[SML] Fixing DMX Cables

[Dale Farmer]

If swapping out the cables chased the gremlins away, then they are 
damaged.   First, ensure they actually are DMX cables, not microphone 
cables with five pin connectors.  (They have different impedences, and 
that actually does matter with DMX as you get into larger and longer 
cable plants)
   You always need another three or five foot DMX cable.   So get some 
new connectors.  What brand of connector is sometimes a religious issue 
right up there with what is the  correct way to coil a cable.   Neutrik 
is a solid brand and readily available.
   Cut out the visibly damaged portion of the cable.  Be generous in 
cutting out the damage, as invisible damage will extend an inch or so 
beyond the visible damage.   Open up the connectors and see what color 
wire goes to which pin.  Write this down.  Don't assume all your cables 
are wired up the same.  Check the existing connectors for damage. 
frequently the flexing of the cable in normal use will cause an 
intermittent open in one of the conductors at or just past the connector 
strain relief.  give each of the individual tug.  If they slide out of 
the cable insulator easily, then that was also one of the problems and 
that whole connector needs to be redone.  Cut back an inch or so and 
start with fresh cable.

If you don't have a soldering setup, there are lots of good soldering 
tutorials on youtube.  For advanced skills, look at the tutorials that 
show NASA rated soldering skills.   Get an exhaust fan and some dryer 
hose.  Set that up to suck the fumes away.  the solder does get hot 
enough that lead fumes are a minor hazard.  the vapors released by the 
flux are rather more hazardous to your health.   Also, don't eat drink 
or smoke while soldering, and wash your hands right after.

   I prefer the 63/37% tin and lead solder with flux core.  It comes in 
various diameters, for this you want one of the smaller ones.  The 
various lead free solders are a lot more difficult to work with, and 
their fluxes I've heard, are alot more toxic.  The 63/37 has the 
smallest solidus zone, and reduces the chance of creating a cold solder 
joint.  ( as the solder cools off, it starts liquid, and then cools to a 
solidus state, which is essentially a slush of mixed solid and liquid 
metals.  Any movement of the wire relative to the connector pin during 
this time will make a cold solder joint.   Once it fully solidifies, it 
can't move.   )  If you drink a lot of coffee or have fine tremors in 
your hands, this is very frustrating.   If the solder joint looks dull 
and not shiny, it's probably a cold solder joint.  Reflow it.

   Soldering irons.  For DMX cables, I prefer a small chisel point. 
think flat blade screwdriver shape.   You have a couple of variables 
with soldering irons.  there is wattage of the heating element.  the 
higher the wattage, the faster the iron will heat up to the temperature. 
   there are temperature controlled irons that will sense the tip 
temperature and turn the heating element on and off as needed.   But 
those are spendy.   cheaper irons just have the heating element on and 
leave it on, those are a bit more difficult to deal with with the small 
wires as they can easily get too hot.    There is also the mass of the 
tip.  Basically, the more mass your soldering iron tip has, this is 
effectively reserve heat so the temperature swings are slower and 
smaller, easing your job.    Tin the tip well and take care of it.  The 
cleaner and smoother your tip is, the better it performs.  Buy spare 
tips and replace them as they get rough and pitted.

Soldering the connectors.  The pins of the DMX connectors are quite 
small in size.  they heat up fast and if you overheat them, it messes up 
the metal plating, and will soften the plastic they are imbedded in, and 
you can easily move an overheated pin in the plastic damaging the entire 
connector.  You will also burn your fingers on the pins and wires as 
they stay quite hot for longer than you would think.
   Tin the connector solder cups and the individual wires.   Get some 
flux, paste or liquid.  Using a toothpick or cotton swab dab a little 
flux inside each solder cup.  Then stick the end of the solder into the 
cup, then touch the hot iron to the outside of the cup just long enough 
to melt the solder.  The goal is to leave a thin layer of solder inside 
the solder cup.  Very thin layer. Your third hand will be useful here.

   Tin the wires.  Strip them back according to the diagram on the 
connector package.  Dab a small amount of flux on the bare wire.  Twist 
the strands together to keep them from sticking out.  Melt a small blob 
of solder on the iron and hold the iron horizontally.  Touch the wire 
with flux to the blob of hot solder and you will see the flux start to 
boil and then the surface tension of the solder will suck the molten 
solder right up the wire.  Pull it away when the solder is about halfway 
up the stripped length.
   Let things cool off a bit.  Assemble the connector on the cable, 
don't forget the strain relief before you solder it up.   check your 
notes on which color goes to which pin.  Slide each wire into the solder 
cup, touch the soldering iron to the cup with just a tiny blob of solder 
on it.  This will melt the solder in the cup, the solder on the iron 
will be drawn in by surface tension. the solder of the wire will melt. 
Withdraw the soldering iron immediately, hold the wire and connector 
immobile as the solder solidifies.  repeat with the rest of the pins. 
Check your work again, making sure you didn't do the wires in reverse 
order.  It's really annoying to find you did that or forgot to put the 
connector strain relief on and have to redo the whole thing.

Assemble the connector and test it again.   Coil it up neatly, take a 
deep breath and a stretch.  Do the next one.


Sorry, I didn't realize this was a hot button of mine.   LOL.

Dale


On 3/28/2024 7:57 AM, Ross via Stagecraft wrote:
> In the hunt for grimlenes in the lighting system, I found that some DMX 
> cables got to close to the fixture & the outer coating melted a little.  
> (Having replaced said cables the grimelines have gone away).
> 
> These cables are roughly 10'.  Is it worth the hasel to find some one to 
> fix them?
> 
> 
> Ross
> 
> 
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