This posting was originally from the SCX Worldwide site.

Fixing your SCX Digital chip: Part 3

Replacing the D2 Diode or Q6 Transistor (Pit Box dimming issues)

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.

All right, it’s time once again for another installment on fixing the chip. This one was a bit more complicated for me in respect to finding where the problem was located (no obvious cracks or burn marks to give it away). However, once it was found and I had the parts to make the fix, it wasn’t any harder than any previous repair. Thus, I’ve done the hard work for you and now it’s just a matter of using a voltmeter & knowing how to solder new parts into place to carry out this fix.

Let’s begin with this video from Bryan Young showing the symptom of the problem.


As you can see in the video, the mere action of adding throttle while this defective car (chip) is on the track produces a symptom of dimming the Pit Box (on low throttle) or resetting it completely (on high throttle).

So after messing around with a few chips that had this issue (thanks to “offdatrack” for letting me use his bad chips as guinea pigs), I was able to isolate the problem down to either the D2 Diode or the Q6 Transistor (not both; only one or the other). For reference, here is a pic showing where they are located on a V1/V2 chip (note that I have removed other pieces on the board in order to provide this un-obstructed view of the bad parts).

Either one of these two pieces will be shorted as a telltale sign that they “could” be damaged. For D2, both ends of this diode would produce a reading of .002 (or basically zero) on the continuity setting of a voltmeter to indicate there is a problem (a good diode of this design would register .655 on the voltmeter’s continuity test). For Q6, the Collector and Emitter (C and E in the diagram below) would produce a .002 reading as well to indicate the same issue of possible damage.

(Diagram courtesy of Toki and SCXWiki)

But here is the problem. The diagnosis of either piece must be done with one of the pieces “removed” from the board. The reason is because when one or the other is damaged, it makes the other unit appear to be shorted as well since the damage of one will basically close the circuit between them and give a false positive to whatever part is actually in good condition (I hope what I just said didn’t confuse anyone).

So to make this a bit simpler, just follow along carefully and hopefully it will all make sense @ the end.

One , do a simple continuity test on both D2 and Q6 while they are still on the board just to make sure the points that I just mentioned produce a .002 reading. If you get these readings, we can continue on. If not, then we’re looking @ the wrong place completely and I just wrote this thread up for nothing.

Two, secure the chip in something like a vice or clamp as shown in the several pics below (not too tight or you will break the chip). Due to the small size of the digital chip, it is difficult to work on if it’s simply not secured down. Besides, you will need both hands to accomplish this repair.

Three, remove just D2 from the board. If anything, based on what I experienced with four of these bad chips, of them had the D2 diode as the problem. Also, D2 is simply easier to reach vs. Q6 (more on that later). So, in order to make it easy to remove D2, it would be best to de-solder the right side power pick up arm as indicated below to give you the space needed to work on the D2 extraction.

Once the arm is removed, take an X-Acto blade in one hand and a soldering iron in the other to “slowly” remove the diode. The trick here will be that you’ll heat up one side of the diode @ a time and place the X-Acto blade under it to create some lifting action as the solder liquefies (see pic below).

This process will take a bit of time since you can only move the piece a few micrometers @ a time per side (you will probably have to add a tiny bit of solder to get the solder liquefied more quickly). Eventually, the X-Acto blade will work its way completely under the diode as you continue to heat up both sides and then it will eventually lift completely away from the chip. As noted in the pic below, the solder pad for D2 should look like this now with no solder bridges touching each side.

Four, retest D2 and Q6 again. Now that D2 diode is removed from the chip, take a voltmeter and perform the continuity test on it again. If you are still reading a .002 on the meter, the diode is damaged. Also, go and test the Collector and Emitter of Q6 (which is still on the board) and you should now have a reading that is now about .510 which means the transistor should be fine. (Go to step 5)

If the readings you get indicate that D2 gives a reading of .655 and Q6 (C and E) gives a reading of .002, this indicates that the diode is good and the transistor is bad. (Go to step 6)

Five (if D2 was the problem), install a new diode. So now that we know D2 is bad, we need to install a replacement unit. With the help of “Toki” of SCXWiki to help identify the part and “offdatrack” for directing me to the only supplier in the US that sold this part, I was able to identify the part as a 100 Volt, 350mA microMELF diode. Here is the link for anyone interested in purchasing them (only 5 cents a piece).

