Resistance Soldering

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junctionmad
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Re: Resistance Soldering

Postby junctionmad » Mon Feb 19, 2018 8:41 am

Jol Wilkinson wrote:Dave,

as the owner of LRM is a close friend of mine, I"ll leave you to work out your own RSU spec. Maplin used to sell a suitable transformer kit on which you could wind your own secondary outputs, but these are no longer available. I wound my own when the LRM unit was being developed but later replaced it with a professionally manufactured LRM transformer which was better.

The carbon probe is the easy bit, making a good probe holder is more difficult, which is why the LRM version is sold as a spare part and has been bought by owners of other makes of RSU.


Pete,

sometimes there isn't a good "connection" between the carbon probe tip and the workpiece. I keep a small piece of card near the return plate and rub the tip lightly on that. If I can write with it it is clean enough to work. The RSU voltage is low and won't always work on dirty or oily metal, so cleanliness is important.

Jol



Thanks re the probe

What I was looking for was the type of current needed to solder brass kits , I dont see the need for for low voltages as 1-2V , I suspect 4V is sufficient. I was have to custom wind the secondary , but that's not difficult

Edit , I'll use a triac based input control to reduce the output rather then custom wind the Traffo , it would seem that 4V may be on the upper end for brass kits , so I will have lower voltages via the electronic input control

Anyways , I have found considerable information on the " net " , I'm going to use a standard 0-6V toroidal Traffo, 50-100VA with electronic control on the input voltage , allowing me to control the output down to very low voltages . I can source carbon rods from welding supplies and modify an old soldering Iron body.

I am a professional EE , so in theory I should know what I'm doing :D
Dave
Last edited by junctionmad on Mon Feb 19, 2018 9:06 am, edited 3 times in total.

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Jol Wilkinson
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Re: Resistance Soldering

Postby Jol Wilkinson » Mon Feb 19, 2018 9:02 am

Dave,

all the RSU available in the UK have had multiple or variable outputs (notably the American Beauty unit for the latter). The Mignon Models unit had a switched primary input, but that only seemed to give +/- 10% output variation and was only available for a relatively short time. The LRM, Graskop, original Exactoscale and others all have/had multiple tapped outputs.

Like "ordinary" soldering, heat conduction away from the point of interest and therefore heat input required varies. So you'll need more current for attaching the last of several layers of smokebox wrapper, but less for a lamp iron on the back of a tender tank. Too much current can "blow" apart very small, thin, etched items, leave "burn" marks or cause local overheating of the metal and distortion.

Jol

junctionmad
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Re: Resistance Soldering

Postby junctionmad » Mon Feb 19, 2018 9:10 am

Jol Wilkinson wrote:Dave,

all the RSU available in the UK have had multiple or variable outputs (notably the American Beauty unit for the latter). The Mignon Models unit had a switched primary input, but that only seemed to give +/- 10% output variation and was only available for a relatively short time. The LRM, Graskop, original Exactoscale and others all have/had multiple tapped outputs.

Like "ordinary" soldering, heat conduction away from the point of interest and therefore heat input required varies. So you'll need more current for attaching the last of several layers of smokebox wrapper, but less for a lamp iron on the back of a tender tank. Too much current can "blow" apart very small, thin, etched items, leave "burn" marks or cause local overheating of the metal and distortion.

Jol


Well I can understand the simplicity of tapped outputs but that requires a custom traffo , these days it's easy to control the power to the input ,

I have a big 500W variac on my lab bench , so I can rig that up to a 100 VA 0-6Vac output toroidal and see what's currents are required, it looks like small parts only need around 2A to solder

Actually on second thoughts a conventional EI transformer would be better as the inrush currents are lower

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Jol Wilkinson
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Re: Resistance Soldering

Postby Jol Wilkinson » Mon Feb 19, 2018 11:14 am

junctionmad wrote:
Well I can understand the simplicity of tapped outputs but that requires a custom traffo , these days it's easy to control the power to the input ,

I have a big 500W variac on my lab bench , so I can rig that up to a 100 VA 0-6Vac output toroidal and see what's currents are required, it looks like small parts only need around 2A to solder

Actually on second thoughts a conventional EI transformer would be better as the inrush currents are lower


Which explains why most people don't build their own RSU's. Lack of knowledge on transfomer design/charactristics and no handy 500W variac.

junctionmad
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Re: Resistance Soldering

Postby junctionmad » Mon Feb 19, 2018 12:03 pm

Jol Wilkinson wrote:
junctionmad wrote:
Well I can understand the simplicity of tapped outputs but that requires a custom traffo , these days it's easy to control the power to the input ,

I have a big 500W variac on my lab bench , so I can rig that up to a 100 VA 0-6Vac output toroidal and see what's currents are required, it looks like small parts only need around 2A to solder

Actually on second thoughts a conventional EI transformer would be better as the inrush currents are lower


Which explains why most people don't build their own RSU's. Lack of knowledge on transfomer design/charactristics and no handy 500W variac.



