Probe problems

Before and after comparison of a pesky grub screw – in the upper of the two holes.

If you browse through my posts over the past six months, you’ll appreciate that I have done a lot of soldering. I’ve assembled 17 brass kits for Edwardian era Great Western Railway rolling stock, and I’ve made significant progress on three S scale freight motors for the Niagara St. Catharines & Toronto Railway.

All of this progress comes at a price, however: I’ve worked my way through the tip on the probe of my resistance soldering unit.

The tip is actually a copper-clad carbon rod, secured into the body of the probe with a grub screw. The tip of this rod gets consumed in the soldering process: One rod will last a long time, but not forever, and as it’s burned away one loosens the grub screw and pulls more rod out of the probe. I have plenty of spare rods, but when I went to remove the remains of the old rod to fit a new one, the grub screw that secures it in place would not budge.

This is not surprising. Building in brass involves a lot of heating and cooling, and the screw and the piece into which it’s threaded are made of different materials so they’ll expand and contract at different rates. In addition, the flux I use – a liquid known as Ruby Fluid – is very effective at prepping metal for soldering, but it’s also quite corrosive and seems to wick and seep everywhere. I suspect these two factors combined to lock that screw in place.

I could not budge that grub screw, and a search online turned up distressing news: it seems the manufacturer of my unit – Hot Tip – recently went out of business. So, finding a replacement probe is not on.

Resistance soldering units are apparently still available from American Beauty but they have a different sized probe. Each model has its advantages and drawbacks – and in keeping with the mantra of “the right tool for the job”, I know some modellers who have both a Hot Tip and an American Beauty on their bench.

I’m certainly considering that myself, but that still leaves me with the question of how to fix my Hot Tip probe.

Fortunately, “I know a guy…”

I discussed the problem with my friend Chris Abbott and he said “send it to me”.

So, I packed up the probe, a replacement tip, and a thank you gift (because I know he would not accept payment from me) and off it went. Chris tried several things over the course of a few days and as the lead photo shows, he was able to remove the grub screw. He’s now sourcing a replacement fastener – we might go with something easier to insert and remove – and will send it back as soon as that work is done.


Going forward, between work sessions I’ll remove the fastener and probe tip, then shoot some contact cleaner on both the fastener and through the hole into which it screws. It’s a few extra steps but it’ll save me grief in the long run.

(If you’re interested in machining, Chris has launched a show on Twitch TV under the name “Mechaform”. You can find it here. I’ve dropped in occasionally and while most of it’s over my head I’ve enjoyed what I’ve seen.)

Speaking of Twitch, I’ve found myself feeling pretty twitchy since the probe packed it in. I have a new appreciation for just how much it means to me to be able to build things, especially when I have a lot of work projects on the go and need to step back from the computer and think about the material I’m writing. A half-hour at the bench clears my mind and often results in a fresh perspective that improves the material for my client.

While I wait for the probe to return home, I’ve been finding projects to work on that don’t require the resistance soldering rig. These include adding smaller details to models, which I can do with a soldering iron – and as noted in a previous post, I have also been prepping to install DCC and Sound in my NS&T freight motors.

Finally, I’m pretty close to finishing NS&T 8, 15, and 19 – close enough, at least, that I can start thinking about what’s next. To that end, I’ve drawn up a master parts list for four more NS&T freight motors – 17, 18, 20, and 21. I’ve given each of these projects its own box to hold photo etches, detail parts, power trucks, and so on. I’ve printed out photos to aid construction of each and sorted these into binders (with plastic sheet protectors, so I can flip through them in the workshop without getting muck all over the pictures). And I’ve placed orders for the parts I’m missing.

Over all, I’m really pleased with my progress on modelling a railway that made a huge impression on me more than 40 years ago, and that has been on my “must-do” list ever since. And a little problem like a grub screw isn’t going to stop me!

Published by Trevor

Lifelong model railway enthusiast and retired amateur shepherd who trained a border collie to work sheep. Professional writer and editor, with some podcasting and Internet TV presenting work thrown in for good measure.

2 thoughts on “Probe problems

  1. I really enjoyed this update. Super happy to read it may be possible to repair the probe you have. Even more happy still that it is another connection to a friend—that’s the most wonderful of all.

    You make a wonderful point about the positive power of being able to invest even just a few minutes into modelling or making something as a break in the day. It’s too easy to wander away from the work on the computer but only as far as a new tab in the same browser window which isn’t a true break. For me, being able to walk from my work computer and phone the few steps to where my modelling bench is or as many steps in the opposite direction to be in the kitchen to make something like some bread or as simple as fresh tea is so good feeling. Often my work is a larger puzzle and since it’s contained in a computer it lacks a tactile element and the feeling of work lacks the form of work done in modelling or cooking—touch is just so important.

    I love this era in your modelling. Each update is a touchpoint on a project that just radiates fascination with cool subjects realised as models. It feels so good to read.



    1. Thanks Chris. I’m fortunate that my career allows me to work from home. The workshop is 26 paces (I counted) from my desk. It’s around a few corners, out of sight, so I’m not tempted to visit more often than necessary while working – and it also feels like I’m in a different place since I can’t see the home office.
      Melting solder into joints, or turning something on the lathe, or spraying paint, or otherwise fettling over a project it’s a wonderful reset for my brain. I think my work is better as a result.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: