A warning. A Mystery Fault - Explained - I think

[Plummet]

At the end of August we went on holiday. We went to stay with friends on an island which does not have a model shop. I took an almost completed Multiplex FubCub with us in the hope of getting some flying in. I took a small toolbox containing everything I thought I might need. (We stay with friends, and my DIY skills (skills???) are often called for while we are there.) I had my soldering iron, and my box of soldering tools and jigs with me.

So, I complete the last bits of assembly, charge up a lipo, bind the receiver, and try the motor.

Nowt - nothing - not a murmur.

I do have another motor available( for my wife's plane, so I cannot steal it) , but its bullet connectors are a different type. I manage to kludge them into contact with the bullet receptacles, and this motor runs perfectly. The ESC is OK.

I check the motor's continuity with a multimeter, and I discover that one of its three leads does not seem to have any connection to the other two. Duff motor I think.

I contact the model shop I bought the stuff from, and they sell me a replacement motor, with the promise of a refund on return of the duff one.

When it eventually arrives I unsolder the bullet connectors, and solder them onto the new motor.

This motor also fails to work! A duff batch, I think.

I contact another model shop and get a replacement from them. It arrives eventually.

I desolder and resolder. This motor at least bleeps, but does not run properly. I examine the connections. On one lead some solder has "tinned" the bullet connector. I assume that the different electrical characteristics of the tinned connector must be effecting the timing of the ESC. I contact another modeller on the island, and scrounge a new set of bullet connectors. I fit them, and all is well.

The model flies well. I got in one flight before time and weather ran out.

We are now back home.

This morning I re-checked the duff motors before taking them back to the shop. They seem to be perfectly OK.

What have I done wrong?

I have a little block of plaster that I have drilled a variety of holes into, These holes are the right size to hold bullet connectors. I have used it quite often. I used it for all my soldering mentioned here.

I use electronic solder, with the flux in the solder. I think that what has happened is that the flux has accumulated in the block's holes, and when I have been soldering the bullets the flux has melted and coated the mating surface of the bullet. Thus, when the bullets are connected the circuit is not complete. When I check with the multimeter, the flux acts as an insulator, and the motor appears duff.

I have to use the same bullets for the second motor, and the same thing happens.

New bullets, and all is well.

The morals of the tale?

1. Do not create a flux bath in which to bathe the bullets while they are being soldered.
2. Get some electronic cleaning solvent and clean the bullets after soldering them.

Plummet

Edited By Plummet on 27/09/2012 16:58:35

[Braddock, VC]

Or get a new piece of plaster each time. I have to do this as I always lose the bit I used last. Now I'm quite glad I did!

Thanks for the headup.

[Radge]

Well done on the investigation anyway, could have got expensive with no results. I use one of these little things, they're cheap enough and great for all sorts of soldering and other jobs when another hand is needed.(and they're really cute)

[Biggles' Elder Brother - Moderator]

Mmmm. Did you actually find resin on the bullets Plummet - or are you surmising what the problem may have been?

You see I've got a couple of problems with this explanation:

1. I've never seen that much resin released from electronic solder!

2. Gold is pretty reluctant to let anything grip to it - that one reason why its used, along with its very low resistance. So I'd be a bit surprised that, even if you did get resin on the bullet, that it would stay on there forming a 100% perfect insulating coating despite having been pluged in and out a couple of times.

Just a thought - I suppose stranger things have happened!

BEB

[Shaunie]

General Garage Solvent (AKA Brake Cleaner) is an excellent solvent for the flux in cored solder. Pretty safe on most plastics but can't vouch for foams though, make your own tests.

I presume Halfrauds sell it, you can buy it in 500ml spray cans from motor factors and probably an awful lot cheaper than whatever special cleaner Maplins or whatever charge. I buy it in 5 litre cans for about 8 quid IIRC (I hardly ever do any brake work as I'm an Autoelectrician, but I clean an awful lot of PCBs with it after repair work and I've never had a problem yet).

Shaun

[John Privett]

Hmmm, interesting... I'm not convinced about the explanation, but I can't come up with a better one!

I had something very similar happen a few years ago with my first electric model - which had a brushed motor. I'd initially checked the motor by connecting it directly to the battery (8-cell NiCad!) and then soldered bullet 4mm connectors to the batteries, ESC leads and motor leads. It was only later that I tried to run the motor via the ESC, but got no response at all. Looking at the soldering of the bullet connectors I couldn't see any bad joints so assumed that the ESC was faulty as the motor still worked when I connected it directly to the battery.

I got a new ESC and wired it up and it worked. I slung the "dead" ESC in a dark corner somewhere where it remained for many months until curiosity got the better of me. For some reason (and I've no idea why I tried it) I resoldered the bullet connectors and was amazed to find it now worked. As I said, the soldered joints looked good (and I might not be the best solderer in the world, but I'm a looooooong way from the worst!) yet at least one of the bullets was not conducting as it should.

The "flux on the outside of the bullet" is an explanation I hadn't thought of until now. It still sounds a bit unlikely - but I can't think of a better one!

[Chris Bott - Moderator]

Here's another unlikely one that really surprised me a while ago.

Somehow, and I don't remember the details how, but I ended up with some pink zap (very thin cyano) running into an already mated and working 4mm gold bullet connection.

Not only did it securely fix the plug into the socket, but it completely insulated the connection too. I put it down to a completely freak one off, put new connectors on and threw the failed pair in the bin.

Just shows though how little it takes.

[Plummet]

Posted by Biggles' Elder Brother - Moderator on 27/09/2012 19:40:33:

<snip>

1. I've never seen that much resin released from electronic solder!

<snip>

BEB

Ah yes, BEB, but you forget that the flux was accumulating in the little holes from a series of solderings. It was not just the flux from one joint.

I do know that it is easy to burn out motors if the connections are poor, so I was very reluctant to run extensive tests.

Using the multimeter the joints seemed OK, but I was stabbing the points of the test leads onto the bullets, and I could just touch bare metal on the wire inside the motor. I must have scratched the flux off during this test, but not in places that touched when the connector was mated.

I intend to do more tests on the motors when time permits.

Plummet

[David Tweddle]

Plaster whats that, I can only afford a borrowed clothes peg.

Ps not a blind hole in sight...

David

[John Hickson]

IPA is a good enough flux remover, especially when combined with an old toothbrush

[Erfolg]

Moral, is to use a piece of Balsa with holes drilled in it for the bullets.Never had any problem, the wood does char with time, but a new piece of balsa would cure the issue, if it becomes more than an observation.

[Gordon Tarling]

Bullet connectors of any kind are very prone to flux ingress, particularly under the spring 'basket'. If that 'basket' isn't free to revolve after you've fitted the bullet to the wire, it's probably got flux under it, so give it a good wash in methylated spirit or similar alcohol until it does revolve easily. On the larger (3.5mm and up) connectors, I usually remove the sprung basket before I solder the bullets on - it can save a lot of hassle later on!