The best methods would really be a DC controlled high current low voltage output with slew rate control on the waveform, This would eliminate almost all arcing and also prevent transformer saturation. IN the past this was expensive, but today , high current MOSFETs are two a penny

I must have a think about a better design that the system that essentially shorts a transformer :D

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Jol Wilkinson
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Re: Resistance Soldering

Postby Jol Wilkinson » Mon Feb 19, 2018 4:19 pm

junctionmad wrote:
Jol Wilkinson wrote:
junctionmad wrote:
Well I can understand the simplicity of tapped outputs but that requires a custom traffo , these days it's easy to control the power to the input ,

I have a big 500W variac on my lab bench , so I can rig that up to a 100 VA 0-6Vac output toroidal and see what's currents are required, it looks like small parts only need around 2A to solder

Actually on second thoughts a conventional EI transformer would be better as the inrush currents are lower


Which explains why most people don't build their own RSU's. Lack of knowledge on transfomer design/charactristics and no handy 500W variac.



The best methods would really be a DC controlled high current low voltage output with slew rate control on the waveform, This would eliminate almost all arcing and also prevent transformer saturation. IN the past this was expensive, but today , high current MOSFETs are two a penny

I must have a think about a better design that the system that essentially shorts a transformer :D


Might that be how the Amercan Beauty unit works? More powerful than the LRM and other UK model supplier units but also considerably more expensive at over three times the price.

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Le Corbusier
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Re: Resistance Soldering

Postby Le Corbusier » Mon Feb 19, 2018 4:45 pm

Jol Wilkinson wrote:
Might that be how the Amercan Beauty unit works? More powerful than the LRM and other UK model supplier units but also considerably more expensive at over three times the price.


Cripes ... and there's me thinking the LRM unit was a bit too pricey to justify at present :shock:

Back to the trusty Antex and concentration on skills refinement :thumb
Tim Lee

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LesGros
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Re: Resistance Soldering

Postby LesGros » Mon Feb 19, 2018 7:22 pm

junctionmad"
... This would eliminate almost all arcing and also prevent transformer saturation...


I am curious about this arcing. The only time I have seen arcing is on the odd occasion, when the footswitch was pressed just before applying the tweezers through being a bit clumsy.
LesG

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David B
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Re: Resistance Soldering

Postby David B » Fri Feb 23, 2018 4:30 pm

chrisf wrote:Resistance Soldering: Bob Alderman. A concise and well – ordered guide to the tool, its use and maintenance. EM Gauge Society Manual 5.7.3 (5)

Chris put a helpful list of articles on the previous page of this thread. I have a copy of Bob Alderman's article which appeared in the Missenden Manual and Gauge O Guild manual as well as the EMGS Chris refers to. For those who don't know of Bob's modelling pedigree, he is a larger gauge modeller, former technical advisor to the GOG and a long-time tutor at Missenden amongst many other activities.

With Bob's permission, I have attached a copy here. Note that it was written more than 20 years ago but is no less relevant today. In his email to me, he says, 'I will emphasize it is a tool that supplements a soldering iron and does not replace it.'.

RSU article Bob Alderman.pdf
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Enigma
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Re: Resistance Soldering

Postby Enigma » Fri Feb 23, 2018 5:35 pm

Hmm, interesting that he does not advocate acid flux, uses Carr's 188 paint and is also happy for the tip to glow. I've not seen my tip glow (yet!).

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David B
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Re: Resistance Soldering

Postby David B » Sat Feb 24, 2018 8:53 am

Enigma wrote:Hmm, interesting that he does not advocate acid flux, uses Carr's 188 paint and is also happy for the tip to glow. I've not seen my tip glow (yet!).

