OK, I was an idot. Now I have a problem.

Discussion in 'FAQs' started by retroguy1953, Feb 9, 2007.

  1. retroguy1953

    retroguy1953 New Member

    OK, I was an idiot. Now I have a problem.

    Hi Folks.

    To make a long story short, I accidentally reconnected a wire harness inside my N scale Burlington Zepher set reversed. There are three small PC boards (one in each car) interconnected by a seven wire cable harness. I totally spaced out on the fact that there was a white dot on the bottom of the connector that I should have lined up with the white dot on the pins - instead I saw a + symbol near the connector on the PC board and lined up the red wire in the harness to the position near the + symbol. Power on, and poof. Smoke. Dead lights (LEDs). Well I'm sure I shorted out and blew at least 3 diodes. The motor still works fine, and the LEDs come on in reverse (the setup is DC, not DCC). So it looks like I blew one side of a power rectifier circuit on each PC board. The diodes are very small and SMT. Looks like these diode modules are actually a pair of diodes in the same chip with a common collector - there are 3 leads, two on one side, one on the other. Of course, there is nothing stamped on the diodes that would help me identify them by part number. I'm wondering if I should attempt to solder in some small 12V diodes or just buy a new set of PC boards from Con-Cor. To top it off, I don't have schematics and I probably never will be able to get them. I have BASIC electronics skills only. These PC boards are pretty simple - a few resistors, a few caps, a few diodes, a few LEDs. They shouldn't be too expensive, but you know how much parts are for these suckers.

    What do you think?


  2. Russ Bellinis

    Russ Bellinis Active Member

    I would get new boards. The diodes and related circuitry reduce the voltage from the 12 volt max track voltage down to 1.5-3 volts for the leds.
  3. Torpedo

    Torpedo Member

    I think that if you are knowledgeable enough to use terms like SMT and common collector in a cogent sentence, then you are certainly knowledgeable enough to determine if you have the abilities and equipment needed to do the repairs. :D

    Besides, if the smoke has already been let out of the boards, can you really do any more damage?hamr
  4. cidchase

    cidchase Active Member

    Doug, I think if you take on re-building those little SMD boards yer a better man than I am!:D

    Unless you can really dope out the ckt I wouldn't even try, and I would see about the availability of new boards before doin' anything, I think. You could really spin your wheels :curse: trying to repair those little b_ds! (IMO) A junker might get you some used boards, if you could find one on ebay or ?? I'm not familiar with the engine, myself.
  5. retroguy1953

    retroguy1953 New Member

    Thanks for the input folks.

    I think Torpedo's advice was pretty basic - at this point, what do I have to loose?

    Well, I had never before attempted to work with surface mount components. The last brain surgery I did was putting together some DYNACO stereo equipment back in the 70's, and back then it was all through the board mounting. I gritted my teeth and fired up the soldering iron. Looking at each diode I could see which side had burned out and that gave me an idea where the current was flowing when the lights were on in the positive train direction. I removed all the diodes. I then jumpered solid wire across each pair of leads following the current path where the diode had burned out - and presto we have lights in the forward direction. I think that the power is rectified on the 12V side, as there are individual resistors inline at each LED, therefore I went to Radio Shack and bought some 20ma 12V diodes. Physically they are a bit larger than the gap between the leads on the board, but they should work. I'm going to pull the solid wire connections one by one and replace them with diodes. If I get the polarity wrong, I should be able to tell right away, and reverse it. Once I have every wire replaced with a diode, I will solder in the remaining diodes for the other side keeping the polarity consistent. That should give me back my lights in reverse direction. Maybe...

    The train is the N Gauge Con-Cor Burlington Zepher set. It consists of 3 connected cars, and is a 1930's streamline design. It is a great little model and quite pricey ($400 retail). It is new (2006) and was produced in a very limited set, so finding junk parts will be nearly impossible. I just know that those PC boards are going to cost an arm and a leg, and I'd rather not shell out a couple of hundred dollars if I can repair the boards for $10 worth of basic components. Of course I run the risk that I will mess it up and need to repace the boards anyways, but it's worth a shot.

    I was curious if any of the members had done similar work on these small PC boards, and what tips or thricks they might suggest.

    Thanks all again for your comments.

  6. cidchase

    cidchase Active Member

    Great site, wholesale pricing for even small quantities. Beats RS all to heck! I think you could find some very small signal diodes (maybe that's what you have already) which would handle the 20 mA, go more easily on your boards and still fit inside the shell.
  7. Torpedo

    Torpedo Member

    Try some 1N914 small signal diodes. Can handle 75ma and 100 volts and are dirt cheap if you buy a roll of 100 of more -- a penny a piece or less -- :thumb: -- or you can pay a buck each at Rip Shack. :cry:

    Search 1N914 price on Google and look for the term "tape reel." They come in a roll with narrow strips of tape on each end of the leads. They are designed for automatic insertion in PC boards, but are just as handy for manual work. :cool:
  8. baldwinjl

    baldwinjl Member

    Magnifier visor. Bright light.
  9. Jim Krause

    Jim Krause Active Member

    and don't forget some heat sinks.

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