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Labjack U6 with three blown chips

U6 reading voltage blown defective U21 U16 D752 multiplexer

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#1 Automotive

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Posted 29 October 2013 - 05:22 AM

Hey guys,

 

I'm struggling a bit with diagnosing a defective Labjack U6. I'll provide as much (visual) info as possible, and hopefully I'll be able to resolve the issue.

 

So, here goes...

 

During a schoolproject I've encountered a defective Labjack U6, which previously was used in an engine dynamometer. Even though I'm normally not active in this area of expertise, it is pretty essential to search for the cause of the defects, because we were lucky enough to receive a spare U6.

We're now working with our last and only U6, so we want to make sure it doesn't blow out immediately or during runtime when hooking it up to the engine dynamometer again.

 

Because that's what had happened to the previous one. We already discoverd that the Labjack was dealing with a lack of voltage (1,17V between VS and GND). PC's couldn't recognize the device, and none of the status LED's lit up during connection. After opening the case three (clearly) blown out chips were visible, marked as; U6, U16 and U21.

 

U16_U21.png    U6.png

 

With help from the functioning U6 I was able to read off the markings from those three chips.

 

U6: D752 TI83 Z54W

U16: 62

U21: AVF8X

 

U16 is described here in an other topic as a control chip, and that should not be relevant to the other two chips blowing out, would it ?

 

U21 I guess would be a '8X' mulitplexer ?

 

U6 I found after extensive search to be a 12-bit digital to analog converter, by Texas Intruments. I've attached the datasheet. Did I get this right, or have I found the wrong part ?

IN the U6 chip's case, the entire left side seems to be blown out. The side which seems to handle the analog input and output voltages from DAC_A and DAC_B according to the pin description in the datasheet.

 

Finally we come to the point where I'm at now, stuck. I'm trying to connect these blowouts with the ports of the Labjack. For use with the dynamometer we have signals like RPM, temperature, load cell. All in all we're tryin to figure out if any of these signals could have caused too much of a voltage on those chips and caused them to blow out.

 

Again this is all done as an important precaution. We can't afford to risk 'lighting up' our second Labjack as well.

 

I'd appreciate any help, suggestions, ideas whatsoever. If there is need of further clarifying things, I'll be glad to explain.

 

 

Attached Files



#2 LabJack Support

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Posted 29 October 2013 - 10:15 AM

U6 is a D/A converter that is part of the system to provide analog outputs DAC0 & DAC1.  U16 is a USB port protector that clamps voltages between all 4 USB lines (5V, GND, D+, D-).  U21 is a current limiting switch that controls inrush current when USB is first connected and also shuts down the device if you try to draw more than 0.5 amps.

 

I suspect that in addition to the 3 chips where you see visible damage, other chips are also damaged.

 

The most likely cause of something like this is a major overvoltage event on the power rails.  Either VS or GND got pushed away from its norm, and the difference between VS and GND became much more than 5V during some event.  The likely suspect is something on a VS or GND terminal.  It might be possible for a severe overvoltage on an I/O terminal to cause this, but all I/O have protection so it is difficult.

 

If you want to share a wiring diagram, or send it to [email protected], we can take a look for further ideas.



#3 Automotive

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Posted 30 October 2013 - 05:47 AM

Thanks for replying.

 

I do have a table that shows what wires were connected to which ports of the Labjack. Strangely enough there wasn't anything connected to the DAC1 and DAC0 ports.

 

Dyno connections.png

 

Note that the table isn't complete. On the third row of the Labjack U6 with the FIO0 and FIO1 ports, an LJTick DAC was attached.

 

Soon we'll be testing the possibility that there would have been an over voltage by trying to run the engine dyno and reading all the voltages from the signals that would normally go to the Labjack.

 

Some voltage readings that I've done in the past on the Labjack can be seen in the attachments. I had already searched through this forum for some reference numbers.

 

USB U6.png    Readings table.png

 

If there is one, would it be possible for me to see an internal wiring diagram of the U6 ? It's nigh on impossible to follow all the connections on the PCB only. ?

 

 

 

 



#4 LabJack Support

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Posted 30 October 2013 - 08:19 AM

A good troubleshooting technique is to remove all connections from the damaged U6, including USB, and use a DMM to measure resistance from VS to GND at any screw terminals.  This will typically read megaohms.  If it reads 18 ohms, things are damaged.  The next step is to remove parts one at a time until the resistance jumps up to 100k+.  You would start by removing the chips that are obviously damaged, then protection diodes (D6-D9, D19, D47, D3), then other ICs.

 

As for measuring voltages, readings from the various testpoints, versus GND, might provide some insight, but probably not since I suspect a bunch of chips are damaged:

 

TP1                 typically 5V

TP2                 typically 5V

TP-VAMP+      typically 13V

TP-VAMP-       typically -13V

TP-5VA+         typically 5V

TP-VADAC      typically 5V

TP-USBV         typically 5V

TP-3.3V           typically 3.3V

 

*TP1 and TP2 are on either side of a 0.1 ohm resistor.  Measure TP1 versus TP2 to calculate supply current.



#5 Automotive

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Posted 04 November 2013 - 01:41 AM

Sorry for the late reply. As far as removing chips from the pcb goes, I'll have to get permission for that since it's not my property, and a chip extractor. I'll have to get back to you on that one.

I did measure the resistance between the VS and GND terminals in current state. They were all around 145 Ohm, with +/- 1 Ohm difference.

 

By the way, could you point me out where I can locate diode D9 ? I found every other one without too much problems but I just can't see this one anywhere on the pcb.

Also, here are the voltage readings from all the diodes (except D9). I did my best to set the measuring points of the test leads on the right places.

 

V_readings diodes.png

 

The voltage readings from all the testpoints are as follows:

TP1                 4,84V

TP2                 4,81V

TP-VAMP+      0,694V

TP-VAMP-       0,146V

TP-5VA+         1,181V

TP-VADAC      1,190V

TP-USBV         1,193V

TP-3.3V           1,200V

 

So it seems TP1 and TP2 are on par with what they should be. Does this tell anything special, maybe one component can be excluded from being defective ?

 

To conclude, until now I had a hunch the cause of all this started with the signal from the Load Cell of the engine dyno. The Load Cell Signal cable was connected to AIN2, and the GND cable obviously to the nearby GND port. As soon as the dyno was turned on the signal roughly red 12V.

 

However, taking a look at appendix A of the user guide it seems that the analog input ports won't be damaged up to a voltage of 20V compared to GND. So I guess this is a dead end...

So the next step will be removing chips until the resistance values jump up too the right value.



#6 LabJack Support

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Posted 04 November 2013 - 09:50 AM

D9 is between the green LED and the first screw terminal block.

 

I am afraid none of the voltage readings shed any light.  Still just suggests that a bunch of chips have been blown.  I doubt that even going through the process of removing parts and attempting repair will shed any light.  I suspect we will simply find that the 3 chips with visible damage, plus many other parts, are blown.  Perhaps a 120VAC line from somewhere got shorted to GND for a moment?

 

I suggest you make a wiring diagram and share it with us.  You need a diagram that literally shows each wire.





Also tagged with one or more of these keywords: U6, reading voltage, blown, defective, U21, U16, D752, multiplexer

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