Resistance soldering units.

Discussion in 'Tools' started by Keith M, Nov 7, 2020.

  1. Keith M

    Keith M Staff Member Moderator

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    For those of us who build brass/white metal kits, an RSU can be a useful but pricey tool, no substitute for a soldering iron, but useful for many small and/or intricate jobs. The best known version, only made in small batches more or less as demand requires is the London Road Models unit, quite a versatile bit of kit with the option of several different voltage/current settings, but accordingly is presently priced at £210+£16 P&P ('cos it's heavy!). That's a lot of dosh, you can build a 4mm model kit for less than that, and go a good way towards more than half the cost of a 7mm one, so anything that might save a good few bob is a welcome change. (especially for 'Tight*rses' like me!).
    So I decided to have a look around, and perusing t'internet, I came across a couple of American sites with suggestions of DIY versions using the transformer from a dead Microwave oven. Going down this route, you remove the entire secondary windings from the high voltage (2Kv) transformer, substituting 3 or 4 turns of heavy gauge welding cable (or similar) to give a 3-4 volt output at much increased amperage, depending on cable diameter/turns ratio.
    I didn't happen to have a deceased microwave to hand, so looking further and way past the American RSU's costing $400/$500, I came across a British Company called "Frost Restorations" who market tools for the Classic car restoration market, they do one at a much more wallet-friendly £99.99 including carriage on 24 hour FedEx, so I took the plunge, and this is what you get.

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  2. Keith M

    Keith M Staff Member Moderator

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    The red button is actually supposed to be a 'foot switch', but rather than switching the unit on/off, it switches the unit from 'Normal' to a 'Boost' setting, not a lot of use as it means the probe part is always 'Live' (only very low voltage, but a high-ish current). The destructions included with the unit say that the output voltage is 6 volts, quite high for modellers needs, but as a retired Electrician, I have a decent digital multimeter which measured the open circuit voltage as 2.53 volts, much more in the range that we as modellers need, maybe the unit has been re-rated after feedback from users, who knows, but it'll do nicely for me.
    The probe being 'Live' all the time is about as much use as a back pocket in a string vest, so the simplest way around that is to do as I did and visit that well known auction site and buy a footswitch, I chose an 'Industrial' all-metal version (under a 'tenner') rather than a cheap plastic one. With a spare 13amp flush switched socket, suitable mounting box, a couple of metres of 3 core flex (at least 1mm squared cross-section) and a plug head, it's simple to set up the socket so that the footswitch switches the supply to the socket on and off as the pedal is pressed. Here's my setup.

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    Last edited: Nov 7, 2020
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  3. Keith M

    Keith M Staff Member Moderator

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    Then you need some form of 'Earth plate' to actually work on, fastening down any brass fret with magnets onto the steel plate, which is the negative electrode. After a root around in my garage, I came across a suitable bit of 3mm steel plate, drilled and screwed to an offcut of timber, and an M8 screw with Nyloc nut at one corner to attach the negative croc clip to. It needn't be anything fancy as long as it does the job, here's mine.

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  4. Keith M

    Keith M Staff Member Moderator

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    I must point out that most of my constructional soldering is carried out using an Iroda gas microflame, which I find more versatile than electric soldering irons, though these have their uses on occasions. Anyway, now it's time to 'have a bash', I chose what I think would be one of the more difficult/delicate tasks that an RSU might be used for as a test, that of attaching lamp brackets to a fret body without 'vaporising' said bracket with too high a current. Using a couple of offcuts of thin brass fret strip, I bent up a mock bracket, tinned the back of it (I'd use solder paste here, but I'm still awaiting delivery!) using "Ersin" multicore Electronics solder, which I normally use for construction, it's a 60/40 Tin/Lead solder, so makes for a stronger joint, once tinned and flux applied (Phosphoric Acid), I held the bracket in place on the strip with the probe, pressed the foot switch for 3 seconds until I could see the solder melt, and that was that, job done and wonder of wonders, NO cleaning up, so definitely a:thumbup: for me!
    Now I'm not gonna say that this unit is as versatile as the London Road or similar versions, 'cos it's obviously not, BUT for the use I'll put it to, as I'm not building models on commission it will be fine for my use. Where the London Road versions are the "Rolls-Royce" of RSU's, the Frost unit is in the "Ford" bracket, the main thing that might take a little getting used to is releasing the footswitch BEFORE you remove the probe from the workpiece, otherwise the resultant 'Flash' might mean an impromptu underwear change!!!
    Here's my 'Baby's first attempt' joint.

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    Keith.
     
  5. paul_l

    paul_l Staff Member Administrator

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    I have Toto's version of this, and a foot pedal as per yours. Now I just have to find them :whatever:

    The earth plate is taken care of, I helped them clear some of the scrap metal from work, so a 12" square piece of 5mm plate is in the cave. Thinking of applying gun metal blue to help hold back the rust.

    Great tutorial Keith.

    Paul
     
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  6. Keith M

    Keith M Staff Member Moderator

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    My bit of steel plate had spent a few years in my garage and was a bit 'crusty' to say the least! It's not perfect, but emery paper followed by rotary wire brush in my Dremel clone brought it back to a reasonable state. The carbon rod fixing into the probe is rather crudely by a screw, so you need to be careful on tightening, enough to grip the carbon rod, but not so much as you break it. I have 3 14" long 'Air Arc Carbon Gouging rods' on order cheaply from China as spare rods, which I'll cut down into shorter and more useable lengths, possibly with ends filed into different shapes according to what the job requires. Only reason for ordering from China is that UK sellers only seem to sell in boxes of 50 rods minimum!
    Keith.
     
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  7. paul_l

    paul_l Staff Member Administrator

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    A pencil sharpener can be used to restore the point on the carbon rod, and nail boards to create chisel tips or custom shapes.

    Paul
     
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