MTL SW1500 - It seems so simple....

[gbcaboose]

Trying to install a Digitrax DN126M2 in a MTL SW1500. I've read the horror stories of the magic smoke on what appears to be a simple install. I disassembled and liberally applied kapton tape anywhere there seemed to be a chance of a short. I put the pieces back together and decided to test it before a put the shell on. I held the frame where each half would touch a rail on the test track and.....nothing. No motor, no light, no snap, crackle, or pop.

I removed the decoder and put the original board back in, touched a 9v battery to the frame. Motor's spinning and the light is on.

I put the decoder in and tried again with the same result as before. I tried to read the decoder on the program track and got nothing. I did notice that every so often the light would flicker when I had the frame halves touching the rails of the test track. Am I missing something here? Shouldn't it work ok without the shell as long as each frame halve is getting power from a rail. I even put anther loco on the track just to be sure it wasn't the throttle.

I suspect the decoder could have shorted, but I thought I would have seen (and smelled) that; Plus, I'm not sure if the light would occasionally flicker if the decoder had shorted.

Any tips or suggestions? I haven't even had the chance to make sure the contact tabs on the shell are going to be a issue [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ] [ Guests cannot view attachments ]

Any help would be appreciated,

Mike

[peteski]

Yes, testing the chassis alone is perfectly ok to do.

Do you have ohmmeter and made sure that the motor leads are not shorting out to the chassis (even with the Kapton tape applied)?

Try to power the loco with decoder installed using the 9V battery. Usually the DC mode is enabled from the facttoyu, so the motor should run.  But the DC mode is not guaranteed to be enabled.

The headlight flashing you see is normal when the decoder first gets power (you seeing it "booting up").  That's normal. But if the decoder is getting power, and the motor is connected, then it should be seen on the programming track.  But if you somehow damaged the motor driving circuit on the decoder, then there is good chance that the rest of the decoder still works ok.

If this is a factory fresh decoder then I assume you  set the throttle to a short address "3" and tried to run it and also tried to turn on the headlights. Correct?  If you use Digitrax system then address "3" is always short.  Other systems can have long or short address "3".

[tehachapifan]

I had one of these chassis where a lower motor lead solder joint poked thru the black insulating tape and was contacting the frame. I had to file-down the joint so it wasn't as sharp and pointy and I then reapplied (Kapton) tape over the joint. I can't recall now what was happening with the loco that made me investigate in the first place, so I can't say this is the same behavior or not.

[gbcaboose]

The decoder is new, fresh out of the package, so I'm using 3 as the address on a digitrax Zephyr.

I tried using a 9v battery and got no response. I'm not sure what the factory setting is as far as DC being enabled.

I got my el-cheapo multi-meter to ohm it out and checked the frame halves with the decoder OUT and it tested good, isolated. I put the decoder in and touched each frame half and got a reading, which I believe is ok (but not sure) because the decoder is bridging the 2 frame halves. But I did notice when I checked the original lightboard with the meter that the 2 contact pads were isolated, so maybe the decoder contact pads should be isolated too. When I checked the decoder directly, the contact pads were not isolated.

Mike

[peteski]

Mike, I don't own one of those "newer" SW locos, and I never did a decoder install on it.

I'm not quite sure if I follow the info you provided.  I don't quite understand the isolated pads on the light board.  When the decoder is installed in the frames, and decoder's pads are touching the frame tabs, then there should not be a short between the frame halves.  The decoder's internal circuitry connected to those pads should be seen as a fairly high resistance (or even open circuit) by your multimeter.

But did you check resistance between both motor leads and both frame halves?  They should not be connected.  If they were, then the motor driver on the decoder would have fried once you put the loco on the DCC-powered track.

[tehachapifan]

Something else I just remembered....

The wire ends that inserted through the holes on the decoder (or light board) on mine seemed like they might be long enough to make contact with the brass worm immediately beneath that insertion point. Thinking this could be a source of a potential short, and/or source of mechanical interference with the worm, I trimmed the length of the bare wire ends a little. That said, if you trim too much, they might not insert securely enough.

[gbcaboose]

Pete,

The "pads" it mentioned are the contact points where the lightboard or decoder contacts the frame.

I rechecked and found I had my multimeter on 200k ohms when I was showing continuity between the frame halves with the decoder installed. When I put it on my "normal setting"( not sure what that symbols called) it showed no continuity. As you can probably guess, I rarely you the multimeter for this purpose. I just puzzles me because this is probably the easiest decoder install I've ever tried, and it's kicking my butt.

Mike

[gbcaboose]

Something else I just remembered....

The wire ends that inserted through the holes on the decoder (or light board) on mine seemed like they might be long enough to make contact with the brass worm immediately beneath that insertion point. Thinking this could be a source of a potential short, and/or source of mechanical interference with the worm, I trimmed the length of the bare wire ends a little. That said, if you trim too much, they might not insert securely enough.

Thanks for that info. I don't think this is my current issue but I'll check it for potential interference with the worm, assuming I get this thing running

Mike

[tehachapifan]

Thanks for that info. I don't think this is my current issue but I'll check it for potential interference with the worm, assuming I get this thing running

Mike

But one thought might be that one or more wire ends is contacting the worm and shorting, causing your failure. I'm not sure to what degree of a short this may cause (maybe just the motor leads to each other if they're both making contact with the worm) and if it would be fatal to the decoder, but I would assume it could. IIRC, that worm (or was it a flywheel) sits just beneath the decoder.

[peteski]

Pete,

The "pads" it mentioned are the contact points where the lightboard or decoder contacts the frame.

I rechecked and found I had my multimeter on 200k ohms when I was showing continuity between the frame halves with the decoder installed. When I put it on my "normal setting"( not sure what that symbols called) it showed no continuity. As you can probably guess, I rarely you the multimeter for this purpose. I just puzzles me because this is probably the easiest decoder install I've ever tried, and it's kicking my butt.

Mike

I think this clarifies things.  In low-ohm range, likely "200" (or in continuity check range) there should not be any continuity between left and right pads (on either the decoder, or the light board).  But if you had the muiltimeter on the 200k range, then it is likely that it would have shown some resistance (continuity) between the left and right pads.  So from that angle, all is well. The left and right pads of either decoder or the light board show no continuity (open circuit) when you are using proper multimeter setting.

I'm still worried about possible short between either of the motor leads and the frames.  Did you check for continuity between both motor leads and the frame halves?  Should be an open circuit.

Yes, this should be an easy and simple decoder install . . . if all goes well.

[gbcaboose]

I checked the continuity between the motor leads and the cosponsoring frame halves and it tested good,open.

I already have another sw1500 on order which I also plan to install a decoder in. I'm planning on getting the 2nd decoder and installing it in the other sw1500 when it arrives. If, and that's a big IF, all goes well with the second install, I'll try swapping the decoders in the 2 in hopes of determining if the decoder is bad, or if it's a frame/motor issue. If my LHS has the decoder in stock, I should have some results by the New Year. Thanks for the help so far. I'll update this once I figure something out.

Thanks again,

Mike

[Ed Kapuscinski]

My advice: CAPTON TAPE EVERYWHERE AND EVERYTHING.

That is all.