I think this shows how we should sometimes test what we read, not necessarily take it as gospel, and experiment. After all, if some people had not done so, we would not have made progress with the hobby. I use an acid flux and find it works very well though, as Bob says and I mentioned elsewhere, it can spit and ping loose bits of solder off when it boils. In the end, we find a way that works for us.

Regarding the glowing tip, this is more likely at higher power settings or if the tip is dirty and the connection higher resistance. I feel it preferable to use the minimum voltage necessary but when you have larger lumps of metal, as Bob does modelling in 7mm and larger scales, you need more power and then the tip is more likely to glow. This can also lead to the metal being burned and pitted. Yesterday I made some new leads for an RSU and when putting the flex in to the banana plug and probe holder, I finished off with my RSU which I had to bump up to 3v. The tip did glow a bit, something it does not do when I use 1.5v or 2v on my 4mm kits.

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Jol Wilkinson
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Re: Resistance Soldering

Postby Jol Wilkinson » Sat Feb 24, 2018 8:56 am

Enigma wrote:Hmm, interesting that he does not advocate acid flux, uses Carr's 188 paint and is also happy for the tip to glow. I've not seen my tip glow (yet!).


I've used an RSU for many years and struggled to get good results with solder paint or solder paste. I always tin the item to be fitted, usually with 145 solder, as I can control the amount of solder in the joint that way and use a liquid flux (Phosflux 12%).

Nor would I recommend a "glowing tip". That indicates a very high temperature at the point of contact, which may well cause surface burning and distortion. We are only trying to reach a temperature sufficient to melt the solder and to get it to flow within the joint.

Several writers have expressed the view that it is the temperature of the carbon probe at the tip that is doing the work (much like a soldering iron tip)and therefore red hot is good. While tip temperature may have an impact, it has always been my understanding that the heat caused by the electrical resistance within the joint is what does the work.

essdee
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Re: Resistance Soldering

Postby essdee » Sat Feb 24, 2018 2:03 pm

As the proud new owner of a LRM RSU, still awaiting unpacking, may I just say how invaluable all this advice and experience is proving. I have still a few more urgent tasks to deal with, but am looking forward to putting this to use, particularly with some rather nice MR clerestory coach kits at the top of my 'bucket' list.

Keep it coming, guys...

Steve

Enigma
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Re: Resistance Soldering

Postby Enigma » Sat Feb 24, 2018 5:03 pm

essdee wrote:As the proud new owner of a LRM RSU, still awaiting unpacking, may I just say how invaluable all this advice and experience is proving. I have still a few more urgent tasks to deal with, but am looking forward to putting this to use, particularly with some rather nice MR clerestory coach kits at the top of my 'bucket' list.

Keep it coming, guys...

Steve

Does this mean that your soldering will be even neater now!!??

essdee
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Re: Resistance Soldering

Postby essdee » Sat Feb 24, 2018 7:00 pm

Ha - thanks Paul. But no - it just means I will (hopefully) be spending a fraction of the time I have spent these last 18 years in laboriously scraping off excess solder?

Actually, much of that saving will also be attributed to following David's meticulous attention to minimising the amount of solder conveyed to a joint, in the first place, as in his recent writings in Finescale Model railway review and S4 News. So, thanks in anticipation, equally, to David, John and Jol, I think.

By the way, I have here an uncompleted Jidenco (yes, that long ago) beer van with slatted sides, LMS I believe; would you like to have that to practise on...NO?!.. well, fine, fine.........

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Guy Rixon
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Re: Resistance Soldering

Postby Guy Rixon » Sat Feb 24, 2018 7:23 pm

Jol Wilkinson wrote:Several writers have expressed the view that it is the temperature of the carbon probe at the tip that is doing the work (much like a soldering iron tip)and therefore red hot is good. While tip temperature may have an impact, it has always been my understanding that the heat caused by the electrical resistance within the joint is what does the work.

I used to believe that, but an experiment changed my mind. Reasoning that a solder-proof metal tip would do as well as a carbon, I got David Eveleigh to turn me up one from aluminium rod. Result: no soldering achieved, on settings and in circumstances that used to work with a carbon. The joint didn't get hot enough to melt the solder or boil the flux. Switching back to a carbon, complete with glowing tip, soldering happened.

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David B
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Re: Resistance Soldering

Postby David B » Sat Feb 24, 2018 7:52 pm

Guy Rixon wrote: Switching back to a carbon, complete with glowing tip, soldering happened.