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With the part in hand, the process of re-soldering this piece requires the use of tweezers and the soldering iron. Place the new diode in the tweezers making sure [u]the black stripe on the diode faces the front of the chip[/u]. Place it on the board as shown in this pic and solder the right end first.

Once you know the right end is secure, the tweezers are no longer necessary and you can easily solder the left end into place. Afterwards, run the voltmeter on all the points discussed earlier and make sure you no longer have any .002 readings. Now re-solder the right side pick up arm and put the chip back together. Place the chip in a car to run it and everything should be fine again.

[color="#FF0000"]Six (if Q6 was the problem)[/color], remove the Q6 transistor. Because of the lack of space available to reach the solder points on Q6, the only way to extract this part is to first remove several other pieces that are in the way (as I hinted in step 3). The parts that need to be removed are illustrated in this pic.

You will have to remove the pick up arms (red), the reed switch (blue), and the oscillator crystal (yellow) [As noted by Krazy Kal, the oscillator crystal is not polarity specific so it can be re-installed in either direction). The pick up arms and reed switch will need to be de-soldered on their respective ends and the oscillator crystal from the bottom of the chip. See below.

Now that it’s all removed, we have plenty of room now to de-solder the Q6 transistor (see the first pic again). As with the removal of the D2 diode, the procedure is really no different for the transistor. You will need the X-Acto blade again to help lift the bad transistor as each of the three pads is heated up one @ a time. Start with the collector (at the front of the chip) and slowly work your way around to the other points while slowly exerting force under the transistor w/ the blade (once again, you'll have to add a bit of solder to get the liquification process started). Eventually, the part will give and you will have it extracted. Clean up any solder residue as noted in the pic below while ensuring none of the pads are bridged by any solder.

The process to install the new transistor will be similar to the diode. The tweezers will be needed again to place and align the new transistor into its proper orientation over its solder pads. Use the tweezers to keep the transistor in place as you re-solder the collector (front of the chip) into place and then lift the tweezers away (the collector end will now hold the transistor in place) to solder the other two connections back in place. Check all three solder points with a voltmeter to ensure you are not getting any .002 readings.

And before I forget, here is the link for the replacement transistor that you can order from DigiKey (94 cents each if you order a minimum of 10 pieces). Once again, thanks to Toki and SCXWiki for providing this information on their website.

Now that the transistor is in place, go back to step 5 to solder back in the “original” D2 diode (not a “new” one since we know the original was not damaged).

Afterwards, re-solder the oscillator crystal (yellow), the reed switch (blue) with the “flat ends” facing up and down, and the pick up arms (red). Put the chip back together and place it in a car to test. It should run if everything was re-assembled properly.

Resolving one last problem (car going full throttle at all times)

It should be noted that a secondary issue related to the Q6 transistor could cause the car to go full throttle at all times.

It is already well documented on SCXWiki but I’ll mention it anyway since this was one of the things I discovered while working on “offdatrack’s” chips. Simply speaking, when a car crashes really hard and it happens to be using an early version of the V1 chip, the Q6 transistor can actually break off from its solder pads. The reason this happens on the early V1’s might have something to do w/ the amount of heat used to solder the parts onto the board which in turn produced weaker connections. Obviously, this problem was discovered early on by SCX so it would be rare for any of the current chips out there to suffer this minor problem.

So, to remedy this issue, simply take the cover off of your V1 chip. Look inside and chances are you will see the Q6 solder pads sitting empty with no transistor in place (or the transistor is sitting beside the pads and obviously disconnected). Go to step 6 and follow the instructions to solder in a new Q6 transistor (or use the old one if it was still there) and everything should work again as it should.

So that is it for this posting. I did end up fixing five chips for “offdatrack” and sent them on their merry way back to Kansas to live out their lives once again in a fast, sleek, slot car. That’s a whole lot better than being thrown into the garbage, wouldn’t you say?