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I have a Mindstorm NXT 2.0 from 2007 (PCB code: 62115.40H) at which the sensors except the simple "Touch sensor" are no longer working. Also the tacho-signal of the motors do no longer work. (The motors work but do not stop after e.g. 5 rotations)

By looking at the schematics of the slightly older NXT (1.0) I could figure out that the problem seems to be caused by a defect in the 4.3V output supply. So I opened the housing and measured at TP79 of my NXT 2.0 the voltage of IPOWERA, IPOWERB, IPOWERC, IPOWERD and POWERMA, POWERMA, POWERMC. The voltage was not having 4.3V while the 5V supply "VCC_RS485" from which the 4.3V output supply is generated was fine.

My problem: The circuit which generates the 4.3V output supply from VCC_RS485 was changed in the NXT-2.0, while I can only find the schematics of the older NXT (1.0). This made it for me impossible to find the defective component (since nothing looks obviously defective)

Does anybody know how to fix the 4.3V output supply of an NXT-2.0?
Or does anybody know where to find the schematics of the NXT-2.0?

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  • I found a schematic from what seems to be the NXT 1.0 here: lego.com/en-us/mindstorms/downloads I am only seeing 5V and 3.3V power supplies listed in the documentation, is the 4.3V in your question a typo?
    – Josh King
    Dec 6, 2017 at 20:49
  • Josh, the 4.3V are not a typo. As you can see in the PDF "LEGO MINDSTORMS NXT Hardware Developer Kit.pdf" available on the link you have mentioned, pin 4 of every sensor and motor connector provides a 4.3V supply voltage for the sensors (including the sensor inside the motors). There is no voltage regulator for the 4.3V. Instead it is derived from voltage VCC_RS485, which has 5V. The circuit for that (of the NXT 1.0) is on page 3 of the schematics. Check TP79.
    – Holger
    Dec 7, 2017 at 18:16

2 Answers 2

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Finally I've found a solution. It is not perfect since I wasn't able to find the schematics of the NXT-2.0 nor could I reverse engineer it. But this workaround seems to work pretty well.

This workaround works at the following conditions:

  • You have an NXT-2.0 (not a NXT-1)
  • The 4.3V voltage for the motors and sensors is no longer there (Voltage at C61 is zero.)
  • The 5.0V voltage VCC_RS485 has still 5.0V (Voltage at C57 is 5V)

As you can see in the image below I soldered a 1N4007 rectifier diode between the positive terminal of capacitor C57 and the positive terminal of C61. The cathode of the diode has to touch C61. Ensure not to touch the test-point T41 close to C57 since this should have 3.3V on NXT-2.0 (while it had 5V on NXT-1).

A silicon rectifier diode causes usually a ~0.7V drop. So it makes 4.3V from the correct 5V source. If you don't have an exact drop of 0.7V don't worry: The NXT motors work also with the Lego-EV3 and according to the spec. of the EV3 that brick sends even 5V directly to the motor-sensors. (So even a wire instead of the diode might work - but I used the diode to stay on the safe side)

To solder the diode you have to open the NXT and to disconnect the battery holder, which requires some desolder work.

Dependent on your skills this might be some tricky work. Please don't blame me if this kills your NXT totally. Only do it if your feeling qualified to do it and if you've understood the explanations above.

PCB bottom side of NXT2.0 with fix for external sensor voltag

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I was in same situation. In my case I fixed the problem replacing C61 capacitor that was shorted. I have found this schematic, but it seems not corresponding to 2.0 also because I can't follow it.

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    Welcome to Bricks! I'm glad that replacing that cap fixed the issue for you! Hopefully this information will be helpful to others as well. How did you identify that this was the problem? Could I just put an ohm meter across the C61 terminals to identify that it is shorted and needs to be replaced?
    – jncraton ♦
    Dec 12, 2019 at 16:38
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    I'm not an electronic professional, but I know that a cap doesn't conduct DC current and electrolitic capacitors are between the more error prone components. So with schematic in hand and the picture that @Holger posted, I put the multimeter in continuity mode and tested C61 cap. It beeped. I was lucky because that was my first try and surely the only problem I could have fixed.
    – eduardofilo
    Dec 13, 2019 at 20:19
  • Great! Thanks for sharing!
    – jncraton ♦
    Dec 13, 2019 at 23:30
  • I'm so glad I found this post! I had left the batteries in the NXT for too long and corrosion had broken some of the connectors, which I had just managed to replace. However, I later realized that I had the sensors and motors issues described above. After seeing this post, I had not much to lose and decided to disconnect the battery holder and test C61. Sure enough, it was shorted and I replaced it quite easily.
    – jfperusse
    Jun 13, 2020 at 4:53

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