What voltage are you using and what are you soldering? A glowing tip does suggest you may have your unit on a higher setting than is necessary . . . or that the point (or metal) is dirty and you are not getting a good connection. A glowing tip is not the norm for resistance soldering.

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Guy Rixon
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Re: Resistance Soldering

Postby Guy Rixon » Sat Feb 24, 2018 8:10 pm

The test was on two pieces of brass etch-scrap, cleaned up and pressed together face-to-face with Carr's 118 solder paste in between. The RSU was the MMS one and I tried all three settings with no obvious result using the aluminium probe. Switching back to the carbon, the unit was initially on the highest setting - too high for the job in hand - and that was when the tip glowed. The return path for the current was through an aluminium baseplate - about 3mm thick - and an M2 corner-post, then via a crocodile clip on the negative lead of the RSU.

I accept that the set-up may not have been ideal, but the test was under very-similar conditions for the metal probe and the carbon. One worked, the other did not. It's hard to see how the metal probe would have higher internal resistance than the carbon, so I'm fairly certain that the full voltage drop was across the joint.

There is one subtlety. If the tip of the probe is rough, then presumably the current is running through a few very-small contact patches and these therefore get very hot. In effect, the joint is being spot-welded in a formation of pinpoints. If the contact patch is larger, then the current is far more spread out and the peak temperature is lower, even though the energy dump is similar. It's possible that the metal probe had too large a contact patch w.r.t the carbon.

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Jol Wilkinson
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Re: Resistance Soldering

Postby Jol Wilkinson » Sat Feb 24, 2018 9:53 pm

Guy Rixon wrote:The test was on two pieces of brass etch-scrap, cleaned up and pressed together face-to-face with Carr's 118 solder paste in between. The RSU was the MMS one and I tried all three settings with no obvious result using the aluminium probe. Switching back to the carbon, the unit was initially on the highest setting - too high for the job in hand - and that was when the tip glowed. The return path for the current was through an aluminium baseplate - about 3mm thick - and an M2 corner-post, then via a crocodile clip on the negative lead of the RSU.

I accept that the set-up may not have been ideal, but the test was under very-similar conditions for the metal probe and the carbon. One worked, the other did not. It's hard to see how the metal probe would have higher internal resistance than the carbon, so I'm fairly certain that the full voltage drop was across the joint.

There is one subtlety. If the tip of the probe is rough, then presumably the current is running through a few very-small contact patches and these therefore get very hot. In effect, the joint is being spot-welded in a formation of pinpoints. If the contact patch is larger, then the current is far more spread out and the peak temperature is lower, even though the energy dump is similar. It's possible that the metal probe had too large a contact patch w.r.t the carbon.


Guy,

I find that a newly sharpened tip quickly rounds oft to a "soft" point. Provided I keep it clean (the writing on a piece of cardboard trck), then I get good contact. So do we have a slightly larger surface area with a slightly softer, more compliant surface than with metal? Probably.

Of course, the AB tweezers contradict this with their copper plated SS tips, but they probably apply a higher contact pressure and achieve good contact and current flow that way.

I don't know what MMS unit power rating or output current is, but I think it had a smaller transformer than the LRM or Exactoscale unit and is possibly less powerful.

Jol

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David B
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Re: Resistance Soldering

Postby David B » Thu Mar 01, 2018 1:58 pm

I tried a metal plate a long time ago and found it a pain, so ditched it and resorted to the croc clip I have mentioned many times. That first plate was too big, heavy, cumbersome and the metal too thick. However, in order to practise what I have been preaching, I shall experiment again with a plate.

First job - to make one! Being more aware now of the pros and cons, especially the latter, I have used a piece of 1.4mm steel which I found in my workshop. The width (50mm) was determined already; length was arrived at with the angle grinder and cutting out existing holes. The two plates in the image are 95mm long. I made a fence from 12mm aluminium angle and 12mm cube neodymium magnets which I have separated with a strip of wood and attached with double sided tape (for now). I have some other magnets with which to hold work. Underneath, I have attached some non-slip rubbery mesh with double sided tape.

How big does the plate need to be? I am reasoning that one is only soldering in a small(ash) area so one does not need a plate so large as to accommodate (say) an entire coach side. Time will tell whether my plates will be large enough but the fence is easily removable should I need a bit more space.

