Friday, August 19, 2016

Control Jumper Repair

One of the little necessities of life if you want to run multiple-unit trains is a reliable control jumper. And fixing ones that go bad is a good project to work on at home.  The CA&E cars all use a GE 9-pin jumper, and we had one in the shop that had been accidentally pulled apart a while back, so I took it home for repair.  Rich Schauer helped with shaping up the damaged plug casting at one end last Saturday.

 So what we have looks like this.  This end appears to be undamaged.  I checked the continuity of each wire, and one thing that immediately popped out was that the wires don't match the GE standard color coding, which is printed on every control circuit, for instance.  Wire #1 is red, #2 is green and white, etc.   You can rely on the color codes for car wiring and usually for the jumpers, in my experience, but here they're random.  That's not a big deal, we just make a new table.  The other thing is that the wires are all stranded copper, such as is used for car wiring.   Control jumpers usually have a special wire in which about a fourth of the strands are steel, for added strength in an application where there's constant bending.  So for unknown reasons this jumper is not quite up to spec, but it should still be usable for museum service.

As you can see from the first picture, the wires all broke near one end, inside the plug.  Each of the solder terminals needs to be cleaned out, and we wind up with a pile of pieces as seen above.  

The easiest way to do this is to have a block of wood with a threaded hole.   The terminal is put in the block, then the solder is melted and the old wire is pulled out.  To attach the good wires, the process is reversed: heat up the terminal, fill it with solder, and insert the bared end of the wire.   The wire will be cold going in, so it needs to be heated for a few seconds to insure a good solder joint.  

And now it looks like this.  Then each terminal is inserted into its proper place in the ceramic casting.

 The holes in the back of the casting are hexagonal, so the solder terminals can't turn.  The connectors in the front of the casting are then tightened into the terminals with a large screwdriver.   It's that easy.  Of course, I recheck all of the connections with a VOM.  

Now we repair to the kitchen.  In order to keep the connections water-tight, they are coated with sealing wax.   So we melt some wax in a tin can placed in boiling water.

 The plug is placed on the floor.  You will note the hole, which has its pipe plug removed.  The melted wax is poured into the plug, filling the holes in the ceramic casting and sealing up the connections.

I've noticed that the two bolts holding the ceramic casting into the socket are usually filled with wax also, so we can do that at the same time.   Check the connections once again, and we're done.

Electrical Testing

Update: Since somebody asked, here are the details on the electrical testing setup used.  A VOM works fine for checking that the wiring is correct, that each pin goes to the corresponding one at the other end.

We also want to check that all wires are isolated from each other and that there's no leakage to ground.  This is best done with a megger, and we have one at IRM.  But it's an expensive instrument for which I would have little use at home, so instead here's my usual test setup using 120V AC that costs essentially nothing.  Note also that 120V RMS has a peak amplitude of 170V, so we're not doing so bad in checking for high-voltage breakdown.

We just have a standard 25W lightbulb and a power cord, with two probes from a former VOM, although almost anything will work as long as you're careful.  One of the probes is connected through the bulb, and the bulb lights when the two probes touch.  Now you want to check that the bulb does not light when the probes are put onto any two different terminals at the same end.  But we can be a lot more sensitive than that.  As a detector, plug a radio into the same outlet and tune it to an AM frequency where there's no station.  It will make a characteristic spark noise when the probes are connected, even for small amounts of leakage.  In the case of this jumper cable, there's a non-negligible capacitance between any two wires, and we're using AC, so you will hear a slight noise when probing for isolation between two wires.  But with a little practice it's easy to hear the difference.


Anonymous said...

Did you use a meter to check that each wire connects to the opposite location as you would expect?

Randall Hicks said...

A VOM works fine for checking continuity, and making sure the wiring is correct. For checking isolation (insulation) I use 120V AC. I'll add a picture and description of the test setup to the post.