Soldering technique

[JHZR2]

Background: dimmer potentiometer on 82 MB 300CD went bad after so many years. Common failure point.

Lighting is somewhat dim in these cars so putting it to max is common; so a jumpering repair is often the best bet.

I cleaned the connections, scuffed the brass, made a jumper with a tight fit in the holes of the potentiometer. Then Inwas ready to solder. I have an elenco 5-40w iron with a pointy tip.

I applied flux then started to heat the unit. The potentiometer is ceramic mostly, I tried heating the brass and the wire. I really had no success.

Used the thinnest gauge 60-40 lead solder I could find. No dice.

I figure in this application because of the hard friction fit of the jumper, the solder is just there for a mechanical support of the connection. But I'm not happy with the quality of the outcome.

I ran the iron at the 650-750F setting. When I stopped, the whole potentiometer was hot to the touch. So heat got in and spread, maybe just not enough heat to instantly warm up the local area.

Any thoughts on how to do this better? Do I need a hotter/more powerful iron? Works great btw.

Thanks!

 

[ARCOgraphite]

Not many options there for a nice jump for high current load. The solderability depends on the metallurgy and cleanliness- not all alloy are solderable. Your effort looks like it is just too cold due to the heavy heatsinking effect of the body, I dont even see any insulation burn on the wire..You didnt mention flux, but I see some rosin residue. Just to note, I've had to preheat high mass bodies in Telcom manufacturing to effect a good solder joint.

If the solder fails, I would make a buss out of small match stick sized piece of tinned sheet copper then mechanically slide it under the staked jumpers that are there, then fold the ends over to secure .

 

[ArrestMeRedZ]

That is one massive heat sink. I'm not sure how you could do a better job without heating the unit up so much it might "pop". A hotter iron or a torch might work. I think I'd just accept the job as it stands.


I fancied myself a decent solderier way back in the last millennium. Then I tried doing a job using the new, environmentally safe solder. I now realize either I know nothing, or that stuff is unworkable. I then went on a scavenger hunt to find as much of the old lead stuff like you used as I would need for my lifetime.

 

[Cujet]

Metcal, MX5200 is what you want for jobs like that. It will rapidly heat the part, without overheating it. (a bit insane on the price though)

The better solder stations are digitally controlled and will pour heat into the solder joint until it reaches the proper temp. Even a $50 one will do this.

 

[wrcsixeight]

I've had issues with cold looking joints like that one on the bottom, using a 140 watt weller gun. Other times the tip seems to take much too long before it can liquify the solder even when it is properly tinned.

Basically if the wire is thick or what I am soldering it to is a large heatsink, I have to remove the tip from the gun, an clean the contacting surfaces within, and then the solder liquifies in seconds of pullng the trigger.

I've also held my 40 watt iron and my 140 watt gun on the same wire, like when soldering 8awg to preheated alligator clamps.

Enough failures and I basically now take no chances with too little heat applied for too long. Surfaces get deoxidized and cleaned with IPA before the past flux comes out, the tips mating surfaces in the iron/gun get cleaned, and the soldering tip has a large liquid solder drip attached to it before it is lowered onto the work for more surface area and heat reservoir.

As said some of the store bought 'safer' solder sold today seems to exist purely to make people curse. Its melting point is too high and seems to be repelled from the surfaces one is trying to solder to.

 

[Win]

Why not just connect the two wires going to the rheostat together, maybe with a wire nut or crimp so you don't have to solder at all?

Since it works fine now, just leave it alone.

If you have to do the repair again, and still want to do a solder repair:

a) yes, a 5-40 watt Elenco iron is not the best choice for this type of task. However,

(edit: the hot air gun stations for SMD / SMT work / rework, are very inexpensive these days, and work really well. Sustained hot air might be a better choice here. You can get a smd / smt rework station with a temp controlled solder iron for $50 ish ).

b) instead of liquid flux, you might get better results with some non corrosive solder paste flux;

c) just use a small jumper wire, maybe a cutoff lead or two from a one watt or greater resistor, so it doesn't steal so much heat from your iron. The wire on the rheostat guts will be quite thin, can't see why the jumper would need to be larger than what is inside the rheostat.

 

[MasterSolenoid]

Once I was soldering copper fittings while installing a Hot Water Tank.
After several attempts, the fittings kept leaking …... I was going crazy.

I finally realized my mistake. I was using electrical solder.
I cleaned things up and used the correct solder (for plumbing) and no more leaks.

Now I have solder & flux for electrical work and separate solder & flux for plumbing.

I recently bought one of those Butane Solder Guns and they work great.
I've only used it for soldering connectors on to wires.

 

[Linctex]

Originally Posted by ARCOgraphite
Your effort looks like it is just too cold due to the heavy heatsinking effect of the body, I dont even see any insulation burn on the wire..


Yes - - you need a solder GUN, not a soldering iron. Something with 500 watts or more.

Harbor freight sells a nice one that is at a good price.

