Author Topic: What did I fry to make all DC voltages and resistance read negaitive ?  (Read 299 times)

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Offline lordvader88

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I have a $10 XL830L which is a clone of a DT830x it seems, with ICL7106 and ICL7107 (1 is LCD driver). On mine they are integrated and under a blob.

This thing runs off 9V, and I was powering it off a 12V SMPS, into a LM317 to get 9V. (I meant to be using a linear transformer, but wasn't)

I was de-soldering a SMD LED with a grounded soldering iron station, and for what ever dumb reason I used the COM DMM tip to get the LED off. I think it was on either 20VDC or buzzer/diode tester. And 5-6 times the LED glowed about as bright as maybe 0.5-1mA would cause. IDK if I also touched the COM tip to the soldering directly.

And then later when I went to use it, all voltages are reading negative. With no input, it reads  ~-6.7v, or -0.67, or -0.07 as I switch voltage ranges. So a 9V battery reads as -6.7+9=2.3V

The resistance reads as negative as well.

Here's the basic schematic for these clones
http://indexxit.com/wp-content/uploads/2017/11/cute-using-multimeter-digital-schematic-diagram-masl-simple-circuit-dt830b-pdf-dt9205a-with-lcd-hd-44780-dt9208a-dmm.gif

and the ICs
https://www.intersil.com/content/dam/Intersil/documents/icl7/icl7106-07-07s.pdf

No sign of damage at all on the SMD stuff.
 

Offline Chris56000

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Hi!

I think there's an internal short or low impedance between Analog Common (Pin 32) and V- (Pin 26) within the '7106 IC - this will pull all the I/P high and reference circuitry to approx. - 6.5 to - 6.8V with respect to the Input Low, which the cause the A/D converter to read what you're getting - as the meter's one of those with an epoxy-blob all you can do is bin it I'm afraid - these things can be bought from China for the price of a Morrison's cafe sandwich (with free postage !) on fleabay these days!

Chris Williams
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 

Offline wraper

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Quote
ICL7106 and ICL7107 (1 is LCD driver).
They are both basically the same ADC except 7106 has LCD driver but 7107 LED driver built it. It's impossible to have both in one circuit.
 


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