FL-12 "weirdness"- won't work right

[nik_n_dad]

Last night Nik left a letter for Santa with some things he wanted for Christmas.  Of course, the missus and I didn't get him any of it.  Oops.  But never fear, I have my "stash".  So I try to do a quickie install of the EM-13 & FL-12s in a Kato N700.  No problem with the motor (after I shim it).  No problem with the lead cab car (headlights on when going forward, tail lights on when going reverse).  Next the tail cab car.  2:00 AM this morning and it's still not working.

The tail light (red) works fine when the train is going forward.  However, when I flip to reverse, the lead cab car switches to tail lights, but the trailing cab shuts off all its lights.  I tried flipping the decoder every which way, programming it for normal and reverse directions. I tried different decoders.  i tried decoders without programming it.  I reset the decoder (writing "8" to CVV 08).  No joy.

Next I take the "behaving" decoder from the lead cab car and put it in the trailing cab car.  Trailing cab car behaves now.  But no matter what I do to the lead car (repeating everything above), I cannot get the headlights to come on.  Ever.

I know I have to be doing something really goofy here, but I can't imagine what it is.

So, Nik got the Dr Tokai i was supposed to get for Christmas, but I still need to get this puppy working!

Any advice?  (No, drinking heavily probably won't help)   

[KenS]

It sounded at first like a placement problem (the pads on the decoder not contacting the pads on the car for one set of LEDs), but if the problem moves with the decoder, it's probably a defective/fried decoder. 

But there is one thing you can try: put the car with the decoder on the programming track and write "8" into CV08.  This will reset the entire decoder to factory defaults (which means you'll need to reprogram anything you changed).  CV08 is normally the manufacturer ID, and will always read 129 for Digitrax/Kato, but all Digitrax decoders (including the Kato's) will reset to factory defaults if you try to write an 8 to that CV. Since you don't get any confirmation on these decoders, do it a couple of times (wait ten seconds or so between each) to make sure it gets it.

If there was somehow a programming error, that should clear it.

The only problem I've ever encountered with the FL-12 is that it defaults to "off" (at least sometimes), and you need to hit Function 0 to turn it on (but that should affect both sets of LEDs connected to the decoder).

[nik_n_dad]

KenS-

I tried 3 different decoders last night, including one with the default settings, straight out of the package. Also did the reset.  I'm puzzled

[KenS]

Weird.

First question: did you test the misbehaving trailing car on DC (with no decoder) and confirm it works?  I presume so, but best to be sure.

Second question: do the misbehaving decoders misbehave in both cars, or only one?  If both, it's probably the decoders (although it could be placement).  If one, it may be the car or the decoder placement.

If it's only a problem with the one car, but multiple decoders it could be that they're all bad. But that wouldn't be my first guess.

More likely is that the decoder isn't seating properly. Sometimes the slot allows the decoder to  be inserted too far. This can look like half the decoder is working because half is contacting the wrong set of pads.
- First, make sure the two decoders are inserted in opposite directions to each other (i.e., looking at the photo below one end has a gold tab with a resistor with silver ends next to it, the other has a gold tab with a multilegged chip next to it; one should be slid into the slot resistor end first, the other chip end first, but both should have the side with the four gold tabs either towards the body or away, whichever works). If they weren't reversed but you saw the described behavior, then you almost certainly had a placement problem. Getting them properly reversed should cause the headlight to come on at both ends if there's a placement problem at one end.
- Now, if one decoder isn't working properly, pull it back slightly (like millimeter or so) using a pair of tweezers.  Do that a couple of times (up to about 2-3 mm).

If that's not it, then my best guess is that there's something wrong in the slot that prevents the decoder from making proper contact with the four pads. The way these decoders work is that the side with one pad on each side (the power pickup) contacts the brass strips that run the length of the chassis. The other side has two pads on each side, and these touch a matching set of pads inside the piece of plastic forming the underside detail of the car. On DC, each brass strip touchs both pads on that side (and polarity causes only one set to light at a time). With DCC, the decoder sits in the middle, but it has to touch the strips (on one side) and the pads (on the other) to work.  If something is wrong with the pads inside the slot, or the decoder is being lifted up at one end and not making contact, this would break the circuit.

If the decoders appear to all be bad, it could be static electricity damage. It's a good idea to work in an environment that minimizes static when working with decoders (no sweaters or cats, wood floor rather than carpet, moderate humidity) and to avoid touching the contacts or exposed metal traces (hard to avoid with these).  I work wearing a grounding wrist strap (which you can get at Radio Shack for a few bucks), which is a safe way to ensure any static on you drains away.