongly advise against coarse grained solder pastes; having used them and found that often the joint fails after a very few years). It is normal in using an RSU to produce joints that have little or no solder visible, making cleaning up almost redundant. The lead solder cream I use was designed for use in the electronics industry, it has no acid based flux but contains 2% silver to aid the flow.