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Fixing your SCX Digital chip: Part 1 (Replacing the 3055L FET)

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  • Fixing your SCX Digital chip: Part 1 (Replacing the 3055L FET)

    This posting was originally from the SCX Worldwide site.

    Fixing your SCX Digital chip: Part 1
    Replacing the 3055L Field Effect Transistor (FET)

    Disclaimer: What you are about to read is in no way implying that everyone should be expected to repair their own digital chip. This is merely a guide for those curious enough and comfortable w/ electronics to explore at their own will. In addition, because SCX does have a two year warranty on everything they make, always contact SCX for warranty coverage on a damaged chip.

    Now that we have that out of the way, let us proceed.

    This topic is in regards to replacing the 3055L Field Effect Transistor (FET) found on the digital chip of an SCX Digital car. This transistor is responsible for providing power to the motor and when damaged will not move the car anymore (that seems logical). Below is a picture of the chip w/ the FET circled in red. The section highlighed in blue is a burn mark created when this FET overheated and died.

    When this chip started going bad in my Seat Cupra GT, the car was having issues getting up to speed and was literally stuttering all over the track until it finally just stopped completely. In addition, a burning smell did start to permeate as the car continued to sit on the track. I did not realize as quickly as I should that the FET (as well as the micoprocessor and motor) was overheating as the car sat there. Thus, the result is pictured below:

    So just as an FYI, when your car stops running but the lights continue to work (the bad FET does not affect lights), pull that car off the track ASAP and feel the underside for excessive heat (you should also smell something burning as well). If it's hot, let it cool down and you will eventually realize it was the FET that died (see my first picture again and notice if your FET has a burn mark).

    Now it's time to find the FET I need to fix this chip.

    Researching this issue wasn't really difficult because there was already information about it at this German website:

    The next step was finding a supplier that had this FET. After some confusion on what I was looking for, I eventually realized that a replacement 3055L was a pretty common part. However, I e-mailed Digikey and had them do additional research to find the exact unit that was closest to the one I needed. The following link will direct you to the exact part if you want to order some for yourself.

    They were not 100% sure if it was going to work for my application but I decided to take a chance anyway since the unit cost was 63 cents each. I ordered five of them along w/ ten reed switches to lower the total cost of the order.

    Time to make the repair.

    The concept of removing a part on a board is really simplistic. Use the the soldering iron to heat up the piece in question, apply the de-soldering tool (I prefer the vacuum pump) @ the point the solder is liquified, and then suck it out. Unfortunately, removing a surface mount transistor on such a small board created some interesting challenges I did not forsee. For one, because the board was so small, it moved around a lot and thus it was near impossible to heat up and desolder @ the same time (unless I had three hands to do the job). Also, surface mounts have the solder sandwiched between the legs of the transistor and the board so getting the solder liquified was difficult.

    The solution was a combination of a utilzing a vise and an X-Acto blade. See pic below:

    Simply put, I used the vise to hold the chip steady. I applied heat to the point circled in red. As I got the solder liquified, I used the X-Acto blade to gently lift that part of the FET away from the board. Now with one side of the FET free to move around, I simply moved it back and forth gently until the three legs circled in green simply broke off. I used my soldering iron to heat up the three "dismembered" legs stuck to the chip and removed them as the solder liquified. I further cleaned up the now exposed solder pads on the chip by removing any excess solder to ensure I once again had a flat surface to mount the new FET.

    The installation of the new FET was quite simple. I just placed the new part on the chip in the exact position it needed to be in and slowly applied solder to re-attach it. It is important to "press down" on this new FET as you are soldering to ensure it it totally flush w/ the board. If the FET is "floating" above the board, it will still work. However, the new part might stick out too much from below and possibly make it difficult to install the chip under the mounting brackets in the car. Here is a picture of the chip w/ the new FET:

    So how does the car perform?

    The moment of truth. Was this FET the correct one to fix my chip? To put it mildly, yes. I ran the car on a simple NASCAR tri-oval (good for max speed) and put the pedal to the metal for several laps. The car actually performed better than it ever had prior to the chip burnout. In addition, to give the new FET a good stress test, I had it push another car to see if the extra amp pull might trigger a problem. No issues whatsoever and of course no overheating (that's very important). Now how's that for saving a $17 computer chip w/ a 65 cent part. (I should note that I never really found out why the original FET burned out in the first place. However, I am assuming it had to do w/ the fact it was an older chip and I found over time that the traction magnet in the car was really set low and thus created more amp pull on the motor. That's what I get for acquiring a used car.)

    In conclusion, I do want to say that I could have easily resolved this issue by simply picking up the phone and calling Bryan Young for a new chip. And of course, he would have been glad to send me one. The purpose of why I did this was partly for education and partly for my feelings about being "green". I simply felt that if I ended up w/ 20 bad chips after five years, there was a good chance I would throw them away and have them rot in a landfill for the next 1,000 years. That did not appeal to me from the perspective of recycling and saving the environment. Thus, fixing the chip and being able to keep using it beyond what might be considered it's useful life made more sense to me. And let's face it, it also means more chips available to keep right on chipping more cars in the future.

    Anyway, I hope most of you found this topic interesting. Part two will be written up in about two weeks time regarding the replacement of reed switches on digital chips.

    Now get back to racing ,

    Last edited by LDFan; 09-10-2013, 12:58 AM.