Friday, July 2, 2010

Mindstorms NXT screen fix

For a while my screen on my Mindstorms NXT had been broken. When I would turn it on and press buttons, it would make correct sounds, but the screen would be blank or sometimes show flashes of lines across the screen. Putting new batteries in and updating the firmware did nothing. I did a search online and this matched the symptoms of other people. They had found a solution:
re-solder 3 capacitors inside the NXT.

Thankfully this procedure only required "tapping" the contact points, which means to re-melt the existing solder so that the connection is better.

Feel free to use this post to learn what to do in this situation. I've tried to include enough information for that to be possible. I get a lot of views of this post every month; there are a lot of people with is problem.
If you've never soldered before, I suggest you watch a couple of videos showing how. Maybe practice soldering two wires together first. But don't be daunted, with some practice and common sense it's really not that hard and pretty fun.
Here are two places that I used as a resource: - 5/3/12 sorry, this link appears to be dead now. Nxtasy has ended and now they have a small site with some forums.

Opening was simple, just remove batteries and unscrew the 4 screws underneath.
Here's what the inside looks like after the cover is removed:

I needed to get at the underside of the daughterboard, the secondary circuit board connected to the screen. The screen is screwed in by the white plastic it is attached to. The daughterboard is attached to the motherboard by pins (two rows of metal leads, left and center in the picture above) and needed to be gently pulled off. The speaker, which is attached to the screen, is attached to the motherboard by wires and can't be taken off completely.

Here's a close up of the insides.

I put the screen back in place without screwing it in and attaching the daughterboard pins. Then I flipped over and taped down the daughterboard. Here's its underside.

Here are the capacitors that are causing the trouble. They are surface mount capacitors, so they are small, compact, and don't have wire leads. Examples of "normal" capacitors can be seen above as large, black cylinders on the motherboard above (look at the "insides" picture).

Tips for tapping/re-soldering the capacitors:
-Be patient and wait for the iron to warm up, it won't really work with a cold iron. Test it by touching it to solder from a reel; it should smoke/burn/sizzle right away and not cool off on the iron.
-You may need to apply some pressure to the solder points to melt the existing solder.
-Don't hold the iron on the circuit board too long! It may not seem like it, but electrical components can get damaged from the heat. That can certainly happen to these small capacitors. Work quickly.
-If you need to apply new solder, go ahead, that might work better.
-Clean up the connection by dragging a clean soldering iron across/by the connection. The iron and the contact points will wick away extra solder to themselves, and you will clear away any bridging mess in the middle. This is especially helpful for the area between the C1 and C3 capacitors.

I had learned my lesson from before to be patient with the soldering iron and wait for it to really heat up. But this time I learned that I needed to apply much more pressure to the contact points to make the solder melt. After that was done, I did a quick test without closing it all back up. It showed more stuff on the screen, but it still was sketchy. I flipped over the daughterboard and tapped the C3 capacitor again, which I didn't think I did a good job on. That did the trick.

Here is the fixed screen. I just laid the rubber buttons on the button contacts so I could operate the NXT.

Hope that helps!

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