 

[Kestas]

True, power is limited on soldering irons. The trick is to get the terminals hot enough above soldering temperature, which is difficult on large parts that act as a heat sink. Only when the terminals are at temperature will you get a good solder weld. I restore old radios, and soldering directly to the chassis is a nearly impossible task.

Also I don't think you cleaned the terminal well enough. Most of the surface needs to be bright and shiny. The brass piece looks heavily tarnished. Flux isn't meant to chemically condition a tarnished surface.

 

[97K15004WD]

I would try a battery welder. I have used those on hard to solder parts with decent success. They are not hard to build and work very well on building battery packs.

 

[willbur]

I would say that you should tin the wires and connectors first, apply separate paste flux, and then solder. Works every time

 

[The_Jeff]

I backfed my w123 lights via the power window switch, or was it the stereo?

Either way they were on whenever the ignition was on. I'm such a butcher.

But you don't know they're on during the daytime because, of course, it's bright out!

You're overthinking this-- I'm sure you can hook a wire to the parking light feed and a wire going off somewhere else through the radio or console.

 

[meep]

for high reliability, we sometimes would use a 10-selector rotary switch instead of a rheostat, and then wire 9 resistors from point to point around the perimeter. You'd have 10 levels instead of infinite, but a high quality switch (harder to find these days) would hold up better over time. Not sure what your space allows, but it could give you options.... note- proper sizing of resistors plays a role...

 

[Donald]

I have always used resin core solder for electrical work and acid flux and solid solder for copper pipe.

 

[Lubener]

Tin your wire first with a solder gun and ditch the pencil iron. It's better suited for smaller work.. You didn't have enough heat to get the solder to flow through the strands and make a nice smooth joint.. Use a 60/40 ROSIN core solder. You also lost some strands in the stripping process.

 

[Lubener]

Originally Posted by MasterSolenoid
Once I was soldering copper fittings while installing a Hot Water Tank.
After several attempts, the fittings kept leaking …... I was going crazy.

I finally realized my mistake. I was using electrical solder.
I cleaned things up and used the correct solder (for plumbing) and no more leaks.

Now I have solder & flux for electrical work and separate solder & flux for plumbing.

I recently bought one of those Butane Solder Guns and they work great.
I've only used it for soldering connectors on to wires.

I have used electrical solder (I also used flux paste), soldered with a propane torch many times over 20 years ago and still do and no leaks ever. Yours failed because the surface was dirty. I always tin the pipe first after cleaning with steel wool, wiping the excess solder off with a damp rag prior to putting the joint together. If its dirty, you will know how well it tins before you even put the joint together.

 

[JetStar]

Your pot looks to be able to handle some power, the use of ceramic tells me that.
As others have stated, its a big heat sink.
The solder didn't flow because it probably didn't get hot enough.
Our rework operators in aerospace electronics will pre-bake the item at 250F to be reworked so that the iron doesn't have to raise the temperature on it's own. That's even with a temperature controlled solder station. Soldering is performed while it's still hot. This also reduces thermal shock on the ceramic. An alternate would be to use a heat gun.

 

[JHZR2]

Originally Posted by eljefino
I backfed my w123 lights via the power window switch, or was it the stereo?

Either way they were on whenever the ignition was on. I'm such a butcher.

But you don't know they're on during the daytime because, of course, it's bright out!

You're overthinking this-- I'm sure you can hook a wire to the parking light feed and a wire going off somewhere else through the radio or console.


This repair is pretty common. The pot slides on two male pins.

Originally Posted by willbur
I would say that you should tin the wires and connectors first, apply separate paste flux, and then solder. Works every time


Thanks!

Originally Posted by Kestas
True, power is limited on soldering irons. The trick is to get the terminals hot enough above soldering temperature, which is difficult on large parts that act as a heat sink. Only when the terminals are at temperature will you get a good solder weld. I restore old radios, and soldering directly to the chassis is a nearly impossible task.

Also I don't think you cleaned the terminal well enough. Most of the surface needs to be bright and shiny. The brass piece looks heavily tarnished. Flux isn't meant to chemically condition a tarnished surface.


Probably can never clean well enough. It was bright where the wires interfaced into the pot.

So when you need to solder to the chassis, what's your approach? Don't and bolt a connection?

Originally Posted by Lubener
Tin your wire first with a solder gun and ditch the pencil iron. It's better suited for smaller work.. You didn't have enough heat to get the solder to flow through the strands and make a nice smooth joint.. Use a 60/40 ROSIN core solder. You also lost some strands in the stripping process.



Thanks. Yes I need an actual gun. Got it. I used 60/40 rosin core lead solder and additional flux.

I'm really not concerned about loss of strands as the wire inside the pot is a single strand thinner than at one strand of Cu I installed. I really wanted a strong, low resistance mechanical joint that solder holds in place. So I purposely cut the strands to just the right number to fit intimately in the hole with good firm contact.