I have also made changes to connecting the plate. Surely if a banana plug is adequate at the RSU end, it should be satisfactory at the plate. I have used an M8 bolt and drilled a 4mm hole, one horizontal and the other vertical, to take a plug. If one is less satisfactory, I can change the bolt.

I would be interested to hear of other people's thoughts and experiences with using a metal plate.

RSU-plates_5311.jpg

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Jol Wilkinson
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Re: Resistance Soldering

Postby Jol Wilkinson » Thu Mar 01, 2018 2:16 pm

David,

I have one of the original Exactoscale plates which does take up a bit of workbench space, but used with magnets I also find it useful as a jig for assembling a variety of things including plastic wagon bodies.

The problems of heat conduction and current transfer are readily resolved. I use aluminium foil between the plate and the work piece for good contact and thin card under the location where I am using the probe to stop heat being sucked away by the plate. The foil also protects the steel plate from flux and is readily replaced with the permission of domestic management.

The banana plug will give better conductivity than a croc clip but not as good as a soldered eyelet bolted to the plate IMO.

Jol

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David B
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Re: Resistance Soldering

Postby David B » Thu Mar 01, 2018 2:42 pm

Jol,

The black plug is the same (apart from colour) as that used on the probe's lead with the LRM unit. Just how important is this connection to the plate given that the actual time we complete the circuit is a matter of seconds. The better the connection, the lower the resistance, but putting this in to the context of actual usage, is this really something to get hung up on?

I accept the shortcomings of the croc clip which can get warm, but that is unusual, again because one's foot is not on the pedal for very long. The clip is an alternative I find useful and it works for me!

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Guy Rixon
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Re: Resistance Soldering

Postby Guy Rixon » Thu Mar 01, 2018 4:23 pm

I also have one of the Exactoscale steel plates and commend Jol's approach for local insulation and rustproofing.

I also have a lightweight plate made of 2mm aluminium on a 3mm MDF backing, with an M2 stud in the corner for the connection. This, with the MMS RSU was part of my travelling modelling kit for a while, the Exactoscale gear being too heaving and bulky to cart around.

A crocodile clip on the M2 post works well most of the time, but it's easy to knock the clip into a position of poor conductivity. If using a clip and finding limited soldering power, I recommend adjusting the clip before going nuclear with the RSU settings.

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Jol Wilkinson
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Re: Resistance Soldering

Postby Jol Wilkinson » Thu Mar 01, 2018 4:54 pm

David,

the aim is to get minimum resistance around the circuit, so that the point where the joint being is - relatively speaking - high resistance, then keeping all the other connections as good as possible should be the aim. The banana plug into the concentric hole in the bolt probably gives more contact area than the cross hole but I would still be happier with a bolted connection.

Standard miniature croc clips as a direct connection onto the model - as originally used when RSU's first appeared in our hobby - aren't a such a good idea as the "teeth" are a set of high resistance points (even if they all make contact with the work). A miniature clamp with good contact area would be better.

Jol

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Flymo748
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Re: Resistance Soldering

Postby Flymo748 » Tue Oct 30, 2018 7:46 am

David B wrote:I tried a metal plate a long time ago and found it a pain, so ditched it and resorted to the croc clip I have mentioned many times. That first plate was too big, heavy, cumbersome and the metal too thick. However, in order to practise what I have been preaching, I shall experiment again with a plate.

First job - to make one! Being more aware now of the pros and cons, especially the latter, I have used a piece of 1.4mm steel which I found in my workshop. The width (50mm) was determined already; length was arrived at with the angle grinder and cutting out existing holes. The two plates in the image are 95mm long. I made a fence from 12mm aluminium angle and 12mm cube neodymium magnets which I have separated with a strip of wood and attached with double sided tape (for now). I have some other magnets with which to hold work. Underneath, I have attached some non-slip rubbery mesh with double sided tape.

I would be interested to hear of other people's thoughts and experiences with using a metal plate.

RSU-plates_5311.jpg


Morning all,

Having done some modelling for the first time in several months whe I was at Missenden a couple of weeks ago, I am now remembering how easy it is to use my RSU.

Following David's idea of having some alternative designs for working on, I'd also like to make a couple of sets of alternative leads. Following the demise of Maplin, that has become more difficult...

Can anyone point me in the direction of some suitable, flexible silicone covered, leads that i could use to build a couple of earthing rigs? I'm thiniing possibly RS online, but I admit that I'm a little lost as to best sources and best specifications.

Cheers
Flymo
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